A new report finds daily exposure to ground-level ozone in cities is associated with an increased risk of death.

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

The findings - based on data from over 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.

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). Australia’s standard is 0.10 ppm of ozone measured over a one hour period.

Recent reviews suggest that 80 per cent 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.

A total of 45,165,171 deaths were analysed 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 per cent increased risk of death, suggesting evidence of a potential direct (causal) association.

This equates to 6,262 extra deaths each year (or 0.2 per cent 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.

Additionally,, 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.

Interventions to further reduce ozone pollution “would provide additional health benefits, even in regions that meet current regulatory standards and guidelines,” the researchers added.

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

The full study is accessible here.