COV4 2006

Long term exposure to respirable volcanic ash on Montserrat: a time series simulation

T. Hincks, R.S.J. Sparks University of BristolW.P. Aspinall Aspinall and Associates and University of BristolP.J. Baxter Dept of Public Health and Primary Care, Univ. CambridgeA. Searl Institute of Occupational Medicine, EdinburghG. Woo Risk Management Solutions

COV4 2006

Aim:Estimate risk of silicosis from cumulative exposure

to cristobalite

• Volcanic activity

• Ash composition

• Deposition and erosion

• Weather conditions

• Human activity / occupation

COV4 2006Risk assessment for

Montserrat

Active since 1995 - high cumulative exposures?

1996 - DfID and DoH research on health risks associated with volcanic ash

silicosis, lung cancer, pulmonary tuberculosis, autoimmune diseases

Health risks?

• Exposure to date

• Continuing volcanic activity

3 - 24 weight % crystalline silica in <10m (inhalable) fraction of ground deposits

Baxter et al. (1999)

preferentially fractionated in PM4 (<4m)Horwell et al. (2003)

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Aleatory uncertainty

eruptive historyash isopach dataair quality monitoringweather data

• Difficult to assess risk in populations

• Information highly uncertain and poorly constrainedprobabilistic risk assessment

Observations

Numerical models ash dispersalweather simulation

Expert elicitation formal and unbiased process for obtaining information from experts

limited data

full extent of system behavior?

Observables represented by probability density functions

Uncertainty and expert judgment

COV4 2006

Time series simulation

Code generates daily exposures for PM10 (<10m) and cristobalite crystalline silica

• 6 occupation groups• 4 sites

• Multiple runs (10,000 runs/simulation)

• Sample from PDF for each parameter• Correlated sampling

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Model structure

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Model structure

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Dome growth

Periodic dome growth function controls replacement of material

frequency of collapses increased probability of vulcanian explosions at high growth rates

Gro

wth

rat

e m

3 s-1

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Rainfall

Rainfall time series simulated as two part process:• Incidence of rain (true/false)• Quantity of rain (24h)

Mean

Standard deviation

Rain depth:

Lognormal distribution with time dependent parameters

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Volcanic activity

6 event categories are considered: significant ash deposits

• 3 - 10 x 106 m3 dome collapse• 10 - 30 x 106 m3 dome collapse• 30 - 50 x 106 m3 dome collapse• 50 - 75 x 106 m3 dome collapse• >75 x 106 m3 dome collapse• Series of 0.4 x 106 m3 vulcanian explosions

Assume event magnitudes and frequencies ~similar to past 10 years activity daily P(event)

Probability of Vulcanian explosions increases after major dome collapses and during periods of high extrusion rate

COV4 2006

Ash deposition

Ash deposits generated with HAZMAP 2-D advection diffusion model for ash transport

(Bonadonna et al. 2002)

• 3 years of daily wind data

• Dome collapse pyroclastic flows down 5 valleys

• Single source Vulcanian explosions

correlated lognormal deposits distributions for 4 locations

COV4 2006Ash removal (wind and

rain)

level Depth (cm)Lifetime

(lower, expected & upper bounds)

Negligible <= 0.1 cm remains on ground

Minor 0.1 - 1.0 cm 1 day / 14 days / 3 mo

Moderate 1.0 - 3.0 cm 1 mo / 6 mo / 12 mo

Major >3.0 cm 1 year / 2 y / 3 y

Approximate with 4 deposit levels

Use beta distribution to represent

variation in deposit lifetime

Expert elicitation for mean, upper and

lower bounds

COV4 2006

Individual exposure

PDF: VARIATION IN EXPOSURE

Beta distribution function of• deposit depth• cristobalite content of ash• occupation

Modified to account for rainfall

High exposure

Gardeners

Public works department

Low exposure

indoor occupations

elderly

dust trak data

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Individual exposure

Sum daily exposure values over 5, 10 and 20 years estimate cumulative exposure risk of silicosis

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Simulated time series

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Simulated time series

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Simulated time series

US NIOSHrecommended limit

US NIOSH recommended limit: 0.05 mg m-3

time weighted average for up to 10 hour work day during 40 h working week

UK HSE recommended maximum occupational exposure to crystalline silica: 0.3 mg m-3

Suggested limit: 0.1 mg m-3 8h time weighted average

UK HSE (2003) suggested limit

COV4 2006

Results: 20 year cumulative cristobalite exposure%

tria

ls e

xcee

ding

exp

osur

e

cumulative cristobalite exposure mg.m-3.year

cumulative cristobalite exposure mg.m-3.year

c

wf

s

COV4 2006

Estimating risk: exposure-response functions for silicosis

Upper limit of risk:

Buchanan et al. 2003Study of silicosis in Scottish coalminers

• High intensity exposure> 0.1 mg m-3

• Heavy ash fall areas only

COV4 2006

Estimating risk: exposure-response functions for silicosis

Upper limit of risk:

Buchanan et al. 2003Study of silicosis in Scottish coalminers

Most analogous:

Hughes et al. 1998occupational exposure for diatomaceous earth workers

exposure intensity affects risk

> 0.5 mg m-3

≤ 0.5 mg m-3

COV4 2006

20 year risk of silicosis

20 years continuous exposure

Hughes et al. (1998) risk function

TYPICAL ADULT

North 0.4 - 0.8 % MODERATE

Salem 1% MODERATE

Cork Hill: 1.1 - 2.5 % HIGH

CMO risk scale

Estimated exposures lie within bottom 20% of Hughes cohort (<100 cases)

RISK?

COV4 2006

20 year risk of silicosis

20 years continuous exposure

Hughes et al. (1998) risk function

TYPICAL ADULT

OUTDOOR WORKER

North 0.4 - 0.8 % MODERATE

Salem 1% MODERATE

Cork Hill: 1.1 - 2.5 % HIGH

CMO risk scale

North 1.3 - 2.4 % HIGH

Salem 2.0 - 3.2% HIGH

Cork Hill: 3 - 5 % HIGH

COV4 2006

Validation & future work

Medical studies• 2000: x-ray survey of 421 high risk workers showed no evidence of chest

abnormalities (< 5 years exposure)

• X-ray survey after 10 years exposure

• Risk to children highly uncertain

limited data - better estimates of cumulative exposure?

applicability of exposure response functions?

Field data• Continuous PM10 measurement

+ weather data

• Personal exposure sampling

• Ash erosion rates - very poorly constrained• Duration of hazard• Implications for lahar and flood hazard assessment

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Validation & future work

Exposure control measures

• dust masks for outdoor

workers in ash affected areas

• minimize exposure during

cleanup operations

• minimize children’s exposure

(clear sports & play areas

after ash fall)

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Further applications

PopocatépetlPM10, ash leachates

Ash-leachates

• water contamination

• risk to livestock

• crop damage

Guadeloupe concerns about contamination of aquifer

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Acknowledgements

Thanks to my PhD supervisors: Steve Sparks, Willy Aspinall and Gordon Woo

Constanza Bonadonna for reconfiguring and running HAZMAP ash dispersal code

DATAAsh data from Clare Horwell, Univ. CambridgePersonal exposure DustTrak data from The Institute of Occupational Medicine Montserrat & Antigua rainfall data from the Montserrat Volcano Observatory and IOMGuadeloupe rainfall data from the Hong Kong Observatory www.hko.gov.hk

CODESCYTHE C++ Statistical Library GNU GPL 2001 A.D. Martin and K.M. QuinnMT19937 Mersenne Twister random number generator 2002 T. Nishimura and

M. Matsumoto

COV4 2006

References

Buchanan, D., B. G. Miller, et al. (2003). "Quantitative relations between exposure to respirable quartz and risk of silicosis." Occupational and Environmental Medicine 60(3): 159-164

Burmaster, D. E. and P. D. Anderson (1994). "Principles of Good Practice for the Use of Monte Carlo Techniques in Human Health and Ecological Risk Assessments." Risk Analysis 14(4): 477-481

Cooke, R. M. (1991) Experts in Uncertainty: Opinion and Subjective Probability in Science. Environmental Ethics and Science Policy Series. Oxford University Press, New York.

Hughes et al. (1998) Radiographic Evidence of Silicosis Risk in the Diatomaceous Earth Industry.

Am. J. Respir. Crit. Care Med., Volume 158, Number 3, 807-814 Horwell, C.J., Sparks, R.S.J., Brewer, T.S., Llewellin, E.W., and Williamson, B.J. (2003). The

characterisation of respirable volcanic ash from the Soufrière Hills Volcano, Montserrat, with implications for health hazard. Bull. Volcanol., DOI: 10.1007/S00445-002-0266-6.

National Institute for Occupational Safety and Health (2002). NIOSH Hazard Review: Health effects of occupational exposure to respirable crystalline silica.