Indoor and roadside exposures to traffic noise and cardiovascular mortality and the role of urban environmental stressors across a high-rise, high-density environment: A case study in Hong Kong

Traffic noise is a common factor associated with elevated cardiovascular risks. Compact environment and building morphology in a high-rise, high-density city can magnify traffic noise in various locations (e.g., roadside and indoor environments). However, no studies have investigated how indoor and roadside traffic noise sepa-rately and jointly influenced cardiovascular risks across a compact environment. Thus, this study applied negative binomial generalized linear mixed models to estimate associations between roadside/indoor exposures to traffic noise and cardiovascular mortality (2006-2015). Stratified analyses were applied to evaluate effect modifications by canopy-layer and surface-layer urban heat island (UHI), night-time light, and greenness. Our results showed that each 1 dB increase in roadside and indoor exposures to traffic noise was positively associated with 1.183 (95% confidence interval (CI): 1.068-1.311) and 1.046 (95% CI: 1.012-1.081) times the risk of all cardiovascular deaths, respectively. When both types of noise were included in the models, the associations of roadside exposure were attenuated and became non-significant whereas the associations of indoor exposure remained consistent. Stronger and significant associations between roadside exposure and cardiovascular mor-tality were observed in areas with more intensive UHI, more severe light pollution, and lower average greenness. Robust impacts regarding indoor exposure to traffic noise were found after roadside exposure was accounted for. In conclusion, urban environment stressors could act synergistically on their adverse effects on cardiovascular outcomes. Built environment interventions should be applied to minimize indoor exposure to traffic noise to reduce cardiovascular risks, despite complex urban morphology.