Diesel exhaust

Diesel exhaust is both an occupational and environmental exposure.

Diesel exhaust causes lung tumors in rats

Studies of workers, with higher exposure than the general population, have indicate that diesel is a lung carcinogen

While workers are more highly exposed than the general population, they are relatively few exposed workers, and the public health burden of lung cancer due to diesel exhaust is higher for the general population.

Environmental exposure.

There are diesel engines in most trucks. Diesel engines also are being used in personal cars, especially in Europe.

Large groups in the general population living in urban areas or close to highways are exposed to DEE, albeit to lower levels than in most occupational settings.

Long distance truck in US, diesel exhaust

Occupational exposure.

Diesel engines were initially used predominantly for heavy duty equipment, with trains converting to diesel locomotives mainly after World War II and with heavy-duty trucks dieselized primarily during the mid to late 1950s. Dieselization of equipment in underground mines occurred mostly in the 1960s –1970s.

Front End Loaders

Haulage Trucks

LHDs

Shuttle Cars

Mantrips

Drills, Roofbolters

Tractors, Dozers, Graders, Scalers, Shovels, Powder Trucks, Gators

DIESEL EQUIPMENT IN MINES

Diesel engine in underground mine around 1980

• The International Agency for Research on Cancer (IARC, part of WHO) – the world authority on what causes cancer - determined diesel fumes to be a definite human lung carcinogen in 2012

IARC says diesel fumes cause cancer

• The findings were based primarily on three studies, two of truckers (low exposure) and one of miners (high exposure), where diesel fumes were measured via elemental carbon – a good surrogate for diesel.

Diesel fumes cause cancer – 3 key studies

• Miners – average elemental carbon exposure

• Overall = 87 µg/m3

• Ever-underground = 128 µg/m3

• Surface only = 2 µg/m3

• Truckers

• Mechanics 26 µg/m3

• Long haul drivers 5 µg/m3

• Dockworkers 2 µg/m3

• General public in cities 1 µg/m3

Exposure in the 3 key studies, average elemental carbon

• Total carbon makes up 80% of the particulate content of diesel exhaust.

• Total carbon is made up of elemental and organic carbon.

• Organic carbon can also come from oil mists and cigarette smoke, but elemental carbon does not.

• Elemental carbon has been chosen by the US MSHA as the best way to measure diesel exhaust in mines.

• There are sources other than diesel fumes for elemental carbon in mines.

Total carbon, elemental carbon, and organic carbon*

* Noll et al. 2007

One of the three studies – the study of diesel-exposed miners - took 20 years to complete. The National Cancer Institute (NCI) and the National Institute for Occupational Safety and Health (NIOSH) initiated the study 1992, which was not published until 2012

Much of the delay was due to legal challenges from an industry group called the Mining Awareness Resource Group (MARG*), and constant interference from members of the US Congress.

*MARG was composed of the National Mining Association, the Salt Institute, Morton International, FMC Wyoming, General Chemical, IMC Fertilizer, Solvay Mineral, Cargill, Stillwater Mining, and Newmont Gold.For more information see Monforton et al., Am J Pub Health 2006

The miner study, a big controversy

For example, in 2012, when IARC was about to consider diesel, industry lawyers representing MARG pressured peer-reviewed journals to not publish the results of the miner study – a study which was key to IARC’s decision that diesel exhaust was a human carcinogen.

The miner study, a big controversy

Miner study at a glance

Followed a cohort 12,315 miners from time of first employment through 1998 to ascertain mortality

8307 ever-underground, 5848 surface-only

Mean year of first exposure = 1971; mean underground tenure = 8 years (after dieselization)

Estimate elemental carbon exposure for each miner during each year of work, using mine’s work history records and some measured exposure of carbon monoxide and elemental carbon

Calculate lung cancer mortality rate by level of exposure

Selected mines had diesel equipment but low levels of silica or radon, which can cause lung cancer

T

PP

P

T

TT

L

S

3 potash mines

3 trona (soda ash) mines

1 salt (halite) mine

1 low-silica limestone mine

Cumulative Elemental Carbon exposure, quartiles*

Ever-underground 1.0 1.5 2.2** 2.2*

Surface only 1.0 1.3 0.7 1.0

Results of the miner study of diesel fumes

*Quartiles approximately <300, 300-600, 600-900, 900+ug/m3 -years

• Estimates of exposure over time were made for each worker in the three studies. Exposure-response data were available for all three studies.

• Exposure-response data quantitatively show the rate of lung cancer with increasing exposure level

Diesel fumes cause cancer – the 3 key studies

• For example, the rate of lung cancer in the general population is about 60/100,000. Suppose exposure to diesel fumes at a level of 20 ug/m3 over 20 years (400 ug/m3 –years) increased the rate 50%, to 90/100,000. We often use cumulative exposure rather than average exposure, because it matters how long you are exposed

Epidemiology stuff about exposure-response

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0 50 100 150 200 250 300 350 400

lung ca.rate/100,000

elemental carbon - years

Lung cancer rate by exposure level

• Epidemiologists often draw a graph with the rate ratio rather than the rate. In our example, the rate ratio for 400 ug/m3-years (an exposure of 20 ug/m3 over 20 years) is 1.5 (90 per 100,000 divided by 60 per 100,000).

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1.05

1.1

1.15

1.2

1.25

1.3

1.35

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1.45

1.5

0 50 100 150 200 250 300 350 400

lung ca.rate ratio

elemental carbon - years

Lung cancer rate ratio by exposure level

More epidemiology stuff about exposure-response

• Worse, epidemiologists often draw a graph with the log of the rate ratio (ln rate ratio) rather than the rate ratio itself (natural log = ln = the power to which 2.84 must be raised to get the rate ratio). Example, log 1.5 = 0.41.

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0.05

0.1

0.15

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0.25

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0.35

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0 50 100 150 200 250 300 350 400

log of rateratio

elemental carbon - years

log of lung cancer rate ratio by exposure level

More epidemiology stuff about exposure-response

• The exposure response data for the 3 studies showed that with more exposure workers got more lung cancer

• This finding helped convince IARC that diesel fumes cause cancer, and also can help set standards for safe exposure.

Diesel fumes cause cancer – the 3 key studies

The three exposure-response curves were similar in the 3 studies

We combined the exposure-response data for the 3 studies, to get the strongest data forrisk assessment.

Combining the 3 key studies

Regulators do risk assessment to determine what level of exposure is permissible – how much risk is involved

Often regulators seek to limit the excess lifetime risk of a disease due to exposure below a certain limit, eg1/1000

They must balance the health benefits agains the cost to industry of lowering permissible exposure limits

Risk assessment

Example: US OSHA just changed the permissible level of silica from 100 ug/m3 to 50 ug/m3, based on exposure-response data which showed that that would substantially decrease the excess risk of workers for lung cancer and silicosis

Risk assessment

*Garshick et al. 2012 (red), Silverman et al. 2012 (yellow), Steenland et al. 1998 (blue)**log rate ratios regressed on mid points of cumulative exposure categories,last category used 1.5*highest cutpoint, test for trend p<0.0001

y = 0.0012x + 0.0017R² = 0.7702

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log RRs

ug/m3-years EC

Pooled exposure response in 3 diesel studies using elemental carbon

Our slope estimate is roughly consistent with the reported slope estimates for EC and lung cancer risk in outdoor air pollution studies, which have much lower exposures.

Using the common exposure-response curve to estimate risk of miners for lung cancer

For each 1 ug/m3-year of elemental carbon exposure, the log rate ratio (RR) for lung cancer would increase by 0.001, or the RR would increase by 0.001.

This gives an RR of about 2 (2 fold risk of lung cancer) for 700 ug/m3-years, which corresponds to an average exposure of about 35 ug/m3 elemental carbon for someone exposed for 20 years (the underground miners had an average exposure of about this level).

More occupational risk assessment

The lifetime risk of lung cancer death in the general population is about 5%

The estimated excess lifetime risk for death from lung cancer death for exposure for 45 years at 1 ug/m3 is 2/1000 (from 0.050 to 0.052) – doesn’t seem like much

But this is slightly above the desired maximum allowed excess risk of 1/1000 in the US for worker (OSHA)

Occupational risk assessment

Given that OSHA seeks to keep excess lifetime risks below 1/1000, this would argue for a permissible exposure level for workers of slightly less than 1 ug/m3.

Currently, with the exception of miners, exposed workers have diesel exhaust exposures in the range 3-13 ug/m3, while members of the public in urban areas are exposed to about 0.8 ug/m3

What would be a good permissible level for miners?

From a health standpoint, permissible levels should be lowered to the same as the general public, ie, 1 ug/m3 of elemental carbon

This must be weighed against current levels in miners, which are likely to be on the order of 50-100 ug/m3

(personal exposure over 8 hours, Debia et al. 2017), in a cost-benefit analysis

Currently US OSHA or MSHA does not have a standard for permissible exposure to elemental carbon in any occupation

“The Canadian province of Ontario has a regulatory time-weighted limit value of 400 µg/m3 of Total Carbon (TC) transposable to elemental carbon (EC) via a conversion factor of 1.3 (i.e. about 310 µg/m3 for EC) for the mining industry”

Debia et al. International Journal of Mining Science and Technology, Volume 27, Issue 4, July 2017, Pages 641–645

Environmental risk assessment

A lifetime of exposure to background levels of 1 ug/m3 of diesel exhaust in ambient air would lead to a lifetime excess risk of about 1% for lung cancer.

While this is a small excess risk, it has important public health consequences because everyone in cities is exposed.

Attributable risk – how many cancer deaths does diesel exhaust cause?

Assume an occupational exposures of 13 ug/m3 elemental carbon (high exposure, 20%), and 4 ug/m3 (low exposure, 80%), and assume that 5% of the population is occupationally exposed.

Assume an average environmental exposure of 1 ug/m3.

Given our estimates of risk per 1 ug/m3, we estimate that

*Environmental exposure is responsible for about 4.5% of lung cancer deaths in the US and England

*Another 1.5% of lung cancer is due to occupational exposures to diesel fumes.

Future?

Epidemiologic studies of truckers to date have evaluated exposures largely from an older engine technology. These older engines will eventually be replaced by newer, cleaner-burning engines designed to reduce exhaust exposures, for both trucks and cars. However, the truck and car fleet will take a long time to turnover.

There will be a need for new epidemiologic investigations to evaluate any potential hazard from exhausts from these new car/truck engines.

Future?

It is not clear whether and when such cleaner-engine technology will also be used in mines and train engines, which have been subject to these same regulatory pressures as cars and trucks.