Low level air pollution is deadlier than current models suggest according to new Canadian research that is improving our understanding of how fine particulate matter affects human health.

The World Health Organization (WHO) estimates 4.2 million people die every year from long-term exposure to fine particulate matter of 2.5 microns or less in diameter (PM2.5). But McGill University epidemiologist Scott Weichenthal and his team have shown that the WHO’s estimate is likely much too low.

That’s because most models assume no risk of death at concentrations below 5 micrograms (ug) per cubic metre. But when the researchers took a second look at levels below that concentration, they found the assumption was wrong.

“The global burden of PM2.5 may be as many as 1.5 million additional deaths around the globe each year because of effects at very-low concentrations that were not previously appreciated,” says Weichenthal, the lead author on the September paper in Science Advances.

Weichenthal and his team looked at Statistics Canada census data on the health and mortality of seven million Canadians gathered over a twenty-five-year period. Then they combined it with levels of outdoor PM2.5 concentrations across the country gathered using satellite imagery, chemical transport models and ground-based air pollution testing stations. They were able to fine-tune the geographic resolution down to populations living within one square kilometre.

Compared to the U.S and Europe, with their greater population density, Canada has low levels of outdoor PM2.5, making it the perfect place to study health impacts at low concentrations. Data collected in this country can now be used to create more accurate models of air pollution and health.

Scientists began to understand the dangers of low levels of air pollution just over a decade ago, says Weichenthal. While air pollution has declined over the last few decades in North America, Europe, and other developed regions, starting in 2012 several large epidemiological studies showed links between poor health and exposure to low concentrations of particulate matter, also known as PM2.5.

PM2.5 particles are microscopic and airborne. Long-term exposure can cause cardiovascular and respiratory disease and cancer. Sources can be natural – dust, ash and sea-spray for example. But the more harmful sources tend to be from human activity such as the combustion of solid and liquid fuels for power generation, domestic heating and in vehicle engines. In Canada, the top five human-related sources are agriculture, industry, power generation, residential combustion, and transportation.

The WHO recently set out new guidelines for annual average outdoor fine particulate air pollution, cutting its earlier recommendations in half, from concentrations of 10 to concentrations of 5 ug per cubic metre.

“The next steps are to stop focussing only on particle mass and start looking more closely at particle composition because some particles are likely more harmful than others,” says Weichenthal. “You could have larger health benefits if you targeted areas exposed to a particularly bad kind of PM2.5.”