Daily exposure to ground level ozone is associated with an increased risk of death, according to analysis of data from 406 cities in 20 countries.
The findings – published in the journal BMJ – suggest that 6000+ deaths each year would have been avoided in these cities if stricter air quality standards had been implemented.
It was 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 the 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”.
Ground level ozone is a highly reactive gas commonly found in urban and suburban environments, formed when pollutants react in sunlight.
Previous studies have found positive associations between ground level ozone and mortality, the researchers say, but differences in study design and quality make it difficult to draw consistent conclusions across different regions.
To try and address this, the international team analysed deaths and environmental measures (weather and air pollutants) in the selected cities, 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 micrograms (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 was analysed. 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 6262 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 World Health Organisation (WHO) guideline.
Smaller but still substantial mortality impacts were found even for ozone concentrations below WHO guideline levels.