Toronto, A new study now claims that mortality risk from air pollution is very high even at very low levels of outdoor particulate matter (PM) 2.5 -- levels which had not previously been recognised as being potentially deadly.

These microscopic toxins cause a range of cardiovascular and respiratory diseases and even cancers, said the study from McGill University in Canada.

The World Health Organisation estimates that over 4.2 million people die prematurely each year due to long-term exposure to fine particulate outdoor air pollution.

"We found that outdoor PM2.5 may be responsible for as many as 1.5 million additional deaths around the globe each year because of the effects at very-low concentrations that were not previously appreciated," said Scott Weichenthal, Associate Professor at McGill University and the lead author on the recent paper in Science Advances.

The researchers arrived at this conclusion by combining health and mortality data for seven million Canadians gathered over a 25-year period with information about the levels of outdoor PM2.5 concentrations across the country.

Canada is a country with low levels of outdoor PM2.5, making it the perfect place to study health impacts at low concentrations.

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

The current US Environmental Protection Agency standard of 12 micrograms per cubic metre is now more than double the value recommended by the WHO.

"One takeaway is that the global health benefits of meeting the new WHO guideline are likely much larger than previously assumed," adds Weichenthal.

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.

"If we can gain a better understanding of this, it may allow us to be much more efficient in designing regulatory interventions to improve population health," said the researchers.