Modification Effect of Temperature on the Association between Ultrafine Particles and Mortality in Eight European Urban Areas

Abstract

Background/Aim: Few studies have explored potential interactions between air temperature and air pollution on mortality and the results are inconsistent. Moreover, whether temperature modifies the effect of ultrafine particles remains unclear. In this study, we investigated the modification of the effects of ultrafine particles on mortality by temperature in eight European urban areas. Methods: We collected daily data on natural, cardiovascular and respiratory mortality, particle number concentrations (PNC, as a proxy for ultrafine particles), fine particles (PM2.5), ozone (O3), and mean air temperature in eight urban areas of Finland, Sweden, Denmark, Germany, Italy, Spain, and Greece during 1999-2013. We first applied Poisson generalized additive models with over-dispersion adjusting for temperature, long-term and seasonal trend, day of the week, influenza epidemics, and population dynamics due to summer vacation and holiday to estimate the interactive effects of temperature categorized into high (>75th percentile), medium, and low (<25th percentile) and PNC on mortality in each city, then pooled the city-level estimates using random-effects meta-analyses. Results: The mortality effects of PNC at lag 6 were generally stronger during periods with high compared to low temperature, though the interactions were not statistically significant. A 10,000 particles/cm3 increase in PNC during periods with high temperature was associated with 1.16% [95% confidence interval (CI): -1.06%; 3.44%], 2.52% [-0.16%; 5.27%] and 0.91% [-3.78%; 5.83%] increases in natural, cardiovascular, and respiratory mortality, respectively. Corresponding estimates for PNC during periods with low temperature were 0.30% [-0.22%; 0.83%], 0.04% [-0.75%; 0.84%], and -0.49% [-2.37%, 1.42%]. This interaction pattern remained when adjusting for other pollutants (PM2.5 and O3). Conclusions: We found that high temperature may modify the acute effects of ultrafine particles on cause-specific mortality.