Is bushfire smoke more harmful to health than other fine particulate matter sources?

By Nicole MacKee
Bushfire smoke may be more harmful to human respiratory health than pollution from other sources, US researchers have reported in Nature Communications.

In the California-based study, researchers found that a 10 microgram per cubic metre increase in fine particulate matter with an aerodynamic diameter 2.5 micrometre or less (PM2.5) from non-bushfire sources increased hospital admissions for respiratory complaints by 1%. The same increase in PM2.5 from bushfire pollution, however, increased respiratory hospital admissions by 1.3 to 10%.

The researchers isolated bushfire-specific PM2.5 using a series of statistical approaches and exposure definitions.

Professor Sotiris Vardoulakis, Professor of Global Environmental Health at the ANU National Centre for Epidemiology and Population Health, Canberra, said the study was significant, but the evidence was still not conclusive.

‘This is an important piece of evidence, but a single epidemiological study,’ he said. ‘There is still no conclusive evidence about the differential toxicity of particles from different sources.’

Professor Vardoulakis pointed to research, in which he was involved, that suggested there was no increased health risk from biomass burning in relation to other pollution sources (Curr Pollut Reports 2019; 5: 353-377). He said further epidemiological and toxicological studies were needed, but the current evidence suggested that particle size might be important in determining toxicity.

‘We think that the smaller the size of the particle, the more harmful they are because they can penetrate deeper into the respiratory system and translocate into the blood stream,’ he said.

The chemical composition of the particles was also important, he said, noting that bushfire smoke may contain chemicals from pesticides and historical soil contamination.

Professor Vardoulakis said it was important to debunk the myth that bushfire smoke was ‘natural’ and therefore not harmful to human health.

‘There is the perception that wood smoke, from our fireplace or in bushfires, is a natural kind of smoke and it’s not harmful to health and this is not the case,’ he said. ‘Based on the current evidence, we shouldn’t treat it as less harmful than industrial pollution or traffic pollution.’ 

And, he said, the frequency and intensity of bushfires was likely to increase as the impacts of climate change – including longer drought periods and more extreme temperatures – were felt.

In a related study, published in Circulation, US researchers reported that long-term exposure to air pollutants – PM2.5, nitrogen dioxide and ozone – was associated with an increased risk of cardiovascular and respiratory hospitalisation among elderly people. The risk was observed even at concentrations below current US and international guidelines.

Professor Vardoulakis said the findings highlighted that there was no safe level for some air pollutants, particularly for elderly populations. ‘We need to minimise exposure to these pollutants as much as possible at all stages of life.’
Nat Commun 2021; 12: 1493; https://doi.org/10.1038/s41467-021-21708-0.
Circulation 2021; 143: 00-00; doi: 10.1161/CIRCULATIONAHA.120.050252.