New York, Mar 6 (IANS): Fine particles in wildfire smoke can be several times more harmful to human respiratory health than particulate matter from other sources such as car exhaust, say researchers.

While this distinction has been previously identified in laboratory experiments, the new study, published in the journal Nature Communications, confirms it at the population level.

This study reveals the risks of tiny airborne particles with diameters of up to 2.5 microns, about one-twentieth that of a human hair.

These particles -- termed PM2.5 -- are the main component of wildfire smoke and can penetrate the human respiratory tract, enter the bloodstream and impair vital organs.

"There is a daily threshold for the amount of PM2.5 in the air that is considered acceptable by the county and the Environmental Protection Agency (EPA)," said researcher Rosana Aguilera from University of California - San Diego.

"The problem with this standard is that it doesn't account for different sources of emission of PM2.5," Aguilera added.

To isolate wildfire-produced PM2.5 from other sources of particulate pollution, the researchers defined exposure to wildfire PM2.5 as exposure to strong Santa Ana winds with fire upwind. A second measure of exposure involved smoke plume data from NOAA's Hazard Mapping System.

A 10 microgram-per-cubic meter increase in PM2.5 attributed to sources other than wildfire smoke was estimated to increase respiratory hospital admissions by 1 per cent. The same increase, when attributed to wildfire smoke, caused between a 1.3 to 10 per cent increase in respiratory admissions.

The research suggests that assuming all particles of a certain size are equally toxic may be inaccurate and that the effects of wildfires -- even at a distance -- represent a pressing human health concern.