High levels of pollution-generated ozone compromise the immune system, killing key bacteria-fighting cells, new research shows.

Scientists at Duke University Medical Center published their findings in the Oct. 1 issue of the Journal of Immunology.

An illustration shows the human lung. When mice were exposed to high ozone levels, critical immune cells that fight foreign invaders in their lungs also died, allowing bacteria to spread.
(CBC)

Exposure to ozone has been linked in previous studies to inflammatory lung disease and death, particularly among people with chronic lung disease.

The researchers found that when they exposed mice to levels of ozone unhealthy to humans and then exposed the same mice to active E. coli bacteria in aerosol form, the animals' lungs became inflamed and damaged, allowing the bacteria to invade.

Critical immune cells that fight foreign invaders in their lungs also died, allowing the bacteria to spread.

The scientists also found that the effect of the inhaled ozone was not limited to the lungs. Mice exposed to ozone were also found to have lower levels of immune system cells circulating in the blood.

"In the mice exposed to ozone, the airways of the lungs were hyperactive and we found higher concentrations of inflammatory cells," said John Hollingsworth, a pulmonologist and lead author of study.

The researchers believe that low ozone levels can actually offer some protection to the lungs.

"Small amounts of inhaled foreign material can be relatively harmless, since they stimulate an appropriate innate immune response that protects the lungs," said Hollingsworth.

But with higher levels of ozone, such as those used in the study, the opposite occurs.

"It appears that ozone causes the innate immune system to overreact, killing key immune system cells, and possibly making the lung more susceptible to subsequent invaders, such as bacteria," Hollingsworth said.

Canada is seeking to lower its ground-level ozone.

In June 2000, the federal, provincial and territorial governments except Quebec signed an agreement known as the Canada-wide Standards for Particulate Matter and Ozone. These standards commit government to significantly reduce particulate matter and ground-level ozone by 2010.