Air pollution, road traffic noise, and greenness in relation to salivary cortisol in young adults

Background Environmental exposures have been linked to stress, but evidence on their influence on its objective marker, salivary cortisol, in young adults is limited. Objective We examined associations of environmental exposures with salivary cortisol levels. Methods The study was conducted within the Swedish birth cohort BAMSE. Exposure to air pollutants (PM 2.5 , PM 10 , Black Carbon, NO x ), road traffic noise, and greenness (i.e. the average Normalized Difference Vegetation Index (NDVI) within 100 m, 300 m and 500 m buffers) one year prior to clinical assessments was estimated at the individual residential address level using validated models. Salivary cortisol levels were measured at age 24 ( n = 1148), and their associations with environmental exposures were estimated using linear regression, adjusted for covariates. Exploratory post hoc subgroup analyses were conducted by sex, tobacco use, body mass index, history of depression, asthma, rhinitis, and eczema. Results In this cross-sectional analysis, no overall associations were observed between environmental exposures and salivary cortisol. However, exploratory post hoc subgroup analyses suggested sex- and season-specific differences. Among males, higher morning cortisol levels were observed in relation to exposure to PM 10 ( β = 0.058; 95% CI = 0.003, 0.112), NO x ( β = 0.070; 95% CI = 0.017, 0.123), and noise ( β = 0.120; 95% CI = 0.024, 0.215), while inverse associations were found with greenness (e.g. for NDVI 500 m β = −0.165; 95% CI = −0.263, −0.067). Similar findings were observed for evening cortisol. In contrast, females exhibited opposite trends. NDVI was significantly positively associated with higher evening cortisol levels during the summer, particularly for NDVI within 100 m ( β = 0.326; 95% CI = 0.063, 0.590), while no seasonal pattern was observed in relation to air pollutants or road traffic noise. Several sex-by-exposure interactions remained significant after false discovery rate correction, whereas season-specific patterns did not. Impact This study investigates how multiple residential environmental exposures, air pollution, road traffic noise, and greenness, relate to salivary cortisol as a biomarker of stress in young adulthood. Using detailed individual-level exposure modelling and a well-characterised population-based birth cohort, we assess both morning and evening cortisol and examine single and combined exposure impact. While overall associations were largely null, exploratory post hoc subgroup analyses suggested potential sex-specific differences in the associations between environmental exposures and salivary cortisol, whereas evidence for seasonal differences was weaker and did not remain statistically significant after correction for multiple testing. The findings highlight the importance of considering multiple environmental exposures and susceptibility factors when evaluating stress-related biological mechanisms in young adults. Significance We found no overall associations between environmental exposures and salivary cortisol in young adults, although exploratory subgroup analyses suggested potential sex-specific differences.