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Led by the London School of Hygiene & Tropical Medicine in the UK and the Fudan University in China, the showed exposure to even a tiny amount of urban air pollution can immediately increase the risk of mortality.

In what is the largest epidemiological assessment to date on the short-term effects of air pollution, the researchers gathered time series data from 652 cities from 24 countries within the period 1986-2015, and applied sophisticated statistical methods to compare daily mortality with levels of particular matter. They found that, on average, a 10 microgr/m³ increase in inhalable (PM₁₀) and fine (PM_2.5) particles is associated with an increase in mortality of 0.44% and 0.68%.

The database has been collected within the Multi-Country Multi-City (MCC) Collaborative Research Network, an international collaboration studying the association between environmental stressors, climate change and health.

Dr Antonio Gasparrini from the London School of Hygiene & Tropical Medicine, coordinator of the MCC Network and one of the senior authors of the article, said: “While the percentage increase in mortality seems small, this risk can lead to a significant excess in number of deaths, given the widespread exposure and the large populations living in urban areas.”

The applications of consistent analytical methods to this international dataset allows a critical comparison of the risk across populations living in different regions. While the researchers identified differences, they were able to measure a positive association in all the 24 countries, independently from the average pollution level and socio-economic setting.

Prof Haidong Kan from Fudan University, the other senior author of the study, said: “The consistency of the estimated risk across multiple countries and population adds evidence on the potential causal link between exposure to air pollution and short-term increase in mortality, and on the associated health and socio-economic burden.”

Interestingly, when the researchers examined the shape of the exposure-response relationship, they were not able to identify a threshold, and found significant increases in mortality even at exposures below the current air-quality guidelines.

Dr Gasparrini added: “The absence of a discernible threshold means that a substantial increase in mortality is likely to happen even at low exposure to particulate matter. These results must be considered when assessing the potential benefits of interventions to reduce urban air pollution, and when revising existing regulatory limits.”

Publication

C. Liu et al. . New England Journal of Medicine. DOI: 10.1056/NEJMoa1817364