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TRANSCRIPT
Confidential: For Review O
nly
Impacts of London's road traffic air and noise pollution on
birth weight: a retrospective population-based cohort study
Journal: BMJ
Manuscript ID BMJ.2017.039236
Article Type: Research
BMJ Journal: BMJ
Date Submitted by the Author: 03-May-2017
Complete List of Authors: Smith, Rachel; Imperial College London, Epidemiology & Biostatistics Fecht, Daniela; UK Small Area Health Statistics Unit, MRC-PHE Centre for Environment and Health, Imperial College London, Department of Epidemiology & Biostatistics Gulliver, John; MRC-PHE Centre for Environment and Health, Imperial
College London, Dept Epidemiology and Biostatistics, School of Public Health Beevers, Sean; MRC-HPA Centre for Environment and Health, King’s College, Environmental Research Group Dajnak, David; MRC-PHE Centre for Environment and Health, Environmental Research Group, King's College London Blangiardo, Marta; Small Area Health Statistics Unit, MRC-HPA Centre for Environment and Health, Imperial College London, Dept Epidemiology and Biostatistics, School of Public Health Ghosh, Rebecca; Small Area Health Statistics Unit, MRC-HPA Centre for Environment and Health, Imperial College London, Dept Epidemiology and Biostatistics, School of Public Health
Hansell, Anna; Imperial College London, Epidemiology and Public Health Kelly, Frank; King's College London Anderson, H Ross; MRC-PHE Centre for Environment and Health, Environmental Research Group, King's College London; Population Health Research Institute, St George’s, University of London Toledano, Mireille; Imperial College London, Epidemiology and Biostatistics
Keywords: birth weight, small for gestational age, air pollution, noise pollution, road traffic, particulate matter
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TITLE: Impacts of London's road traffic air and noise pollution on birth weight: a
retrospective population-based cohort study
AUTHORS: Rachel B Smith, research associate1, Daniela Fecht, research fellow
2, John
Gulliver, senior lecturer1, Sean D Beevers, senior lecturer
3, David Dajnak, deputy manager
3,
Marta Blangiardo, senior lecturer1, Rebecca E. Ghosh, research associate
2, Anna L Hansell,
reader & assistant director2, Frank J Kelly, professor
3, H Ross Anderson, emeritus
professor3,4
and Mireille B Toledano, reader1§
AFFILIATIONS:
1MRC-PHE Centre for Environment and Health, Department of Epidemiology and
Biostatistics, School of Public Health, Imperial College London, St Mary’s Campus, Norfolk
Place, London, W2 1PG, UK; 2UK Small Area Health Statistics Unit, MRC-PHE Centre for
Environment and Health, Department of Epidemiology and Biostatistics, School of Public
Health, Imperial College London, W2 1PG, UK; 3
MRC-PHE Centre for Environment and
Health, Environmental Research Group, School of Biomedical Sciences, King's College
London, Franklin-Wilkins Building, 150 Stamford Street, London, SE1 9NH, UK; 4
Population Health Research Institute, St George’s, University of London, Cranmer Terrace,
London, SW17 0RE, UK.
§Corresponding author
ADDRESS CORRESPONDENCE TO: Dr Mireille B Toledano, MRC-PHE Centre for
Environment and Health, Department of Epidemiology and Biostatistics, School of Public
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Health, Imperial College London, St Mary’s Campus, Norfolk Place, London, W2 1PG, UK,
Tel: +44 (0)2075943298, E-mail: [email protected]
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Abstract
Objective: To investigate the relationship between exposure to both air and noise pollution
from road traffic and birth weight outcomes.
Design: Retrospective population-based cohort study
Setting: Greater London and surrounding counties up to the M25 motorway (2,317 km2),
UK, 2006-2010
Participants: 572,910 live singleton births occurring within the M25 motorway between
2006-2010
Main outcome measures: Term birth weight, term low birth weight (LBW), small-for-
gestational age (SGA) at term
Results: Average air pollutant exposures across pregnancy were 41µg/m3 nitrogen dioxide
(NO2), 73µg/m3 nitrogen oxides (NOX), 14 µg/m
3 particulate matter with aerodynamic
diameter <2.5 µm (PM2.5), 23 µg/m3 particulate matter with aerodynamic diameter <10 µm
(PM10) and 32 µg/m3 ozone (O3), and average day (LAeq,16hr) and night-time (Lnight) noise
levels were 58 dB and 53 dB respectively. Interquartile range increases in NO2, NOX, PM2.5,
PM10 and source-specific PM2.5 from traffic exhaust (PM2.5 traffic-exhaust) and traffic non-exhaust
(brake/tyre wear and re-suspension) (PM2.5 traffic-non-exhaust) were significantly associated with
mean term birth weight reductions of 7 to 13g, 2-6% increased odds of term LBW (except
PM10), and for PM2.5 traffic-exhaust and PM2.5 2-3% increased odds of term SGA. Only PM2.5
traffic-exhaust was consistently associated with reduced birth weight after adjustment for each of
the other air pollutants. Air pollutant associations were robust to adjustment for road traffic
noise. Significant trends of decreasing birth weight across increasing road traffic noise
categories were observed, but were attenuated to null when adjusted for primary traffic air
pollutants.
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Conclusions: Our findings suggest that air pollution from road traffic in London, which is
dominated by diesel exhaust emissions, is adversely affecting fetal growth. Primary PM2.5
from vehicle exhausts appears to be driving these associations in this study. An apparent
relationship with noise appears to reflect confounding by traffic air pollutant co-exposures.
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Introduction
Air pollution is a major public health issue. It has been associated with reduced fetal
growth,(1) via which it may have extensive and permanent influences on the life course.(2)
A key contributor to urban ambient pollution is road traffic, and critically, vehicle emissions
are released close to people. Urban particulate matter includes a large contribution from
outside the urban area, and locally-emitted particles. Exposure is complex, close to roads an
individual would be exposed to more primary exhaust and non-exhaust (brake/tyre wear and
re-suspension) particles, but further away to more nitrate and secondary organic aerosol as a
proportion of their total particulate dose.
However, traffic also produces noise, which has been associated with e.g. hypertension and
cardiovascular disease.(3) Research on noise and birth outcomes is more limited, but a
possible effect on low birth weight has been suggested.(4)
Evidence about the relative roles of air and noise pollution on birth weight is limited and
inconsistent.(5-7) To address health impacts of traffic effectively these need to be better
understood. In this study, the largest to date, we investigate long-term exposure to both
traffic-related air and noise pollution during pregnancy in relation to birth weight.
Methods
Births data
The study boundary was the M25, an orbital motorway encompassing most of Greater
London (2,317 km2). 671,509 singleton births occurring within the M25 during 2006-2010
were extracted from the UK National Births/Stillbirth registers held at the UK Small Area
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Health Statistics Unit (SAHSU) and supplied by the Office for National Statistics. These
registers provide routinely collected data on all births in the country, including date of birth,
birth weight, sex and mother’s age. We appended gestational age and baby’s ethnicity from
the NHS Numbers for Babies (NN4B) dataset, with 99.2% linkage.
Birth addresses were geocoded using Quick Address Software.(8) Births in middle layer
super output areas (MSOAs) overlapping the M25 were excluded (N=7,493) because area-
level covariates would reflect populations inside and outside the study area. We obtained
2011 census output area (COA)-level data as follows: Carstairs deprivation index (9) from
UK Census 2011 standardised across COAs in study area; and 2014 tobacco
expenditure/week (population ≥16 years) from CACI Ltd, as a smoking proxy.
Air pollution exposures
Average monthly concentrations of nitrogen dioxide (NO2), nitrogen oxides (NOX), ozone
(O3), particulate matter with diameter <2.5µm (PM2.5) and <10µm (PM10), and PM2.5 from
traffic exhaust (PM2.5 traffic-exhaust) and traffic non-exhaust (PM2.5 traffic-non-exhaust = brake/tyre
wear and re-suspension) were estimated at 20m x 20m grid resolution across the study area,
using dispersion modelling (KCLurban).(10) NO2, NOX, PM2.5 traffic-exhaust and PM2.5 traffic-non-
exhaust are primary traffic pollutants (i.e. locally-emitted). PM2.5 and PM10 are dominated by
regional and long-range particles and secondary particles formed through atmospheric
chemical reactions but also include particles from primary traffic sources, whilst O3 is a
regional, secondary pollutant - these are more homogeneously distributed than primary traffic
pollutants. The KCLurban model uses Atmospheric Dispersion Modelling System v4 and
road source model v2.3; data on emissions from the London Atmospheric Emissions
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Inventory (LAEI);(11) empirically derived NO-NO2-O3 and PM relationships; and hourly
meteorological information.(10) Further details of the KCLurban model are available at the
study project website.(12) Using a Geographic Information System, each birth address was
linked to the pollutant model grid according to its geocoded XY coordinates and assigned
monthly air pollutant concentrations for a specific 20m x 20m grid cell. For each birth
record, we calculated the time-weighted average concentrations for NOX, NO2, PM2.5 traffic-
exhaust, PM2.5 traffic-non-exhaust, PM2.5, PM10 and O3 across pregnancy and each trimester
(Trimester 1 defined as days 1-93, Trimester 2 as days 94-186 and Trimester 3 as day 187 to
day preceding delivery). The time-weighting was based on the proportion of the pregnancy
or trimester in each calendar month.(13, 14) To define trimesters, gestation length (available
as ‘completed’ weeks of pregnancy) was converted to days, and 4 days (rounded up from the
mid-point 3.5 days) was added to adjust for potential underestimation where true gestation
length was not an exact number of completed weeks.
Noise exposures
A-weighted road traffic noise levels were modelled to 0.1 dB(A) resolution for all geocoded
birth addresses using the TRAffic Noise EXposure (TRANEX) model(15): LAeq,16hr (average
sound level 0700-2300 hours); Lnight (2300-0700); Lday (0700-1900); Leve (1900-2300); Lden
(logarithmic composite of Lday, Leve, and Lnight with 5 dB(A) added to the Leve and 10 dB(A)
added to Lnight). We modelled noise for one mid-point year (2007) and applied these values
to other years for the same address locations because temporal variability in noise over the
study period was negligible.(15) Noise could not be estimated for 4.5% of births due to
receptor placement issues,(15) however these births were randomly distributed. We flagged
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addresses exposed to Lday>50dB(A) from railways or aircraft (Heathrow Airport and City
Airport). Railway and City Airport noise data were from Environmental Noise Directive
strategic noise mapping (2006 annual average), and Heathrow Airport noise from annual
average contours (2001) from the Civil Aviation Authority.
Outcomes/Exclusions
Term low birth weight (LBW) was defined as birth weight<2500g and gestation≥37 weeks,
and SGA as birth-weight-for-gestational-age <10th centile by sex/ethnicity.
We initially excluded births with gestational age <24 or >44 weeks (n=1083), missing or
implausible (<200g or >9000g) birth weight (n=5,747), and missing gestational age
(n=6,617). Birth weight outliers were then identified and excluded according to Tukey’s rule
(16) (i.e. values >2*IQR below the first quartile and above the third quartile for each
gestational week) both overall and separately according to sex and ethnicity (White, Asian,
Black, Other) for the calculation of sex-ethnicity-specific birth weight-for-gestational-age
centiles. Stillbirths were retained at this stage, because excluding stillbirths overestimates
centiles for gestation <28 weeks by up to 30%.(17) 0.58% of the observations overall were
identified as outliers. We calculated smoothed sex-ethnicity-specific birth weight-for-
gestational-age centile curves according to the LMS method (18) using LMSChartMaker
Light V.2.54 software (19) which has been used in previous research.(20-23) The software
has a maximum data input of 100,000 records, so a subsample of 100,000 was randomly
selected if the number of records for a given sex/ethnicity subgroup exceeded this.
Representativeness of these 100,000 samples for their particular subgroup with respect to
exposures/potential confounders was checked and confirmed. We defined SGA as birth-
weight-for-gestational-age <10th centile by sex/ethnicity (to account for constitutional
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differences in birth weight by sex and ethnic group, and thus better identify pathologically
small infants). We did not calculate centiles/SGA for the ethnic group ‘Other’, as it does not
represent a meaningful homogeneous ethnic group for analysis. We excluded birth weight
outliers (n=3,712), stillbirths (n=3,597), births missing noise exposure (n=30,622) and
missing ethnicity (n=39,728), leaving 572,910 live births eligible for birth weight analyses,
and 499,862 for SGA analyses.
Statistical Methods
Air pollutant exposures were analysed as continuous measures, rescaled to interquartile range
(IQR) increments and pollutant-specific increments [NO2 (10 µg/m3), NOX (20 µg/m
3), PM2.5
traffic-exhaust (1 µg/m3), PM2.5 traffic-non-exhaust (1 µg/m
3), PM2.5 (5 µg/m
3), PM10 (10 µg/m
3), O3 (10
µg/m3)]. As all noise metrics were highly correlated (ρ≥0.997), we limited analysis to one
day-time (LAeq,16hr) and one night-time (Lnight) metric. Noise metrics were right skewed, so
were categorised (LAeq,16hr <55dB (reference), 55-<60dB, 60-<65dB, ≥65dB; and Lnight <50dB
(reference), 50-<55dB, 55-<60dB, 60-<65dB, ≥65dB).
We analysed continuous birth weight using linear regression, and LBW/SGA using logistic
regression. We analysed birth weight and SGA overall and limited to term births. All
models were adjusted for maternal age (<25, 25-29, 30-34, ≥35); birth registration type
(within marriage, joint-same address, joint-different address, sole registered); Carstairs
deprivation quintile; tobacco expenditure; season and year of birth; and a random intercept
for MSOA. Birth weight and LBW were also adjusted for sex, gestational age (linear and
quadratic terms), and baby’s ethnicity (White, Asian, Black, Other). We ran two-air-
pollutant models for birth weight, assessing collinearity using the variance inflation factor
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(VIF). In joint air pollutant-noise models we further adjusted air pollutants for noise, and
vice versa.
We ran sensitivity analyses on joint air pollutant-noise models evaluating possible effect
modification by ethnicity (exposure x ethnicity interaction term); stratifying by term/preterm;
and excluding those exposed to aircraft/railway noise >50dB. All analyses were conducted in
Stata version 13.
Patient involvement: As this was a registry-based study there was no patient involvement.
Results
5.68% and 9.47% of births were classified as LBW and SGA respectively (Table 1). Over
the study period 2006-2010, mean birth weight increased, pregnancy average exposures to
NO2, NOX, PM2.5 traffic-exhaust , PM2.5, and PM10 decreased but increased for PM2.5 traffic-non-exhaust
and O3 , and the proportion of births with high noise exposures increased (Table 1), the latter
reflecting change in spatial distribution of birth addresses over time, as noise modelling was
not time-varying. Air pollutant exposures were positively correlated (0.46-1.00), except with
O3 (-0.46 to -0.77); day and night-time noise were very highly correlated (~1.00), and noise
was positively correlated with air pollutants (0.15-0.50) except O3 (~-0.15) (Supplementary
Table 1). Deprivation, tobacco expenditure, ethnicity, birth registration type, maternal age,
season and year were significantly associated with outcomes and exposures (Supplementary
Tables 2 and 3).
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Air pollution
In single-pollutant adjusted models, IQR increases in exposure to NO2, NOX, PM2.5 traffic-
exhaust, PM2.5 traffic-non-exhaust, PM2.5 and PM10 during pregnancy were significantly associated
with term birth weight reductions of 7 to 13g (Figure 1/Supplementary Table 4) and increased
odds of term LBW (except PM10)( Supplementary Table 5). Only PM2.5 traffic-exhaust and PM2.5
were significantly associated with increased odds of term SGA (Supplementary Table 6).
Protective associations were observed for O3. In two-air-pollutant models, only PM2.5 traffic-
exhaust was consistently associated with a significant reduction in term birth weight when
adjusted for each of the other pollutants (Figure 1). The model with PM2.5 traffic-exhaust and
PM2.5 traffic-non-exhaust is not presented, due to multicollinearity. After adjustment for PM2.5 traffic-
exhaust, no other pollutant demonstrated a significant reduction in birth weight: NO2, NOX and
O3 were attenuated to null, whilst PM2.5 and PM10 became positively associated with birth
weight.
Noise
In adjusted models, high day- and night-time noise exposures were associated with
significant reductions in term birth weight, with a significant linear trend. Lnight≥65dB (vs.
reference <50dB) was associated with a -13g (95% CI: -18, -8) mean difference in term birth
weight (Supplementary Table 4). There were no significant associations between noise and
term LBW or SGA in adjusted models (Supplementary Tables 5 and 6).
Air pollution and noise
Air pollutant associations with term birth weight, LBW and SGA were robust to adjustment
for noise (Figures 2 and 3/Table 2), with only small changes to coefficients/ORs. In these
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fully adjusted models, a per IQR increase (0.35 µg/m3) in PM2.5 traffic-exhaust was associated
with the largest birth weight reduction of -13g (95% CI: -15, -10), and 4% and 2% increased
odds of term LBW and SGA respectively. PM2.5 traffic-exhaust was the only air pollutant to be
significantly associated with term SGA after adjustment for noise.
Associations between night-time noise and term birth weight were attenuated to null when
adjusted, in turn, for primary traffic-related air pollutants (NO2, NOX, PM2.5 traffic-exhaust, PM2.5
traffic-non-exhaust), however a significant association remained for high night-time noise, albeit
with some attenuation, after adjustment for PM2.5, PM10, and O3 in turn (Figure 2). Similarly,
high day-time noise was not associated with reduced birth weight after adjustment for air
pollutants, except O3 (Figure 3).
Findings were very similar for birth weight and SGA when not limited to term births (not
shown).
Primary traffic air pollutant exposures in all trimesters were inversely associated with term
birth weight (p<0.001), and 3rd
trimester effect sizes were largest (Supplementary Table 7),
but there was little difference in magnitude between trimesters for other outcomes (not
shown). First and 3rd
trimester PM2.5, and 1st trimester PM10 were also significantly inversely
associated with term birth weight.
Compared to unadjusted analyses, effect sizes were generally reduced in single/joint pollutant
adjusted models (Table 2, Supplementary Tables 4-6). Given the strong relationship between
exposures and COA-level deprivation, we ran birth weight models without adjustment for
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Carstairs to check for overadjustment, however there were only small changes in birth weight
coefficients (<1g) and the pattern of results was unchanged (not shown). The inclusion of a
random intercept for MSOA (to models adjusted for all other covariates described) resulted in
considerable attenuation of associations between air pollutants and term birth weight (-18 to -
28% for primary traffic air pollutants, and -35 to -49% for pollutants including regional/urban
background contributions), but relatively small changes to associations for noise, term LBW
or SGA.
All sensitivity analyses were conducted on joint air pollutant-noise models. Noise analyses
were largely unchanged after excluding those exposed to aircraft/rail noise >50dB (not
shown). We noted significant interactions between ethnicity and both primary traffic air
pollutant and noise exposures in term birth weight analyses (not shown). Ethnicity-noise
interactions were still present after excluding those exposed to high aircraft/rail noise (Lday)
>50dB (which like ethnicity is spatially patterned across London). In ethnicity-stratified
models, inverse relationships between term birth weight and traffic-related air pollutants held
across all ethnic strata, but associations were generally strongest in White, then Black, then
Asian strata. In models adjusted for primary traffic-related air pollution (PM2.5 traffic-exhaust),
noise was not associated with reduced term birth weight in any ethnic strata (not shown).
Discussion
To our knowledge, this is the largest UK study on air pollution and birth weight, and the first
UK study and largest study worldwide of birth weight and noise exposure. We observed that
long-term exposure during pregnancy to NO2, NOX, PM2.5 overall and specifically from
traffic exhaust and non-exhaust sources, and PM10, were all associated with a detrimental
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impact on growth in utero, across London. However, our findings from two-pollutant models
suggest that it is PM2.5 traffic-exhaust, i.e. near-road exposure to fine particulate matter emitted
from vehicle exhausts (and emitted at much higher levels from diesel vs. petrol vehicles), that
is driving the air pollutant associations with reduced birth weight in this study setting, and
that apparent adverse effects of PM2.5 are mediated by PM2.5 traffic exhaust. Whilst road traffic
noise was inversely related to birth weight, this relationship was not robust to adjustment for
primary traffic air pollutants, including PM2.5 traffic-exhaust, and appeared to reflect confounding
by primary traffic air pollutant co-exposures.
This study, which is by far the largest on air pollution and noise to date, benefits from
address-level exposure assessment. For noise particularly, this represents an advance on
previous studies which have estimated noise exposure with lower spatial precision, e.g.
postcodes,(6) 50m/250m buffers around maternal address,(5) or based on road proximity,(24)
and consequently reduces potential exposure misclassification. We avoided selection bias by
using all birth registration data. Individual-level smoking/deprivation data were unavailable,
but we adjusted for these factors at COA-level and for birth registration type, which relates to
individual-level qualifications/housing tenure,(25) and thus socio-economic status. However,
we cannot exclude the possibility of residual confounding by maternal/passive smoking. We
could not account for residential mobility during pregnancy (~16% in UK(26)), nor
exposures away from maternal residence (e.g. workplace, transport), indoor air pollution, or
exposure modification due to behaviours (e.g. opening windows) or building characteristics
(e.g. bedroom façade). These could contribute to exposure misclassification. Third trimester
exposures may be least prone to exposure measurement bias from residential mobility, as
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exposures were assigned to birth address, which may partly explain stronger associations for
third trimester primary traffic air pollutant exposures.
Our single-air-pollutant model findings are consistent with recent meta-analyses which report
reduced mean birth weight and increased risk of LBW associated with NO2,(1) PM2.5,(27)
and PM10.(1, 28) Meta-analysis results for O3 are less clear: -10.0g (95% CI: -32.4, 12.4)
reduction in birth weight and OR for LBW of 1.01 (95% CI: 0.82, 1.25) per 20ppb increase in
pregnancy exposure to O3.(1) To our knowledge, only three Californian studies, have
examined source-specific PM2.5 and birth weight. Converted to the same IQR (0.35 µg/m3)
scale as our PM2.5 traffic-exhaust analyses, these studies each report 2% increased odds of term
LBW for PM2.5 from diesel and 3-4% increased odds for PM2.5 from gasoline,(29-31)
consistent in magnitude with our OR for term LBW of 1.04 per IQR increase. Reduced birth
weight/LBW has been associated with certain chemical constituents of PM2.5: potassium,
zinc, nickel, titanium, vanadium, organic carbon, nitrate, and elemental carbon.(27)
Elemental carbon is a major fraction of diesel exhaust particulate matter,(32) so our findings
for PM2.5 traffic-exhaust could also reflect adverse effects of elemental carbon. To our
knowledge, no previous study has reported two-pollutant models including source-specific
PM2.5. Our finding that adverse effects of PM2.5 upon birth weight, which have been widely
reported, may be mediated by PM2.5 traffic-exhaust is an important and novel contribution to
scientific knowledge.
The most recent systematic review of noise exposure and birth weight found “evidence
supportive of associations between LBW and noise exposure” but that this was
inconsistent,(4) based on 10 occupational studies, 4 aircraft noise studies, and 2 traffic noise
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studies. Three previous studies have examined long-term air pollution and noise exposures
jointly.(5-7) Our findings are consistent with a small cohort study (n=6438) in Barcelona,
which found elevated risks of term LBW and SGA associated with noise (non-significant)
and air pollution exposures (strongest for PM2.5, PM2.5-10, and PM10 and significant for 3rd
trimester for term LBW) in single-exposure adjusted models, but in a joint-exposure model
term LBW risk was associated with 3rd
trimester PM2.5 (per 3.6µg/m3 OR 1.31 [95% CI: 1.07,
1.61]), but not noise (per 6.7dB(A) OR 0.89 [95% CI: 0.71, 1.12]).(5) Term birth weight was
not associated with NO2, NOX, or road traffic noise, in either fully-adjusted single-exposure
models or joint-exposure models in the Danish National Birth Cohort (n=75,166).(7) Our
findings contrast with a registry based study in Vancouver (n=68,238), which found
associations between all transportation (road traffic/railway/aircraft) noise (Lden) and reduced
birth weight/LBW which remained after adjustment for PM2.5, PM10 and primary road traffic
air pollution (NO2, NO), however associations for air pollution exposure were attenuated to
null by adjustment for transportation noise.(6) Road traffic noise demonstrated similar
associations with birth weight (in single-exposure models, but road traffic noise adjusted for
air pollution was not analysed).(6) We, however, found that adjustment for PM2.5 or PM10
did not fully control for confounding of noise by air pollution while adjustment for primary
traffic air pollutants did. This should be noted by other researchers investigating potential
health effects of road traffic noise. Compared to London, the noise distribution in Vancouver
was wider (Lden mean 60.2/range 6.2–89.0 dB(A)), mean air pollution exposures were lower
and with less contrast in Vancouver (PM2.5 mean 4.1/range 0-11.3; NO2 mean 33.7/range: 0–
64.5) and Denmark (NO2 median 11.0 µg/m3, 5
th-95
th percentiles: 7.1–26.3 µg/m
3), and air
pollutant-noise correlations were lower in Vancouver (correlations with road traffic noise:
0.05 for NO2, 0.09 for PM2.5; and all transportation noise: 0.18 for NO2, 0.16 for PM2.5), but
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higher in Denmark (0.47 for NO2).(6, 7) These differences, which could reflect differences in
pollutant sources, may contribute to the contrasting findings from Denmark/Vancouver
compared to our study.
Biological mechanisms via which air pollution or noise may impair fetal growth are not
established. Hypothesised mechanisms for air pollution are oxidative stress; endocrine
disruption; changes to maternal-placental blood flow and oxygen/nutrition transfer;(33)
placental mitochondrial damage;(34) and placental growth/function,(35) whilst those for
noise are stress-triggered endocrine/immune response disruption, plasma catecholamine
increase/placental blood flow decrease,(4) hypertension,(36) and sleep disturbance.(37)
Convincing evidence that maternal passive smoking during pregnancy is causally related to
reduced birth weight,(38) strongly supports the biological plausibility of an association
between ambient air pollution and reduced birth weight, by analogy.
In conclusion, this study suggests that air pollution from road traffic, and specifically locally-
emitted PM2.5 from vehicle exhausts, is having a detrimental impact upon babies’ health in
Greater London, before they are born. There is limited evidence, and no consensus, as to the
effect of traffic-related noise on birth weight. Our results, from by far the largest study to
date, found no independent effect of traffic-related noise on birth weight, and suggest any
apparent relationship reflects traffic air pollutant co-exposures. Our findings should be
broadly generalisable to other UK and European cities/urban areas with comparable exposure
levels and profiles. At city scale, environmental health policies aimed at reducing locally-
emitted particulate matter, e.g. by tackling diesel vehicle emissions, could reduce the burden
of LBW, SGA, and subsequent morbidity. With the annual number of births projected to
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continue increasing in London,(39) the absolute health burden will increase at the population
level, unless air quality in London improves.
What is already known on this subject:
● Road traffic pollution comprises not only air pollutants such as NO2 and particulate
matter, but also noise.
● There is a large body of research demonstrating associations between maternal exposure
to ambient air pollution during pregnancy and reduced birth weight, low birth
weight/small for gestational age.
● The relationship between road traffic noise and birth weight is unclear, and research
examining traffic-related air pollutant and noise co-exposures together is very limited, so
the extent to which observed air pollution associations might be attributable to road traffic
noise is poorly understood.
What this study adds:
● We demonstrate evidence for reduced birth weight specifically in relation to the air
pollution profile of London, which has 19% of all births in England and Wales (40) per
year.
● Our findings suggest that it is exposure to fine particulate matter from road traffic exhaust
emissions specifically that is driving the air pollutant associations with reduced birth
weight in London, but that there is no independent effect of traffic-related noise on birth
weight.
● This study provides local evidence for UK clinicians caring for and advising pregnant
women and for UK policymakers, which suggests reducing exposure to traffic-related air
pollution, and particularly diesel exhaust emissions, could reduce the burden of low birth
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weight, small for gestational age, and subsequent morbidity, and ultimately give babies in
urban environments a healthier start in life.
Acknowledgements: We thank Margaret Douglass and Peter Hambly of the SAHSU
database team for technical support and the TRAFFIC study group for their constructive
comments. CACI tobacco expenditure data is © Copyright 1996-2014 CACI Limited. We
attest that we have obtained appropriate permissions and paid any required fees for use of
copyright protected materials.
Funding: This work was funded by the UK Natural Environment Research Council, Medical
Research Council, Economic and Social Research Council, Department of Environment,
Food and Rural Affairs, and Department of Health (NE/I00789X/1, NE/I008039/1) through
the cross-research council Environmental Exposures & Health Initiative. The work of the UK
Small Area Health Statistics Unit (SAHSU) is funded by Public Health England as part of the
MRC-PHE Centre for Environment and Health, funded also by the UK Medical Research
Council (MR/L01341X/1). The funders had no role in the study design; in the collection,
analysis and interpretation of data; in the writing of the report; and in the decision to submit
the article for publication.
Contributors: MBT, JG, HRA, SDB, FJK contributed to study conception and design; DF,
JG, SDB, DD, HRA and FJK to exposure assessment; and RG, DF and ALH to acquisition of
health and confounder data. RBS contributed to study design, wrote the statistical analysis
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plan, conducted the data analyses, and drafted the initial report. MBT contributed to the
statistical analysis plan, the data analyses and initial drafting of the report. MB contributed to
the study design and statistical analysis plan. All authors contributed to interpreting the
analyses and critically revising the article, approved the final draft, and agree to be
accountable for all aspects of the work. MBT is the guarantor of this paper. All authors had
full access to all of the data in the study and can take responsibility for the integrity of the
data and the accuracy of the data analysis.
Competing Interests: All authors have completed the ICMJE uniform disclosure form at
www.icmje.org/coi_disclosure.pdf and declare: no support from any organisation for the
submitted work other than those detailed above; no financial relationships with any
organisations that might have an interest in the submitted work in the previous three years; no
other relationships or activities that could appear to have influenced the submitted work.
Copyright: The Corresponding Author has the right to grant on behalf of all authors and does
grant on behalf of all authors, a worldwide licence to the Publishers and its licensees in
perpetuity, in all forms, formats and media (whether known now or created in the future), to
i) publish, reproduce, distribute, display and store the Contribution, ii) translate the
Contribution into other languages, create adaptations, reprints, include within collections and
create summaries, extracts and/or, abstracts of the Contribution, iii) create any other
derivative work(s) based on the Contribution, iv) to exploit all subsidiary rights in the
Contribution, v) the inclusion of electronic links from the Contribution to third party material
where-ever it may be located; and, vi) licence any third party to do any or all of the above.
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Ethical approval: The study uses SAHSU data, supplied from the Office for National
Statistics; data use and link between UK National Births and Stillbirth register data and NHS
Numbers for Babies (NN4B) was covered by approval from the National Research Ethics
Service - 12/LO/0566 and 12/LO/0567- and by the Health Research Authority Confidentiality
Advisory Group (HRA-CAG) for Section 251 support (HRA – 14/CAG/1039).
Transparency: The lead author (the manuscript's guarantor) affirms that the manuscript is an
honest, accurate, and transparent account of the study being reported; that no important
aspects of the study have been omitted; and that any discrepancies from the study as planned
(and, if relevant, registered) have been explained.
Data sharing: Health data are available from the data providers on application with
appropriate ethics and governance permissions, but we do not hold data provider, ethics, or
governance permissions to share the dataset with third parties.
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Figures
Figure 1: Adjusted mean difference in term birth weight (g), associated with IQR
increases in air pollutants, in single- and two- air pollutant models
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All models in Figure 1 are adjusted for sex, maternal age, birth registration type, tobacco
expenditure (COA-level), Carstairs quintile (COA-level), individual-level ethnicity,
gestational age as linear and quadratic terms, season of birth, year, and random intercept for
MSOA in addition to including the air pollutants shown above. NO2 and NOX were not
entered into the same model together as they were too highly correlated. PM2.5 and PM10 were
not entered into the same model together as PM2.5 is a substantial subset of PM10 (>50% by
mass).
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Figure 2: Adjusted mean difference in term birth weight (g), associated with night-time
noise (Lnight) and air pollutants, in single-exposure and joint-exposure models
Lnight is night-time noise. All models in Figure 2 are adjusted for sex, maternal age, birth
registration type, tobacco expenditure (COA-level), Carstairs quintile (COA-level),
individual-level ethnicity, gestational age as linear and quadratic terms, season of birth, year,
and random intercept for MSOA, in addition to including the air pollutant and/or noise
metrics shown above.
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Figure 3: Adjusted mean difference in term birth weight (g), associated with day-time
noise (LAeq,16hr) and air pollutants, in single-exposure and joint-exposure models
LAeq,16hr is day-time noise. All models in Figure 3 are adjusted for sex, maternal age, birth
registration type, tobacco expenditure (COA-level), Carstairs quintile (COA-level),
individual-level ethnicity, gestational age as linear and quadratic terms, season of birth, year,
and random intercept for MSOA in addition to including the air pollutant and/or noise metrics
shown above.
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Table 1: Characteristics of the study population (n=572,910), and distribution of pregnancy outcomes and exposures.
Variable N Mean
birth
weight (g)
LBW
(%)
term
LBW
(%)
SGA*
(%)
Mean pregnancy average concentration (µg/m3) % exposed ≥65dB
NO2 NOX PM2.5
traffic-
exhaust
PM2.5
traffic-non-
exhaust
PM2.5 PM10 O3 LAeq,16hr Lnight
Total population 572,910 3330 5.68 2.55 9.47 40.6 72.5 0.6 0.7 14.4 23.1 31.9 14.2 6.3
Infant sex
Male 293,322 3386 5.25 2.05 9.49 40.6 72.5 0.61 0.73 14.4 23.1 31.9 14.2 6.2
Female 279,588 3271 6.13 3.08 9.46 40.6 72.4 0.61 0.73 14.4 23.1 31.9 14.2 6.3
Maternal age
<25 107,450 3252 6.80 3.31 12.56 40.8 73.0 0.62 0.73 14.5 23.2 31.7 15.8 7.1
25-29 148,347 3311 5.73 2.77 9.62 40.5 72.3 0.61 0.73 14.4 23.1 31.9 15.3 6.9
30-34 178,961 3364 5.01 2.18 8.52 40.4 72.1 0.61 0.72 14.4 23.0 32.0 13.6 6.0
>=35 138,152 3367 5.62 2.23 8.20 40.6 72.7 0.61 0.72 14.4 23.1 31.8 12.3 5.3
Ethnicity
White 301,426 3415 4.25 1.71 9.50 39.8 70.5 0.59 0.70 14.3 22.9 32.3 12.9 5.5
Asian 99,773 3135 8.78 5.11 9.58 40.9 73.1 0.62 0.74 14.4 23.1 31.7 15.0 6.4
Black 98,687 3275 7.06 2.76 9.29 42.0 76.2 0.66 0.78 14.6 23.5 31.0 15.3 7.2
Other 73,024 3318 5.45 2.32 41.4 74.6 0.64 0.76 14.5 23.3 31.5 16.6 7.8
Birth registration
Within marriage† 366,767 3342 5.20 2.48 8.48 40.6 72.6 0.61 0.73 14.4 23.1 31.8 13.8 6.0
Sole registration 39,090 3240 8.15 3.43 12.67 41.4 74.5 0.64 0.75 14.6 23.3 31.4 15.7 7.4
Joint/same addr. 111,924 3358 5.45 2.20 10.17 40.0 70.9 0.60 0.71 14.4 23.0 32.3 14.8 6.7
Joint/different addr. 55,129 3258 7.58 3.21 12.59 40.8 73.1 0.63 0.74 14.4 23.1 31.7 14.5 6.5
Season of birth
Winter 138,297 3317 5.93 2.66 9.66 39.7 70.3 0.61 0.72 13.9 22.6 30.9 14.4 6.4
Spring 141,415 3328 5.71 2.59 9.65 43.4 79.9 0.68 0.79 14.8 23.8 27.3 13.9 6.0
Summer 146,600 3337 5.59 2.50 9.34 41.9 76.0 0.63 0.75 15.0 23.8 33.0 14.1 6.3
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Variable N Mean
birth
weight (g)
LBW
(%)
term
LBW
(%)
SGA*
(%)
Mean pregnancy average concentration (µg/m3) % exposed ≥65dB
NO2 NOX PM2.5
traffic-
exhaust
PM2.5
traffic-non-
exhaust
PM2.5 PM10 O3 LAeq,16hr Lnight
Autumn 146,598 3337 5.50 2.48 9.27 37.3 63.7 0.54 0.65 13.9 22.2 36.2 14.3 6.4
Year of birth
2006 108,190 3317 6.08 2.79 10.23 42.3 77.6 0.72 0.71 16.1 25.1 30.4 13.7 6.0
2007 113,092 3324 5.82 2.57 9.55 40.6 71.7 0.63 0.69 14.8 24.1 34.2 13.9 6.1
2008 113,213 3330 5.74 2.58 9.35 42.1 77.7 0.63 0.76 14.5 23.5 30.6 14.1 6.2
2009 116,454 3336 5.51 2.50 9.24 41.0 73.0 0.60 0.77 14.0 22.6 27.4 14.2 6.3
2010 121,961 3340 5.29 2.36 9.08 37.2 63.2 0.51 0.69 12.9 20.5 36.5 14.8 6.6
Carstairs quintile
1 - least deprived 89,401 3418 3.82 1.55 7.72 37.3 64.1 0.51 0.61 14.1 22.5 33.7 9.2 2.6
2 97,039 3380 4.56 2.03 8.81 39.3 69.3 0.57 0.69 14.3 22.8 32.6 13.6 5.6
3 106,936 3337 5.53 2.43 9.37 40.4 71.9 0.60 0.72 14.4 23.0 32.0 15.2 6.7
4 126,815 3303 6.28 2.89 10.07 41.1 73.8 0.63 0.74 14.5 23.2 31.6 15.9 7.2
5 - most deprived 152,719 3263 7.08 3.30 10.56 43.0 78.6 0.70 0.81 14.7 23.6 30.5 15.2 7.9
London
Inner 183,575 3332 5.62 2.52 9.87 45.1 84.3 0.78 0.88 14.8 24.0 29.4 17.1 9.5
Outer 389,335 3329 5.71 2.57 9.29 38.4 66.9 0.54 0.65 14.2 22.7 33.1 12.8 4.8
Abbreviations: LBW, low birth weight (<2500g); SGA, small-for-gestational-age. *SGA, % out of a total 499537 for whom sex-ethnicity-
specific SGA calculated. †includes civil partnerships.
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Table 2: Joint air-pollutant-noise models: Mean difference (95% CI) in birth weight (g) and OR (95%CI) for LBW and SGA associated
with air and noise pollution amongst term births
Joint air pollutant-noise model* Joint air pollutant-noise model* Joint air pollutant-noise model*
Term birth weight (g) Term LBW Term SGA
Exposure N MD (95%CI) p-value OR (95% CI) p-value N OR (95% CI) p-value
NO2 (per IQR) 540,365 -10.52 (-12.71, -8.33) <0.001 1.03 (1.00, 1.06) 0.043 471489 1.01 (0.99, 1.03) 0.176
NOX (per IQR) 540,365 -10.18 (-12.35, -8.00) <0.001 1.03 (1.00, 1.06) 0.029 471489 1.01 (1.00, 1.03) 0.161
PM2.5 traffic-exhaust (per IQR) 540,365 -12.68 (-15.04, -10.32) <0.001 1.04 (1.01, 1.08) 0.009 471489 1.02 (1.00, 1.04) 0.025
PM2.5 traffic-non-exhaust (per IQR) 540,365 -7.46 (-9.32, -5.61) <0.001 1.02 (1.00, 1.05) 0.087 471489 1.01 (0.99, 1.02) 0.388
PM2.5 (per IQR) 540,365 -11.24 (-14.88, -7.60) <0.001 1.06 (1.01, 1.12) 0.022 471489 1.03 (1.00, 1.06) 0.054
PM10 (per IQR) 540,365 -5.54 (-8.31, -2.76) <0.001 1.03 (0.99, 1.07) 0.176 471489 1.00 (0.98, 1.03) 0.761
O3 (per IQR) 540,365 4.48 (2.45, 6.50) <0.001 0.96 (0.94, 0.99) 0.013 471489 0.99 (0.98, 1.01) 0.413
LAeq,16hr
<55dB 157491 Reference Reference 138696 Reference 55-<60 dB 265603 3.95 (1.18, 6.71) 0.005 0.97 (0.93, 1.01) 0.181 232346 0.99 (0.96, 1.01) 0.242
60-<65 dB 40755 -0.45 (-5.19, 4.29) 0.852 1.01 (0.94, 1.08) 0.851 35334 1.01 (0.97, 1.05) 0.766
>=65dB 76516 4.35 (0.21, 8.49) 0.039 0.97 (0.91, 1.04) 0.389 65113 0.99 (0.95, 1.02) 0.423
p for trend 540365 0.113 0.535 471489 0.581
Lnight
<50dB 162260 Reference Reference 142880 Reference 50-<55 dB 257045 2.15 (-0.62, 4.92) 0.128 0.98 (0.94, 1.02) 0.392 224864 0.99 (0.97, 1.02) 0.638
55-<60 dB 40256 -0.54 (-5.29, 4.21) 0.824 0.99 (0.92, 1.07) 0.868 34960 1.02 (0.98, 1.06) 0.376
60-<65 dB 46994 1.71 (-2.86, 6.29) 0.462 1.00 (0.93, 1.07) 0.933 40344 0.99 (0.95, 1.03) 0.675
>=65dB 33810 0.97 (-4.77, 6.71) 0.739 0.97 (0.89, 1.06) 0.527 28441 1.01 (0.96, 1.06) 0.736
p for trend 540365 0.726 0.715 471489 0.803
*Models are adjusted as follow:
Term birth weight: Adjusted model covariates: sex, maternal age, birth registration type, tobacco expenditure (COA-level), Carstairs quintile
(COA-level), individual-level ethnicity, gestational age as linear and quadratic terms, season of birth, year (linear term) and random intercept for
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MSOA.
Term LBW: Adjusted model covariates: sex, maternal age, birth registration type, tobacco expenditure (COA-level), Carstairs quintile (COA-
level), individual-level ethnicity, gestational age as linear and quadratic terms, season of birth, year (linear term) and random intercept for
MSOA.
Term SGA: Adjusted model covariates: maternal age, birth registration type, tobacco expenditure (COA-level), Carstairs quintile (COA-level),
season of birth, year (linear term) and random intercept for MSOA.
*All air pollution estimates are adjusted for daytime noise (LAeq,16hr), and noise estimates are adjusted for traffic-related air pollution exposure
(PM2.5 traffic-exhaust).
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Supplementary Table 1: Air pollution and road traffic noise exposures in the study population (N=572,910) and their correlations
Air pollutants (pregnancy average) (µg/m
3) Noise (annual average 2007)
(dB)
NO2 NOX PM2.5 traffic-
exhaust
PM2.5 traffic-
non-exhaust
PM2.5 PM10 O3 LAeq,16hr Lnight Lday Leve Lden
Summary statistics Mean 40.6 72.5 0.61 0.73 14.4 23.1 31.9 58.1 53.2 58.4 56.8 59.8
SD 6.7 18.0 0.24 0.25 1.4 2.3 6.2 5.2 5.4 5.2 4.9 5.2
Min 20.6 26.6 0.16 0.20 10.3 13.6 4.9 54.7 49.6 55.0 53.7 56.3
IQR 8.6 23.7 0.35 0.29 2.2 3.0 8.4 3.5 3.9 3.6 3.1 3.7
Max 191.6 543.7 7.10 6.59 26.6 48.8 55.8 86.0 80.0 86.4 84.4 86.7
Spearman’s
correlation
NO2 1.00
NOX 1.00 1.00
PM2.5 traffic-exhaust 0.95 0.95 1.00
PM2.5 traffic-non-
exhaust 0.90 0.89 0.93 1.00
PM2.5 0.68 0.69 0.68 0.46 1.00
PM10 0.77 0.78 0.78 0.58 0.95 1.00
O3 -0.76 -0.77 -0.68 -0.66 -0.46 -0.53 1.00
LAeq, 16hr 0.31 0.29 0.38 0.47 0.15 0.23 -0.14 1.00
Lnight 0.33 0.32 0.41 0.50 0.16 0.24 -0.15 1.00 1.00
Lday 0.30 0.29 0.38 0.47 0.15 0.23 -0.14 1.00 1.00 1.00
Leve 0.31 0.30 0.38 0.48 0.15 0.23 -0.14 1.00 1.00 1.00 1.00
Lden 0.32 0.31 0.40 0.50 0.15 0.24 -0.15 1.00 1.00 1.00 1.00 1.00
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Supplementary Table 2: Mean difference (95% CI) in birth weight (g) and OR (95%CI) for LBW and
SGA associated with potential confounding factors (univariate linear/logistic regression) amongst all live
singleton births
Birth weight (g) LBW SGA (sex/ethnicity-specific)
Variable N MD (95%CI) p-value N cases OR (95% CI) p-value N cases OR (95% CI) p-value
Gestational Age (weeks) 572,910 171.14 (170.54, 171.74) <0.001 32,532 0.39 (0.39, 0.39) <0.001
Tobacco expenditure (£) (COA) 572,910 -17.59 (-18.32, -16.87) <0.001 32,532 1.07 (1.07, 1.08) <0.001 47360 1.05 (1.04, 1.05) <0.001
Sex
Male 293,322 Reference 15,389 Reference
Female 279,588 -114.96 (-117.79, -112.13) <0.001 17,143 1.18 (1.15, 1.21) <0.001
Maternal age
<25 107,450 Reference 7305 Reference 11575 Reference
25-29 148,347 58.75 (54.45, 63.05) <0.001 8495 0.83 (0.81, 0.86) <0.001 12528 0.74 (0.72, 0.76) <0.001
30-34 178,961 111.46 (107.32, 115.61) <0.001 8969 0.72 (0.70, 0.75) <0.001 13404 0.65 (0.63, 0.67) <0.001
>=35 138,152 114.58 (110.21, 118.95) <0.001 7763 0.82 (0.79, 0.84) <0.001 9853 0.62 (0.60, 0.64) <0.001
p for trend 572,910 <0.001 <0.001 <0.001
Ethnicity
White 301,426 Reference 12,825 Reference
Asian 99,773 -280.32 (-284.18, -276.46) <0.001 8760 2.17 (2.11, 2.23) <0.001
Black 98,687 -140.59 (-144.47, -136.71) <0.001 6970 1.71 (1.66, 1.76) <0.001
Other 73,024 -97.67 (-102.03, -93.31) <0.001 3977 1.30 (1.25, 1.34) <0.001
Birth in marriage
Within marriage† 366,767 Reference 19,073 Reference 27319 Reference
Sole registration 39,090 -102.07 (-107.79, -96.35) <0.001 3185 1.62 (1.56, 1.68) <0.001 4224 1.57 (1.51, 1.62) <0.001
Joint/same addr. 111,924 16.04 (12.37, 19.71) <0.001 6095 1.05 (1.02, 1.08) 0.001 9982 1.22 (1.19, 1.25) <0.001
Joint/different addr. 55,129 -84.11 (-89.02, -79.21) <0.001 4179 1.50 (1.44, 1.55) <0.001 5835 1.55 (1.51, 1.60) <0.001
Year
2006 108,190 Reference 6583 Reference 9666 Reference
2007 113,092 7.06 (2.48, 11.64) 0.003 6578 0.95 (0.92, 0.99) 0.008 9406 0.93 (0.90, 0.96) <0.001
2008 113,213 13.58 (9.01, 18.16) <0.001 6503 0.94 (0.91, 0.97) 0.001 9208 0.90 (0.88, 0.93) <0.001
2009 116,454 19.20 (14.65, 23.75) <0.001 6415 0.90 (0.87, 0.93) <0.001 9401 0.89 (0.87, 0.92) <0.001
2010 121,961 22.88 (18.38, 27.38) <0.001 6453 0.86 (0.83, 0.89) <0.001 9679 0.88 (0.85, 0.90) <0.001
p for trend 572,910 <0.001 <0.001 <0.001
Season
Winter 138,297 Reference 8195 Reference 11646 Reference
Spring 141,415 11.09 (7.02, 15.17) <0.001 8074 0.96 (0.93, 0.99) 0.015 11932 1.00 (0.97, 1.03) 0.938 Summer 146,600 20.51 (16.48, 24.55) <0.001 8193 0.94 (0.91, 0.97) <0.001 11912 0.96 (0.94, 0.99) 0.008
Autumn 146,598 20.49 (16.45, 24.52) <0.001 8070 0.92 (0.90, 0.95) <0.001 11870 0.96 (0.93, 0.98) 0.001
Carstairs quintile (COA)
1 - least deprived 89,401 Reference 3418 Reference 6244 Reference
2 97,039 -38.31 (-43.28, -33.35) <0.001 4423 1.20 (1.15, 1.26) <0.001 7537 1.16 (1.12, 1.20) <0.001
3 106,936 -80.98 (-85.84, -76.12) <0.001 5913 1.47 (1.41, 1.54) <0.001 8649 1.24 (1.19, 1.28) <0.001
4 126,815 -115.96 (-120.64, -111.28) <0.001 7959 1.68 (1.62, 1.76) <0.001 10916 1.34 (1.30, 1.38) <0.001
5 - most deprived 152,719 -155.46 (-159.97, -150.94) <0.001 10,819 1.92 (1.84, 2.00) <0.001 14014 1.41 (1.37, 1.46) <0.001
p for trend 572,910 <0.001 <0.001 <0.001
Inner/Outer London
Inner 183,575 Reference 10,308 Reference 15670 Reference Outer 389,335 -3.69 (-6.74, -0.64) 0.018 22,224 1.02 (0.99, 1.04) 0.156 31690 0.94 (0.92, 0.95) <0.001
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Supplementary Table 3: Mean difference (95% CI) in exposure metric associated with potential confounding factors (univariate linear
regression) amongst all live singleton births NO2 (pregnancy average, µg/m3) NOX (pregnancy average, µg/m3) PM2.5 traffic-exhaust (pregnancy average,
µg/m3)
PM2.5 traffic-non-exhaust (pregnancy
average, µg/m3)
Variable N MD (95%CI) p-value MD (95%CI) p-value MD (95%CI) p-value MD (95%CI) p-value
Gestational Age (completed weeks) 572,910 0.02 (0.01, 0.03) <0.001 0.06 (0.03, 0.08) <0.001 0.00 (0.00, 0.00) 0.004 0.00 (-0.00, 0.00) 0.184
Tobacco expenditure (£) (COA) 572,910 0.79 (0.78, 0.79) <0.001 2.03 (2.01, 2.05) <0.001 0.03 (0.03, 0.03) <0.001 0.03 (0.03, 0.03) <0.001
Sex
Male 293,322 Reference Reference Reference Reference
Female 279,588 -0.02 (-0.05, 0.02) 0.300 -0.05 (-0.14, 0.05) 0.322 -0.00 (-0.00, 0.00) 0.265 -0.00 (-0.00, 0.00) 0.483
Maternal age
<25 107,450 Reference Reference Reference Reference
25-29 148,347 -0.27 (-0.32, -0.22) <0.001 -0.71 (-0.85, -0.57) <0.001 -0.01 (-0.01, -0.01) <0.001 -0.01 (-0.01, -0.01) <0.001
30-34 178,961 -0.35 (-0.40, -0.29) <0.001 -0.87 (-1.01, -0.73) <0.001 -0.01 (-0.02, -0.01) <0.001 -0.01 (-0.02, -0.01) <0.001
>=35 138,152 -0.14 (-0.19, -0.08) <0.001 -0.30 (-0.45, -0.16) <0.001 -0.01 (-0.01, -0.01) <0.001 -0.01 (-0.01, -0.01) <0.001
p for trend 572,910 <0.001 <0.001 <0.001 <0.001
Ethnicity
White 301,426 Reference Reference Reference Reference
Asian 99,773 1.07 (1.02, 1.12) <0.001 2.61 (2.48, 2.74) <0.001 0.03 (0.03, 0.03) <0.001 0.04 (0.04, 0.04) <0.001
Black 98,687 2.25 (2.21, 2.30) <0.001 5.74 (5.61, 5.87) <0.001 0.07 (0.07, 0.08) <0.001 0.08 (0.08, 0.08) <0.001
Other 73,024 1.59 (1.54, 1.65) <0.001 4.06 (3.92, 4.21) <0.001 0.05 (0.05, 0.06) <0.001 0.06 (0.06, 0.06) <0.001
Birth in marriage
Within marriage† 366,767 Reference Reference Reference Reference
Sole registration 39,090 0.74 (0.67, 0.81) <0.001 1.90 (1.71, 2.09) <0.001 0.03 (0.03, 0.03) <0.001 0.03 (0.03, 0.03) <0.001
Joint/same address 111,924 -0.66 (-0.70, -0.61) <0.001 -1.70 (-1.82, -1.58) <0.001 -0.02 (-0.02, -0.02) <0.001 -0.02 (-0.02, -0.02) <0.001
Joint/different address 55,129 0.21 (0.15, 0.27) <0.001 0.52 (0.35, 0.68) <0.001 0.01 (0.01, 0.01) <0.001 0.01 (0.01, 0.01) <0.001
Year
2006 108,190 Reference Reference Reference Reference
2007 113,092 -1.74 (-1.79, -1.69) <0.001 -5.82 (-5.97, -5.68) <0.001 -0.09 (-0.09, -0.09) <0.001 -0.02 (-0.03, -0.02) <0.001
2008 113,213 -0.24 (-0.29, -0.19) <0.001 0.14 (-0.01, 0.28) 0.059 -0.08 (-0.08, -0.08) <0.001 0.05 (0.04, 0.05) <0.001
2009 116,454 -1.27 (-1.32, -1.21) <0.001 -4.54 (-4.68, -4.40) <0.001 -0.12 (-0.12, -0.12) <0.001 0.06 (0.06, 0.06) <0.001
2010 121,961 -5.05 (-5.11, -5.00) <0.001 -14.41 (-14.55, -14.27) <0.001 -0.21 (-0.21, -0.21) <0.001 -0.03 (-0.03, -0.02) <0.001
p for trend 572,910 <0.001 <0.001 <0.001 <0.001
Season
Winter 138,297 Reference Reference Reference Reference
Spring 141,415 3.74 (3.69, 3.78) <0.001 9.61 (9.48, 9.74) <0.001 0.07 (0.06, 0.07) <0.001 0.07 (0.07, 0.07) <0.001
Summer 146,600 2.22 (2.17, 2.27) <0.001 5.75 (5.62, 5.87) <0.001 0.02 (0.01, 0.02) <0.001 0.03 (0.03, 0.03) <0.001
Autumn 146,598 -2.41 (-2.45, -2.36) <0.001 -6.57 (-6.70, -6.45) <0.001 -0.08 (-0.08, -0.08) <0.001 -0.06 (-0.06, -0.06) <0.001
Carstairs quintile (COA)
1 - least deprived 89,401 Reference Reference Reference Reference
2 97,039 2.04 (1.99, 2.10) <0.001 5.16 (5.01, 5.32) <0.001 0.07 (0.07, 0.07) <0.001 0.08 (0.07, 0.08) <0.001
3 106,936 3.10 (3.04, 3.16) <0.001 7.82 (7.67, 7.98) <0.001 0.10 (0.10, 0.10) <0.001 0.11 (0.11, 0.11) <0.001
4 126,815 3.84 (3.79, 3.90) <0.001 9.67 (9.52, 9.82) <0.001 0.12 (0.12, 0.12) <0.001 0.14 (0.13, 0.14) <0.001
5 - most deprived 152,719 5.69 (5.64, 5.74) <0.001 14.49 (14.34, 14.63) <0.001 0.19 (0.19, 0.19) <0.001 0.20 (0.20, 0.20) <0.001
p for trend 572,910 <0.001 <0.001 <0.001 <0.001
Inner/Outer London
Inner 183,575 Reference Reference Reference Reference
Outer 389,335 -6.71 (-6.75, -6.68) <0.001 -17.44 (-17.53, -17.35) <0.001 -0.25 (-0.25, -0.24) <0.001 -0.23 (-0.23, -0.22) <0.001
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Supplementary Table 3 cont/d.
PM2.5 (pregnancy average, µg/m3) PM10 (pregnancy average, µg/m3) O3 (pregnancy average, µg/m3) LAeq,16hr (dB) Lnight (dB)
Variable MD (95%CI) p-value MD (95%CI) p-value MD (95%CI) p-value MD (95%CI) p-value MD (95%CI) p-value
Gestational Age (weeks) 0.01 (0.01, 0.01) <0.001 0.02 (0.01, 0.02) <0.001 -0.02 (-0.03, -0.02) <0.001 0.00 (-0.00, 0.01) 0.266 0.00 (-0.00, 0.01) 0.399
Tobacco expenditure (£) (COA) 0.08 (0.08, 0.08) <0.001 0.17 (0.16, 0.17) <0.001 -0.42 (-0.42, -0.41) <0.001 0.15 (0.14, 0.15) <0.001 0.18 (0.17, 0.19) <0.001
Sex
Male Reference Reference Reference Reference Reference
Female -0.01 (-0.01, 0.00) 0.173 -0.00 (-0.02, 0.01) 0.455 -0.00 (-0.03, 0.03) 0.981 0.01 (-0.02, 0.03) 0.666 0.01 (-0.02, 0.03) 0.674
Maternal age
<25 Reference Reference Reference Reference Reference
25-29 -0.08 (-0.09, -0.07) <0.001 -0.13 (-0.15, -0.11) <0.001 0.20 (0.15, 0.25) <0.001 -0.07 (-0.11, -0.03) 0.001 -0.09 (-0.13, -0.04) <0.001
30-34 -0.10 (-0.11, -0.09) <0.001 -0.19 (-0.21, -0.18) <0.001 0.27 (0.22, 0.32) <0.001 -0.37 (-0.41, -0.33) <0.001 -0.39 (-0.43, -0.35) <0.001
>=35 -0.09 (-0.10, -0.08) <0.001 -0.17 (-0.18, -0.15) <0.001 0.10 (0.05, 0.15) <0.001 -0.58 (-0.62, -0.54) <0.001 -0.61 (-0.65, -0.56) <0.001
p for trend <0.001 <0.001 <0.001 <0.001 <0.001
Ethnicity
White Reference Reference Reference Reference Reference
Asian 0.10 (0.09, 0.11) <0.001 0.20 (0.19, 0.22) <0.001 -0.66 (-0.71, -0.62) <0.001 0.35 (0.31, 0.38) <0.001 0.37 (0.33, 0.40) <0.001
Black 0.26 (0.25, 0.27) <0.001 0.52 (0.51, 0.54) <0.001 -1.34 (-1.38, -1.29) <0.001 0.45 (0.41, 0.49) <0.001 0.53 (0.49, 0.57) <0.001
Other 0.16 (0.15, 0.18) <0.001 0.34 (0.32, 0.35) <0.001 -0.82 (-0.87, -0.77) <0.001 0.62 (0.58, 0.66) <0.001 0.67 (0.62, 0.71) <0.001
Birth in marriage
Within marriage† Reference Reference Reference Reference Reference
Sole registration 0.13 (0.11, 0.14) <0.001 0.25 (0.22, 0.27) <0.001 -0.43 (-0.50, -0.37) <0.001 0.38 (0.33, 0.44) <0.001 0.42 (0.37, 0.48) <0.001
Joint/same address -0.07 (-0.08, -0.06) <0.001 -0.12 (-0.14, -0.11) <0.001 0.42 (0.38, 0.46) <0.001 0.19 (0.15, 0.22) <0.001 0.19 (0.15, 0.22) <0.001
Joint/different address 0.01 (-0.01, 0.02) 0.364 0.04 (0.02, 0.06) 0.001 -0.10 (-0.16, -0.05) <0.001 0.13 (0.09, 0.18) <0.001 0.16 (0.11, 0.21) <0.001
Year
2006 Reference Reference Reference Reference Reference
2007 -1.28 (-1.28, -1.27) <0.001 -0.97 (-0.99, -0.96) <0.001 3.75 (3.71, 3.79) <0.001 0.04 (-0.01, 0.08) 0.091 0.04 (-0.01, 0.08) 0.097
2008 -1.56 (-1.57, -1.56) <0.001 -1.56 (-1.57, -1.55) <0.001 0.18 (0.14, 0.22) <0.001 0.07 (0.03, 0.11) 0.001 0.07 (0.03, 0.12) 0.002
2009 -2.11 (-2.12, -2.10) <0.001 -2.45 (-2.46, -2.43) <0.001 -2.99 (-3.03, -2.95) <0.001 0.10 (0.06, 0.14) <0.001 0.10 (0.06, 0.15) <0.001
2010 -3.19 (-3.20, -3.18) <0.001 -4.59 (-4.60, -4.57) <0.001 6.07 (6.03, 6.12) <0.001 0.19 (0.14, 0.23) <0.001 0.19 (0.15, 0.24) <0.001
p for trend <0.001 <0.001 <0.001 <0.001 <0.001
Season
Winter Reference Reference Reference Reference Reference
Spring 0.91 (0.91, 0.92) <0.001 1.24 (1.22, 1.26) <0.001 -3.57 (-3.61, -3.53) <0.001 -0.07 (-0.11, -0.03) <0.001 -0.07 (-0.11, -0.03) <0.001
Summer 1.10 (1.09, 1.11) <0.001 1.19 (1.17, 1.21) <0.001 2.07 (2.03, 2.11) <0.001 -0.03 (-0.06, 0.01) 0.161 -0.03 (-0.07, 0.01) 0.135
Autumn -0.04 (-0.05, -0.03) <0.001 -0.35 (-0.36, -0.33) <0.001 5.26 (5.23, 5.30) <0.001 -0.01 (-0.05, 0.02) 0.493 -0.01 (-0.05, 0.02) 0.459
Carstairs quintile (COA)
1 - least deprived Reference Reference Reference Reference Reference
2 0.16 (0.15, 0.18) <0.001 0.37 (0.35, 0.39) <0.001 -1.10 (-1.15, -1.04) <0.001 0.83 (0.78, 0.88) <0.001 0.89 (0.85, 0.94) <0.001
3 0.25 (0.24, 0.27) <0.001 0.56 (0.54, 0.58) <0.001 -1.74 (-1.80, -1.69) <0.001 1.07 (1.03, 1.12) <0.001 1.17 (1.12, 1.21) <0.001
4 0.34 (0.33, 0.35) <0.001 0.75 (0.73, 0.77) <0.001 -2.17 (-2.22, -2.12) <0.001 1.17 (1.13, 1.22) <0.001 1.30 (1.25, 1.34) <0.001
5 - most deprived 0.56 (0.55, 0.57) <0.001 1.18 (1.16, 1.20) <0.001 -3.18 (-3.23, -3.13) <0.001 1.20 (1.16, 1.24) <0.001 1.41 (1.37, 1.45) <0.001
p for trend <0.001 <0.001 <0.001 <0.001 <0.001
Inner/Outer London
Inner Reference Reference Reference Reference Reference
Outer -0.63 (-0.64, -0.62) <0.001 -1.29 (-1.30, -1.28) <0.001 3.68 (3.64, 3.71) <0.001 -0.80 (-0.83, -0.77) <0.001 -1.05 (-1.08, -1.02) <0.001
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Supplementary Table 4: Mean difference in term birth weight (g) associated with air and noise pollution (with 95% confidence
intervals)
Unadjusted single-exposure model Adjusted single-exposure model
Term birth weight (g) Term birth weight (g)
Exposure N MD (95%CI) p-value MD (95%CI) p-value
NO2 (per IQR) 540,365 -20.67 (-22.30, -19.04) <0.001 -10.97 (-12.98, -8.96) <0.001
NOX (per IQR) 540,365 -20.15 (-21.81, -18.49) <0.001 -10.74 (-12.76, -8.73) <0.001
PM2.5 traffic-exhaust (per IQR) 540,365 -23.09 (-24.89, -21.29) <0.001 -12.43 (-14.51, -10.35) <0.001
PM2.5 traffic-non-exhaust (per IQR) 540,365 -17.24 (-18.68, -15.80) <0.001 -7.41 (-8.96, -5.86) <0.001
PM2.5 (per IQR) 540,365 -14.61 (-16.58, -12.65) <0.001 -12.94 (-16.41, -9.47) <0.001
PM10 (per IQR) 540,365 -14.38 (-16.08, -12.69) <0.001 -7.27 (-9.84, -4.70) <0.001
O3 (per IQR) 540,365 14.53 (12.81, 16.25) <0.001 5.12 (3.11, 7.12) <0.001
LAeq,16hr
<55dB 157,491 Reference Reference
55-<60 dB 265,603 -3.80 (-6.74, -0.87) 0.011 2.02 (-0.73, 4.78) 0.150
60-<65 dB 40,755 -14.63 (-19.76, -9.49) <0.001 -4.70 (-9.39, -0.02) 0.049
>=65dB 76,516 -19.88 (-23.95, -15.81) <0.001 -5.23 (-9.00, -1.45) 0.007
p for trend 540,365 <0.001 0.001
Lnight
<50dB 162,260 Reference Reference
50-<55 dB 257,045 -6.63 (-9.56, -3.70) <0.001 0.08 (-2.67, 2.83) 0.956
55-<60 dB 40,256 -14.62 (-19.76, -9.47) <0.001 -4.64 (-9.33, 0.06) 0.053
60-<65 dB 46,994 -17.33 (-22.17, -12.49) <0.001 -4.34 (-8.76, 0.09) 0.055
>=65dB 33,810 -31.65 (-37.17, -26.13) <0.001 -12.66 (-17.77, -7.55) <0.001
p for trend 540,365 <0.001 <0.001
Adjusted model covariates: sex, maternal age, birth registration type, tobacco expenditure (COA-level), Carstairs quintile (COA-level),
individual-level ethnicity, gestational age as linear and quadratic terms, season of birth, year (linear term) and random intercept for MSOA.
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Supplementary Table 5: Odds of term LBW (<2500g and ≥37 weeks gestation) associated with air and noise pollution (OR, 95%
confidence intervals)
Unadjusted single-exposure model Adjusted single-exposure model
Term LBW Term LBW
Exposure N N cases OR (95% CI) p-value OR (95% CI) p-value
NO2 (per IQR) 540,365 13,804 1.10 (1.07, 1.12) <0.001 1.03 (1.00, 1.06) 0.025
NOX (per IQR) 540,365 13,804 1.09 (1.07, 1.12) <0.001 1.03 (1.01, 1.06) 0.017
PM2.5 traffic-exhaust (per IQR) 540,365 13,804 1.10 (1.08, 1.13) <0.001 1.04 (1.01, 1.07) 0.006
PM2.5 traffic-non-exhaust (per IQR) 540,365 13,804 1.07 (1.05, 1.09) <0.001 1.02 (1.00, 1.04) 0.050
PM2.5 (per IQR) 540,365 13,804 1.10 (1.08, 1.13) <0.001 1.06 (1.01, 1.12) 0.013
PM10 (per IQR) 540,365 13,804 1.09 (1.07, 1.12) <0.001 1.03 (0.99, 1.07) 0.105
O3 (per IQR) 540,365 13,804 0.92 (0.90, 0.95) <0.001 0.96 (0.93, 0.99) 0.009
LAeq,16hr
<55dB 157,491 3,956 Reference Reference
55-<60 dB 265,603 6,719 1.01 (0.97, 1.05) 0.721 0.98 (0.94, 1.02) 0.334
60-<65 dB 40,755 1,093 1.07 (1.00, 1.14) 0.052 1.02 (0.95, 1.10) 0.517
>=65dB 76,516 2,036 1.06 (1.00, 1.12) 0.032 1.01 (0.95, 1.07) 0.775
p for trend 540,365 0.012 0.572
Lnight
<50dB 162,260 4,032 Reference Reference
50-<55 dB 257,045 6,546 1.03 (0.99, 1.07) 0.214 0.99 (0.95, 1.03) 0.664
55-<60 dB 40,256 1,054 1.06 (0.99, 1.13) 0.126 1.01 (0.94, 1.09) 0.771
60-<65 dB 46,994 1,253 1.07 (1.01, 1.15) 0.027 1.02 (0.95, 1.09) 0.554
>=65dB 33,810 919 1.10 (1.02, 1.18) 0.013 1.03 (0.95, 1.11) 0.510
p for trend 540,365 0.002 0.349
Adjusted model covariates: sex, maternal age, birth registration type, tobacco expenditure (COA-level), Carstairs quintile (COA-level),
individual-level ethnicity, gestational age as linear and quadratic terms, season of birth, year (linear term) and random intercept for MSOA.
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Supplementary Table 6: Odds of term small-for-gestational-age (SGA) associated with air and noise pollution (odds ratios with 95%
confidence intervals)
Unadjusted single-exposure model Adjusted single-exposure model
Term sex-ethnic-specific SGA Term sex-ethnic-specific SGA
Exposure N N cases OR (95% CI) p-value OR (95% CI) p-value
NO2 (per IQR) 471489 44966 1.05 (1.04, 1.07) <0.001 1.01 (1.00, 1.03) 0.076
NOX (per IQR) 471489 44966 1.05 (1.04, 1.07) <0.001 1.01 (1.00, 1.03) 0.071
PM2.5 traffic-exhaust (per IQR) 471489 44966 1.07 (1.06, 1.08) <0.001 1.02 (1.01, 1.04) 0.009
PM2.5 traffic-non-exhaust (per IQR) 471489 44966 1.03 (1.02, 1.05) <0.001 1.01 (1.00, 1.02) 0.144
PM2.5 (per IQR) 471489 44966 1.08 (1.07, 1.10) <0.001 1.03 (1.00, 1.06) 0.024
PM10 (per IQR) 471489 44966 1.07 (1.05, 1.08) <0.001 1.01 (0.99, 1.03) 0.428
O3 (per IQR) 471489 44966 0.96 (0.94, 0.97) <0.001 0.99 (0.97, 1.01) 0.308
LAeq,16hr
<55dB 138696 13113 Reference Reference 55-<60 dB 232346 22008 1.00 (0.98, 1.03) 0.859 0.99 (0.97, 1.01) 0.417
60-<65 dB 35334 3458 1.04 (1.00, 1.08) 0.058 1.02 (0.98, 1.06) 0.463
>=65dB 65113 6387 1.04 (1.01, 1.07) 0.011 1.00 (0.97, 1.04) 0.771
p for trend 471489 0.004 0.567
Lnight
<50dB 142880 13406 Reference Reference 50-<55 dB 224864 21351 1.01 (0.99, 1.04) 0.256 1.00 (0.98, 1.02) 0.902
55-<60 dB 34960 3433 1.05 (1.01, 1.09) 0.012 1.03 (0.99, 1.07) 0.205
60-<65 dB 40344 3899 1.03 (1.00, 1.07) 0.088 1.00 (0.96, 1.04) 0.902
>=65dB 28441 2877 1.09 (1.04, 1.13) <0.001 1.03 (0.99, 1.08) 0.140
p for trend 471489 <0.001 0.156
Adjusted model covariates: maternal age, birth registration type, tobacco expenditure (COA-level), Carstairs quintile (COA-level), season of
birth, year (linear term) and random intercept for MSOA.
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Supplementary Table 7: Adjusted trimester-specific air pollutant associations with term
birth weight (models adjusted for road traffic noise co-exposure)
(To allow comparison between trimesters for a given pollutant we present analyses per
specific µg/m3 increments, as the IQR varied between pregnancy and trimester-specific
averages).
Exposure
Exposure window
Term birth weight (g)
MD (95%CI) p-value
NO2 (per 10 µg/m3) Pregnancy -12.17 (-14.71, -9.64) <0.001
Trimester 1 -5.54 (-7.38, -3.69) <0.001
Trimester 2 -5.49 (-7.29, -3.68) <0.001
Trimester 3 -7.28 (-9.12, -5.44) <0.001
NOX (per 20 µg/m3) Pregnancy -8.58 (-10.41, -6.75) <0.001
Trimester 1 -3.76 (-5.07, -2.44) <0.001
Trimester 2 -3.98 (-5.29, -2.68) <0.001
Trimester 3 -5.14 (-6.48, -3.80) <0.001
PM2.5 traffic-exhaust (per 1 µg/m3) Pregnancy -36.44 (-43.22, -29.67) <0.001
Trimester 1 -22.04 (-27.52, -16.55) <0.001
Trimester 2 -23.93 (-29.40, -18.45) <0.001
Trimester 3 -26.65 (-32.40, -20.90) <0.001
PM2.5 traffic-non-exhaust (per 1 µg/m3) Pregnancy -25.74 (-32.14, -19.34) <0.001
Trimester 1 -14.48 (-19.68, -9.28) <0.001
Trimester 2 -17.10 (-22.31, -11.89) <0.001
Trimester 3 -18.94 (-24.21, -13.67) <0.001
PM2.5 (per 5 µg/m3) Pregnancy -25.95 (-34.35, -17.55) <0.001
Trimester 1 -6.11 (-9.51, -2.71) <0.001
Trimester 2 -2.24 (-5.51, 1.04) 0.180
Trimester 3 -4.01 (-7.45, -0.58) 0.022
PM10 (per 10 µg/m3) Pregnancy -18.23 (-27.36, -9.10) <0.001
Trimester 1 -11.82 (-16.85, -6.80) <0.001
Trimester 2 -1.86 (-6.33, 2.60) 0.414
Trimester 3 -1.55 (-6.01, 2.91) 0.496
O3 (per 10 µg/m3) Pregnancy 5.36 (2.94, 7.79) <0.001
Trimester 1 1.27 (-0.35, 2.89) 0.124
Trimester 2 2.24 (0.66, 3.82) 0.005
Trimester 3 3.31 (1.69, 4.94) <0.001
Birth weight models adjusted for: sex, maternal age, birth registration type, tobacco
expenditure (COA-level), Carstairs quintile (COA-level), individual-level ethnicity,
gestational age as linear and quadratic terms, season of birth, year (linear term), random
intercept for MSOA and daytime noise (LAeq,16hr). N for term birth weight = 540,365.
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