Adjusted mortality rate ratios (MRR) of lung cancer as a function of synthetic metalworking fluid (MWF) after stratification by history of exposure to biocide (in synthetic MWF) as estimated by Cox regression using penalized splines in the incident hires subcohort of workers hires on or after start of follow-up. Models were adjusted for straight and soluble MWF (mg/m3 per year), race, gender, plant, calendar year, and years since retirement. Straight, soluble and synthetic MWF, calendar year, and years since retirement were treated as non-linear terms if degrees of freedom were selected by Akaike information criterion (AIC) >1.0 and if P<0.10. Rug plots represent the distribution of exposure to synthetic MWF among workers never exposed (top) and ever exposed (bottom) to biocide.