Breast Cancer Incidence and Exposure to Metalworking Fluid in a Cohort of Female Autoworkers

Abstract

Breast cancer is the leading cancer diagnosed among women, and environmental studies have produced few leads on modifiable risk factors for breast cancer. Following an Institute of Medicine recommendation for occupational studies of women highly exposed to potential breast cancer risk factors, we took advantage of an existing cohort of 4,503 female autoworkers in Michigan exposed to metalworking fluid (MWF), complex mixtures of oils and chemicals widely used in metal manufacturing worldwide. Cox proportional hazards models were fit to estimate hazard ratios for incident breast cancer (follow-up, 1985-2013) and cumulative exposure (20-year lag) to straight mineral oils (a known human carcinogen) and water-based soluble and synthetic MWF. Because the state cancer registry began decades after the cohort was defined, we restricted our analyses to subcohorts of women hired closer to the start of follow-up. Among those hired after 1969, the hazard ratio associated with a 1 interquartile-range increase in straight MWF exposure was 1.13 (95% confidence interval: 1.03, 1.23). In separate analyses of premenopausal breast cancer, defined by age at diagnosis, the hazard ratio was elevated for exposure to synthetic MWF (chemical lubricants with no oil content), possibly suggesting a different mechanism in the younger women with breast cancer. This study adds to the limited literature regarding quantitative chemical exposures and breast cancer risk.

Figure 1.

Number of active workers (gray vertical bars) and annual exposure prevalence to straight (blue line), soluble (red line), and synthetic (green line) metalworking fluids (MWF) by year among female members of the United Autoworker-General Motor incidence cohort who were alive in 1985 (n = 4,503), Michigan, 1940–1994.

Figure 2.

Adjusted hazard ratios for premenopausal breast cancer incidence as a smoothed function of 20-year lagged cumulative synthetic metalworking fluid (MWF) as estimated in a Cox regression model using penalized splines (2 degrees of freedom) based on a cohort of female workers in the United Autoworker-General Motor incidence cohort who were alive in 1985, Michigan, 1985–2013. Premenopausal breast cancer was defined as diagnosis by four select age cut points. The 95% confidence interval (95% CI) is shown for the model using age 55 years as the cut point. Models used age as the time scale and adjusted for cumulative exposure to straight and soluble MWF, year of hire, calendar year, race, and manufacturing plant. Graph truncated at the 99th percentile of synthetic fluid exposure among cases (0.60 mg/m³-years). The rug plot indicates exposure of the cases 55 years old or younger.