Exposure to Small Particle Air Pollution Linked to Ischemic Heart Disease in Aluminum Workers

Occupational exposure to air pollution less than 2.5 microns in diameter (PM2.5) is associated with an increased risk of ischemic heart disease (IHD) among aluminum smelter and fabrication workers, a new study by COEH researchers at UC Berkeley concludes.

The longitudinal study predicted that after 15 years, the risk of IHD for smelter workers was 2.9 percent higher in those always exposed above, compared to always below the 10th percentile (0.16 mg/m3) of exposure to PM2.5. For fabrication workers, where the 10th percentile was 0.06 mg/m3, the estimated difference in the risk of IHD was 2.5 percent.

“In the smelter worker population, we predicted that 4 percent would have been diagnosed with IHD if they had been exposed at the lowest levels compared to 6.9 percent if exposed at the higher level. So this 2.9 percent absolute increase is actually a 77 percent relative increase. In the fabrication worker population, the 2.5 percent increase is equivalent to a 45 percent relative increase,” reports lead author Daniel Brown, a research data analyst in the School of Public Health.

PM2.5 poses a greater health risk than larger, coarse particles over 2.5 microns in diameter, reports the U.S. Environmental Protection Agency. Due to their smaller size, they can be inhaled and stored deep in vulnerable areas of the lung. IHD narrows the arteries that bring oxygen and blood to the heart muscle. It is the leading cause of death globally, notes the World Health Organization. Heart disease is responsible for one in four deaths each year in the United States.

“Our study provides more evidence that there is a causal relationship between inhaling small particulates at levels seen in an occupational environment and the subsequent development of heart disease,” says Brown.

A total of 13,529 workers from one of 11 aluminum smelter or fabrication plants were followed for the incidence of IHD for more than two years between January 1996 and December 2012, starting two years from their date of hire. The two-year lag allowed researchers to remove pre-existing cases of IHD from the cohort. All participants were enrolled in the company health plan.

Published in the journal, Epidemiology, the study is the first to apply targeted minimum loss estimation (TMLE) in the field of occupational and environmental health – a doubly robust statistical method of achieving an unbiased estimate of disease and exposure.

With the TMLE method, researchers were able to take into account time-varying confounding in cohorts affected by the Healthy Worker Survivor Effect (HWSE), where workers with better health may remain in their job and accrue more exposure to PM2.5 compared to workers who move to lower exposure jobs or transition out of the workforce due to a change in health status, which can bias study results.

“Basically, workers who get sicker as they get older, possibly due to some of the exposure in the workplace, tend to leave work or selectively reduce their exposure by switching to different jobs,” explains Brown. “That process makes it difficult to get meaningful estimates of the effect of the exposures that we’re looking at. In order to adjust for that process, we really need to use these newer causal techniques.”

Using TMLE, researchers also took into account personal characteristics that change over time, such as smoking and Body Mass Index (BMI) two risk factors for heart disease. The study collected smoking and BMI data at occupational medicine clinics onsite at each facility. The results of the study remained robust to the removal of these variables. “Yes, they’re certainly strong predictors of heart disease, but in this setting, they didn’t act as confounders,” confirms Brown.

Brown’s co-authors from COEH include Sadie Costello, Elizabeth Noth, Katherine Hammond, and Ellen Eisen. Additional authors are Mark van der Lann and Maya Petersen from UC Berkeley and Mark Cullen from the Stanford University School of Medicine.

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