Background Secondhand smoke (SHS) is a leading preventable cause of illness, disability and mortality. There is a lack of quantitative analyses on socioeconomic disparities in SHS; especially, it is not known how socioeconomic disparities have changed in the past two decades in the USA.

Objectives To examine socioeconomic disparities and long-term temporal trends in SHS exposure among US never-smoking adults aged ≥20 years.

Methods 15 376 participants from the National Health and Nutrition Examination Survey (NHANES) 1999–2010 were included in the analysis of socioeconomic disparities; additional 8195 participants from NHANES III 1988–1994 were included in the temporal trend analysis. SHS exposure was assessed using self-reported exposure in the home and workplace as well as using serum cotinine concentrations ≥0.05 ng/mL. Individual socioeconomic status (SES) was assessed using poverty-to-income ratio.

Results During the period 1999–2010, 6% and 14% of participants reported SHS exposure in the home and workplace, respectively; 40% had serum cotinine-indicated SHS exposure. Individual SES was strongly associated with SHS exposure in a dose–response fashion; participants in the lowest SES group were 2–3 times more likely to be exposed to SHS compared with those in the highest SES group. During the period 1988–2010, the prevalence declined over 60% for the three types of SHS exposure. However, for cotinine-indicated exposure, the magnitudes of the declines were smaller for lower SES groups compared with higher SES groups, leading to widening socioeconomic disparities in SHS exposure.

Conclusions SHS exposure is still widespread among US never-smoking adults, and socioeconomic disparities for cotinine-indicated exposure have substantially increased in the past two decades.