When it comes to air pollution, the most commonly followed guidelines are those set by the WHO. These guidelines set short-term (24-hour) and long-term (annual) exposure limits for a range of common airborne pollutants. These guidelines are backed by a range of research and are considered quite conservative compared to other scales, such as that set by the EPA.

However, in a study released just a few days ago, it’s been found that PM2.5 may be harmful in concentrations even lower than the guidelines suggest. Previously, the WHO estimated that 4.2 million people died prematurely each year due to outdoor air pollution. This estimate was made in 2016 and hasn’t been updated since.

The study suggests this number might be wrong, and the deaths caused by PM2.5 might be significantly higher. This is because PM2.5 may be significantly more harmful than previously thought - even at concentrations considered ‘very low’. 

This news is worrying because, if true, it adds another 1.5 million annual deaths to the estimated annual mortality due to PM2.5. These extra deaths previously went unnoticed because the impacts of PM2.5 at very low concentrations were not previously appreciated.

The researchers behind the study analysed health and mortality data from seven million Canadians over 25 years to arrive at this conclusion. Since Canada has relatively low levels of PM2.5, it was the perfect country for the study to be carried out.

Findings from the impacts of low concentrations of PM2.5 in Canada were then applied to the scale used worldwide to determine deaths due to PM2.5. This lead to a large increase in the estimated global mortality from PM2.5 - taking the estimated deaths from 4.2 million to around 5.7 million.

Please refer to the study here if you’re interested in reading more findings.