The effects of Sulphur dioxide on acute mortality and years of life lost are modified by temperature in Chengdu, China

The effect modification of meteorological factors on the association between ambient Sulphur dioxide (SO₂) and mortality is critical for designing intervention policy. Existing studies did not result in consistent conclusions on the effect modification, and Years of life lost (YLLs) was rarely used as a health impact indicator to examine the modifying effect. This study aims to estimate the mean air temperature and relative humidity modification effects on the impact of SO₂ on daily mortality and YLLs in Chengdu, China. Mortality, YLLs, air pollution and meteorological data were collected for 2011-2014. Three analytical approaches based on generalized additive models (GAMs) were used, including bivariate response surface model, product term model, and stratification model. We found that the effects of SO₂ on mortality and YLLs depended on temperature at various lags, but did not depend on relative humidity. SO₂ exhibited larger adverse effects on mortality in high temperature level (22.8-29.4°C) days than in low temperature level (-0.3-9.3°C) days, with a 10μg/m³ increment in SO₂, non-accidental death increased by 0.8% (0.001, 0.015)at low temperature level, but increased by 1.4% (0.005, 0.024) at high temperature level. On the contrary, SO₂ showed greater adverse effects on YLLs in low temperature days than in high temperature days, with a 10μg/m³ increment in SO₂, non-accidental YLL increased by 40.580 (31.478, 49.682) at high temperature level, but increased by -2.703 (-14.668, 9.261) at low temperature level. We concluded that the effect of SO₂ on mortality and YLLs may depend on temperature in Chengdu, China. Our results highlight the importance of considering the interaction between SO₂ and temperature on health outcomes in future research. Also, policy makers should enhance the emission control of SO₂ in extreme temperature days in Chengdu.