Daily exposure to ground level ozone in cities worldwide is associated with an increased risk of death, according to the largest study of its kind, published in an open-access paper in The BMJ. The findings—based on data from more than 400 cities in 20 countries across the world—show that more than 6,000 deaths each year would have been avoided in the selected cities if countries had implemented stricter air quality standards.

Ground level ozone is a highly reactive gas commonly found in urban and suburban environments, formed when pollutants react in sunlight.

Current air quality thresholds (in micrograms per cubic meter of ambient air) range from 100 μg/m³ (WHO), 120 μg/m³ (European Union directive), 140 μg/m³ (US National Ambient Air Quality Standard), and 160 μg/m³ (Chinese Ambient Air Quality Standard).

Overall and country specific excess mortality (%) associated with ozone by specific ranges defined between thresholds consistent with current air quality standards. (No excess mortality associated with ozone was found in Australia, as daily ozone levels were below the maximum background level of 70 µg/m³). 100 µg/m³, World Health Organization guideline; 120 µg/m³, European Union directive; 140 µg/m³ (about 0.070 parts per million); National Ambient Air Quality Standard (NAAQS) in the US; 160 µg/m³ Chinese Ambient Air Quality Standard (CAAQS). Vicedo-Cabrera et al.

Recent reviews suggest that 80% of the world’s population in urban areas are exposed to air pollution levels above the WHO threshold.

Most previous studies have found positive associations between ground level ozone and mortality, but differences in study design and quality make it difficult to draw consistent conclusions across different regions.

To try to address this, an international research team analyzed deaths and environmental measures (weather and air pollutants) in 406 cities in 20 countries, with overlapping periods between 1985 and 2015.

Using data from the Multi-City Multi-Country Collaborative Research Network, they derived daily average ozone levels (above a maximum background level of 70 μg/m³), particulate matter, temperature, and relative humidity at each location to estimate the daily number of extra deaths attributable to ozone.

A total of 45,165,171 deaths were analyzed in the 406 cities. On average, a 10 μg/m³ increase in ozone during the current and previous day was associated with a 0.18% increased risk of death, suggesting evidence of a potential direct (causal) association.

This equates to 6,262 extra deaths each year (or 0.2% of total mortality) in the 406 cities that could potentially have been avoided if countries had implemented stricter air quality standards in line with the WHO guideline.

Smaller but still substantial mortality impacts were found even for ozone concentrations below WHO guideline levels, supporting the WHO initiative of encouraging countries to revisit current air quality guidelines and enforcing stronger emission restrictions to meet these recommendations, say the researchers.

This is an observational study, so it can’t establish cause, and the researchers point to some limitations. For example, areas such as South America, Africa, and the Middle East were unrepresented or not assessed, and differences in monitoring and data collection between countries may have affected the accuracy of their estimates.

Nevertheless, they say their results suggest that ozone related mortality “could be potentially reduced under stricter air quality standards.”

Moreover, interventions to further reduce ozone pollution “would provide additional health benefits, even in regions that meet current regulatory standards and guidelines,” they add.

Resources

• Vicedo-Cabrera Ana M, Sera Francesco, Liu Cong, Armstrong Ben, Milojevic Ai, Guo Yuming et al. (2020) “Short term association between ozone and mortality: global two stage time series study in 406 locations in 20 countries” BMJ doi: 10.1136/bmj.m108