Modification of heat-related effects on mortality by air pollution concentration, at small-area level, in the Attica prefecture, Greece

Abstract

Background: The independent effects of short-term exposure to increased air temperature and air pollution on mortality are well-documented. There is some evidence indicating that elevated concentrations of air pollutants may lead to increased heat-related mortality, but this evidence is not consistent. Most of these effects have been documented through time-series studies using city-wide data, rather than at a finer spatial level. In our study, we examined the possible modification of the heat effects on total and cause-specific mortality by air pollution at municipality level in the Attica region, Greece, during the warm period of the years 2000 to 2016.

Methods: A municipality-specific over-dispersed Poisson regression model during the warm season (May-September) was used to investigate the heat effects on mortality and their modification by air pollution. We used the two-day average of the daily mean temperature and daily mean PM₁₀, NO₂ and 8 hour-max ozone (O₃), derived from models, in each municipality as exposures. A bivariate tensor smoother was applied for temperature and each pollutant alternatively, by municipality. Α random-effects meta-analysis was used to obtain pooled estimates of the heat effects at different pollution levels. Heterogeneity of the between-levels differences of the heat effects was evaluated with a Q-test.

Results: A rise in mean temperature from the 75th to the 99th percentile of the municipality-specific temperature distribution resulted in an increase in total mortality of 12.4% (95% Confidence Interval (CI):7.76-17.24) on low PM₁₀ days, and 21.25% (95% CI: 17.83-24.76) on high PM₁₀ days. The increase on mortality was 10.09% (95% CI: - 5.62- 28.41) on low ozone days, and 14.95% (95% CI: 10.79-19.27) on high ozone days. For cause-specific mortality an increasing trend of the heat effects with increasing PM₁₀ and ozone levels was also observed. An inconsistent pattern was observed for the modification of the heat effects by NO₂, with higher heat effects estimated in the lower level of the pollutant.

Conclusions: Our results support the evidence of elevated heat effects on mortality at higher levels of PM₁₀ and 8 h max O_3. Under climate change, any policy targeted at lowering air pollution levels will yield significant public health benefits.

Keywords: Air pollution; Air temperature; Effect modification; Mortality.