London School of Hygiene & Tropical Medicine

Daily exposure to ground level ozone in cities worldwide is associated with an increased risk of death, finds the largest study of its kind published in The BMJ.

The findings - based on almost 50 million deaths 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.

Antonio Gasparrini, Professor of Biostatistics and Epidemiology at the London School of Hygiene & Tropical Medicine and study lead author, said:

“We are currently living in dangerous levels of air toxicity, especially in cities.  The effects of air pollution on human health are tangible, but avoidable.

“Our study highlights the urgent need for stricter regulations on emissions, and a global commitment to reduce ozone pollution. Even in countries that adhere to guideline levels, ground level ozone is a potential direct cause for increased deaths. Air quality guidelines need updating to limit the impact of climate change on global health.”

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).

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 and address this, an international research team analyse 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 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% increased risk of death, reinforcing the 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 would have been avoided if countries had implemented stricter air quality standards in line with the WHO guideline.

What’s more, 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 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.

Publication

Ana M Vicedo-Cabrera et al. 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