Short-Term Association between Sulfur Dioxide and Mortality: A Multicountry Analysis in 399 Cities

Abstract

Background: Epidemiological evidence on the health risks of sulfur dioxide (${\mathrm{SO}}_2$) is more limited compared with other pollutants, and doubts remain on several aspects, such as the form of the exposure-response relationship, the potential role of copollutants, as well as the actual risk at low concentrations and possible temporal variation in risks.

Objectives: Our aim was to assess the short-term association between exposure to ${\mathrm{SO}}_2$ and daily mortality in a large multilocation data set, using advanced study designs and statistical techniques.

Methods: The analysis included 43,729,018 deaths that occurred in 399 cities within 23 countries between 1980 and 2018. A two-stage design was applied to assess the association between the daily concentration of ${\mathrm{SO}}_2$ and mortality counts, including first-stage time-series regressions and second-stage multilevel random-effect meta-analyses. Secondary analyses assessed the exposure-response shape and the lag structure using spline terms and distributed lag models, respectively, and temporal variations in risk using a longitudinal meta-regression. Bi-pollutant models were applied to examine confounding effects of particulate matter with an aerodynamic diameter of $\le 10\mu m$ (${\mathrm{PM}}_{10}$) and $2.5\mu m$ (${\mathrm{PM}}_{2.5}$), ozone, nitrogen dioxide, and carbon monoxide. Associations were reported as relative risks (RRs) and fractions of excess deaths.

Results: The average daily concentration of ${\mathrm{SO}}_2$ across the 399 cities was $11{.7\mu g/m}^3$, with 4.7% of days above the World Health Organization (WHO) guideline limit ($40{\mu g/m}^3$, 24-h average), although the exceedances occurred predominantly in specific locations. Exposure levels decreased considerably during the study period, from an average concentration of $19.0{\mu g/m}^3$ in 1980-1989 to $6.3{\mu g/m}^3$ in 2010-2018. For all locations combined, a $10\text{-}{\mu g/m}^3$ increase in daily ${\mathrm{SO}}_2$ was associated with an RR of mortality of 1.0045 [95% confidence interval (CI): 1.0019, 1.0070], with the risk being stable over time but with substantial between-country heterogeneity. Short-term exposure to ${\mathrm{SO}}_2$ was associated with an excess mortality fraction of 0.50% [95% empirical CI (eCI): 0.42%, 0.57%] in the 399 cities, although decreasing from 0.74% (0.61%, 0.85%) in 1980-1989 to 0.37% (0.27%, 0.47%) in 2010-2018. There was some evidence of nonlinearity, with a steep exposure-response relationship at low concentrations and the risk attenuating at higher levels. The relevant lag window was 0-3 d. Significant positive associations remained after controlling for other pollutants.

Discussion: The analysis revealed independent mortality risks associated with short-term exposure to ${\mathrm{SO}}_2$, with no evidence of a threshold. Levels below the current WHO guidelines for 24-h averages were still associated with substantial excess mortality, indicating the potential benefits of stricter air quality standards. https://doi.org/10.1289/EHP11112.

Figure 1.

Geographical location of the 399 urban areas and related average annual concentrations of ${\mathrm{SO}}_2$ (in ${\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$) within the study period 1980–2018. Data can be found in Table S1. Note: ${\mathrm{SO}}_2$, sulfur dioxide.

Figure 2.

Box plot of the distribution of the average concentration of ${\mathrm{SO}}_2$ (in ${\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$) across cities for each year. The horizontal line identifies the current limit of daily concentration of ${\mathrm{SO}}_2$ in the WHO guidelines ($40{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$). The y-axis is represented in a logarithmic scale. The analysis includes data from 399 cities within the study period 1980–2018. Note that a different set of countries contributes to each study period (see Table 1 for details). Note: ${\mathrm{SO}}_2$, sulfur dioxide; WHO, World Health Organization.

Figure 3.

Country-specific and pooled relative risks (RRs, with 95% CIs) for mortality corresponding to a $10\text{-}{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$ increase in ${\mathrm{SO}}_2$ over lag 0–3 d. The analysis includes data from 399 cities within the study period 1980–2018. Data can be found in Table S3. Note: CI, confidence interval; ${\mathrm{SO}}_2$, sulfur dioxide.

Figure 4.

Secondary analysis on the short-term association between ${\mathrm{SO}}_2$ (per $10\text{-}{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$ increase) and mortality. (A) Pooled exposure–response curve obtained using a linear term (dashed line) and a quintic polynomial (continuous line, with 95% confidence intervals), with a bar representing the percentage of studies contributing to the specific exposure range. (B) Pooled lag–response curve obtained using a natural spline with knots at lags 1 and 3, plus intercept. (C) Analysis of temporal variation of the pooled relative risk (RR) associated with a $10\text{-}{\mathrm{\mu }\mathrm{g}/\mathrm{m}}^3$ increase in ${\mathrm{SO}}_2$ over lag 0–3 d. Shaded areas represent the 95% confidence intervals. The analysis includes data from 399 cities within the study period 1980–2018. Summary data on city-specific exposure distributions can be found in Table S1. Note: ${\mathrm{SO}}_2$, sulfur dioxide.