Researchers say there is "no safe level" of PM2.5 air pollution after a large-scale study found a significant association between cardiac arrest and exposure to fine particles – levels of which have frequently soared beyond "hazardous" in south-eastern Australia as a result of bushfire smoke in recent months.

The Sydney University-led study published in The Lancet Planetary Health on Tuesday analysed Japanese air quality data against 249,372 cases of out-of-hospital cardiac arrests, and found even short-term exposure to low levels of PM2.5 comes with elevated risk for people over the age of 65.

Hazardous bushfire smoke blanketing Sydney CBD in December. Credit:Bloomberg

The study found the risk of cardiac arrest increased by 4 per cent for every 10-unit increase in PM2.5 levels – but more than 90 per cent of out-of-hospital cardiac arrests occurred at levels below the Australian standard of 25 micrograms per cubic metre (µg/m3). In other words, at air quality levels deemed "fair", "good" or "very good" under Australian standards.

With the survival rate for cardiac arrests outside hospital at about 12 per cent in NSW and 10 per cent globally, the study's authors said "a global approach to tackle this crucial health issue is necessary for our planet".