The diesel exhaust in miners study: I. Overview of the exposure assessment process

Our plain-English summary of The diesel exhaust in miners study: I. Overview of the exposure assessment process, by P.A. Stewart, J.B. Coble, R. Vermeulen, et al (The Annals of Occupational Hygiene, 2010).

The study

• Authors: P.A. Stewart, J.B. Coble, R. Vermeulen, P. Schleiff, A. Blair, J. Lubin, M. Attfield, D.T. Silverman
• Journal: The Annals of Occupational Hygiene
• Published: 2010;54(7):728-46
• Cited by: 52 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

This report provides an overview of the exposure assessment process for an epidemiologic study that investigated mortality, with a special focus on lung cancer, associated with diesel exhaust (DE) exposure among miners. Details of several components are provided in four other reports. A major challenge for this study was the development of quantitative estimates of historical exposures to DE. There is no single standard method for assessing the totality of DE, so respirable elemental carbon (REC), a component of DE, was selected as the primary surrogate in this study. Air monitoring surveys at seven of the eight study mining facilities were conducted between 1998 and 2001 and provided reference personal REC exposure levels and measurements for other agents and DE components in the mining environment. (The eighth facility had closed permanently prior to the surveys.) Exposure estimates were developed for mining facility/department/job/year combinations. A hierarchical grouping strategy was developed for assigning exposure levels to underground jobs [based on job titles, on the amount of time spent in various areas of the underground mine, and on similar carbon monoxide (CO, another DE component) concentrations] and to surface jobs (based on the use of, or proximity to, diesel-powered equipment). Time trends in air concentrations for underground jobs were estimated from mining facility-specific prediction models using diesel equipment horsepower, total air flow rates exhausted from the underground mines, and, because there were no historical REC measurements, historical measurements of CO. Exposures to potentially confounding agents, i.e. respirable dust, silica, radon, asbestos, and non-diesel sources of polycyclic aromatic hydrocarbons, also were assessed. Accuracy and reliability of the estimated REC exposures levels were evaluated by comparison with several smaller datasets and by development of alternative time trend models. During 1998-2001, the average measured REC exposure level by facility ranged from 40 to 384 μg m⁻³ for the underground workers and from 2 to 6 μg m⁻³ for the surface workers. For one prevalent underground job, 'miner operator', the maximum annual REC exposure estimate by facility ranged up to 685% greater than the corresponding 1998-2001 value. A comparison of the historical CO estimates from the time trend models with 1976-1977 CO measurements not used in the modeling found an overall median relative difference of 29%. Other comparisons showed similar levels of agreement. The assessment process indicated large differences in REC exposure levels over time and across the underground operations. Method evaluations indicated that the final estimates were consistent with those from alternative time trend models and demonstrated moderate to high agreement with external data.

In plain English

This report described how researchers estimated past diesel exhaust exposure for the Diesel Exhaust in Miners Study. That study examined deaths, especially from lung cancer, at eight US non-metal mining facilities (potash, trona, salt and limestone). This report did not give death results. Respirable elemental carbon (REC), a diesel exhaust component, served as a stand-in for exposure. In 1998–2001 surveys at seven facilities, average REC levels were 40 to 384 µg/m³ for underground workers and 2 to 6 µg/m³ for surface workers. Earlier underground levels were back-calculated from diesel horsepower, ventilation and historical carbon monoxide measurements; surface levels were mostly held constant. In most underground operations, estimated levels peaked in the mid-1970s to early 1980s. Estimated 1976–1977 carbon monoxide levels differed from measurements taken at that time by a median of 29%. Asbestos was assessed only as a possible confounding exposure. With only 26 asbestos measurements, the authors scored asbestos exposure by job instead; about 40% of the cohort was rated potentially exposed. The authors concluded that the estimates were credible, while noting that bias and imprecision are inherent in any estimation.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Methods report describing the exposure assessment for an epidemiologic study (a cohort study and a nested case-control study of lung cancer); no mortality results are reported here
• Who was studied: Workers who ever held a non-supervisory, non-administrative job for at least 1 year at eight US non-metal mining facilities: potash in New Mexico, trona in Wyoming, rock salt in Ohio, and limestone in Missouri. The period ran from the first diesel use (1947–1967, depending on the facility) through 1997. There were 12,382 subjects, or 12,315 after 67 were excluded for invalid birth dates and other criteria.
• Exposure: Diesel exhaust, represented by respirable elemental carbon (REC), a component of diesel exhaust used as a surrogate. REC was estimated for each facility, department, job and year. Respirable dust, silica, radon, asbestos and non-diesel PAHs were also assessed as potential confounders.
• Main results:
  • In the 1998–2001 surveys, average measured REC levels by facility ranged from 40 to 384 µg/m³ for underground workers and from 2 to 6 µg/m³ for surface workers.
  • For most underground operations, estimated historical REC levels followed an inverted U shape over time, with the highest estimates typically in the mid-1970s to early 1980s. In Facility A, underground levels kept rising throughout the study period.
  • The highest estimated underground REC level, as a percentage of the 1998–2001 reference level, ranged from 100% (Facility A) to 685% (Facility G).
  • Modeled 1976–1977 underground carbon monoxide estimates were compared with 1976–1977 measurements that were not used in the modeling. The overall median relative difference was 29%.
  • In Facility B, 1994 REC estimates for two underground jobs differed from 1994 measurements by 6% and 10%.
  • Estimated REC levels for surface jobs were 1–2 µg/m³ for group A (no or very limited contact with diesel equipment), 2–4 µg/m³ for group B, and 4–11 µg/m³ for group C (operating heavy diesel equipment, driving forklifts indoors, or repairing diesel equipment). For most surface jobs these 1998–2001 estimates were also used as the historical estimates, because surface exposure was treated as constant over time.
  • Asbestos was among the agents assessed because they could confound the epidemiologic results.
  • Only 26 asbestos measurements were available, so asbestos exposure was scored by job rather than measured. Pipe fitters were scored 3. Auto and diesel mechanics, boiler operators, power operators and bricklayers were scored 2. Dryer operators, kiln operators, maintenance workers and mechanics were scored 1. All other jobs were scored 0.
  • Eleven personal asbestos measurements from the 1980s were all below 0.1 fiber per cc. These measurements were shown for information only and were not used to categorize workers.
  • About 40% of the cohort was identified as potentially exposed to asbestos, and about 40% to PAHs.
  • Average measured levels of radon, asbestos and PAHs were less than 5% of the MSHA standards.
• The authors' conclusion: The authors concluded that the estimates were credible and could be used in the epidemiologic analysis.
• Limitations the authors state:
  • No gold standard was available, so the estimates could be compared with other data but not formally validated.
  • There were no REC measurements before the 1990s, so historical REC levels had to be estimated indirectly using carbon monoxide.
  • Work histories did not show which mine face a worker worked at, so all underground workers with the same job in the same year got the same estimate.
  • Variability between and within individual workers could not be evaluated.
  • The estimates were not adjusted for respirator use.
  • The authors stated that the estimates are not free of error.

Topics

Medical terms in this study, explained in our Lexicon: Lung Cancer, Air Sampling, Confounding.

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