A new study shows that when isoprene, a substance given off by trees that protects their leaves, combines with man-made nitrogen oxides present in air pollution, it produces more of the very small air-borne particles that can penetrate our lungs and damage health.

Senior author Jason Surratt, assistant professor of environmental sciences and engineering at the Gillings School of Global Public Health, University of North Carolina at Chapel Hill, Chapel Hill, and colleagues, write about their findings in a paper published online this month in the Proceedings of the National Academy of Sciences.

In a statement released this week, Surratt says:

"The work presents a dramatic new wrinkle in the arguments for reducing man-made pollutants worldwide."

There has been much speculation about how trees might contribute to air pollution. We have known for some time that they produce and give out out isoprene, a molecule that is plentiful in the air and protects leaves from being damaged by oxygen and temperature changes.

It was in 2004 that researchers first suggested, contrary to popular opinion, that this substance also contributes to particulate matter: particles of air pollution that are small enough to get lodged in lungs, and can result in asthma and lung cancer, and also damage to other tissues. And of course there is also damage to the environment.

But exactly how isoprene helps produce particulate matter has been somewhat of a mystery: until now.

In their study, Surratt and colleagues discovered one particular mechanism by which isoprene contributes to the production of air-borne, health-damaging small particles.

They found that when isoprene is exposed to the sun it alters its chemical structure and reacts with man-made nitrogen oxides to produce particulate matter.

Nitrogen oxides, for instance nitrogen dioxide (NO2) and nitric oxide (NO), are formed when fuels like oil, gas and coal are burned at a high temperature. They are emitted by road vehicles, planes, coal and other fuel-based power plants. Nitrogen oxides are also emitted by factories that manufacture explosives.

Surratt says:

"Isoprene evolved to protect trees and plants, but because of the presence of nitrogen oxides, it is involved in producing this negative effect on health and the environment."

What does he propose?

"We certainly can't cut down all the trees," he adds, "but we can work on reducing these man-made emissions to cut down the production of fine particulate matter."

Now that Surratt and colleagues have discovered the chemistry behind isoprene's contribution to air pollution, researchers can put this into their air quality models to make better predictions about air pollution and its effects on climate. This should then help environment agencies formulate regulations about climate change and protect public health.

In another study published this month, researchers show how over time, increased exposure to air pollution is linked to faster "hardening" of the arteries.

Written by Catharine Paddock PhD