Objectives This study aimed to determine whether night shift work was associated with exposure to carcinogenic hazards, independent of occupation and industry.

Methods We conducted a cross-sectional analysis of 16 250 job records reported by 7023 participants. Participants provided lifetime job histories, including shift schedules and exposures to hazardous agents. Carcinogenic agents included established carcinogens classified by the International Agency for Research on Cancer, as well as broader exposure categories that may potentially contain carcinogenic constituents. Jobs involving ≥1 and ≥7 night shifts/month were compared with day-shift jobs. Occupations and industries were coded using National Occupational Classification (2016) and North American Industry Classification System (2017) codes. Generalized linear mixed models with random intercepts for participant, occupation code (2-digit level), and industry code (2-digit level) were used to estimate odds ratios (OR) for exposures associated with night- versus day-shift work, adjusting for biological sex and job start year.

Results Night shifts were associated with greater odds of any hazardous exposure [≥1 night shift/month: OR 3.1, 95% confidence interval (CI) 2.6–3.8; ≥7 night shifts/month: OR 3.1, 95% CI 2.5–3.8]. Significant associations were observed for multiple agents, including asbestos, degreasing agents, diesel engine exhaust, gasoline engine exhaust, ionizing radiation, mineral/cutting/lubricating oil, paints/stains/varnish, welding fume, and wood dust.

Conclusions Night-shift work was associated with an increased likelihood of self-reported exposure to multiple potentially carcinogenic agents, independent of broad occupational and industry groupings. Future research with more granular task-level information and quantitative exposure assessment will be important for clarifying the mechanisms underlying these differences.

Key terms carcinogen; carcinogenic substance; co-exposure; day shift; exposure; hazard; night shift; night work; occupational exposure; shift work; shift worker

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