In people with asthma, a small increase in coarse particulate matter in outdoor air raised bad cholesterol and increased the count of inflammation-linked white blood cells, among other changes, according to researchers at the University of North Carolina at Chapel Hill School of Public Health.

The study, published in Environmental Health Perspectives, found that when adult asthma sufferers were exposed to a 1 microgram per cubic meter increase in coarse particulate matter in ambient air their triglyceride levels increased by nearly 5 percent. Elevated levels of triglycerides have been shown to increase the risk of coronary heart disease.

"Our results indicate that susceptible people really need to pay attention to air pollution warnings and stay inside when the air pollution is bad," study leader Dr. Karin Yeatts said in a statement. "This is particularly the case for people with asthma."