Ozone pollution may increase risk of death

A short-term association exists between ozone pollution and mortality, a multi-country study has found, noting that ozone-related deaths can be prevented under stricter air quality standards in line with the World Health Organization (WHO) guidelines.

“[I]nterventions to further reduce ozone pollution would provide additional health benefits, even in regions that meet current regulatory standards and guidelines,” the researchers said.

This two-stage, time-series analysis was conducted in 406 cities in 20 countries, with overlapping periods between 1985 and 2015. Data were obtained from the database of Multi-City Multi-Country Collaborative Research Network. Deaths for external or all causes only registered in each city within the study period were included in the analysis.

In total, 45,165,171 deaths were analysed across 406 cities. On average, an increase of 10 µg/m³ in ozone during the current and previous day correlated with mortality (overall relative risk, 1.0018, 95 percent confidence interval [CI], 1.0012–1.0024). Differences were found across countries, with estimates ranging from >1.0020 in the UK, South Africa, Estonia and Canada to <1.0008 in Mexico and Spain. [BMJ 2020;368:m108]

Exposure to ozone greater than maximum background levels of 70 µg/m³ resulted in short-term excess mortality of 0.26 percent (95 percent CI, 0.24–0.28 percent). This corresponded to annual excess deaths of 8,203 (95 percent CI, 3,525–12,840) across 406 cities. The excess persisted at 0.20 percent (95 percent CI, 0.18–0.22 percent) when restricting to days above the WHO guideline of 100 µg/m³, which corresponded to annual excess deaths of 6,262 (95 percent CI, 1,413–11,065).

Above more lenient thresholds for air quality standards, excess mortality stood at 0.14 percent in Europe, 0.09 percent in America and 0.05 percent in China.

“These findings have important implications for the design of future public health actions, particularly … in relation to the implementation of mitigation strategies to reduce the impacts of climate change,” the researchers said.

The short-term association between ozone and mortality was supported by previous epidemiological and experimental studies, which suggested several pathophysiological mechanisms such as systemic inflammation and haemostatic alterations, among others. [Epidemiology 2016;27:211-220; JAMA Intern Med 2017;177:1344-1353]

Larger associations were found in previous single-country or multi-country studies, including a subset of countries analysed in the present study. [J Epidemiol Community Health 2009;63:960-966; Air Qual Atmos Health 2013;6:445-453; Environ Res 2017;155:235-241; JAMA 2004;292:2372-2378; Am J Respir Crit Care Med 2010;182:376-384]

“Differences in the definition of the exposure variable (eg, moving average, single lag) and modelling approach could explain these discrepancies in the magnitude of the association,” the researchers said.

In addition, the present study showed evidence on the potential benefits of stricter clean air policies. However, earlier studies reported that most air quality standards in place are not compliant to the WHO air quality guideline and that 80 percent of the world’s population in urban areas are exposed to air pollution levels above the threshold. [Int J Public Health 2017;62:453-462; https://www.who.int/phe/health_topics/outdoorair/databases/cities/en/]