The Minnesota Pollution Control Agency issued an air quality health advisory for the Twin Cities Monday.

People with heart and lung diseases were urged to avoid strenuous activity because of an above-average level of fine particle pollution.

This is the fifth time this year air quality in the Twin Cities has reached an unsafe level, according to the Minnesota Pollution Control Agency.

Peter Raynor, a professor of public health, said weather conditions often play a role in air-quality conditions. Monday’s fog could have meant there wasn’t much wind and air movement in the area.

The fine particles, which have a diameter of 2.5 micrometers or less, can take weeks to disperse because of their small size, according to the Minnesota Department of Health’s Web site.

“That’s really small particles,” Raynor said.

Robert Moffitt, spokesman for the American Lung Association of Minnesota, said the particles can become embedded in a person’s lungs and cause irritation.

The elderly, children and people with lung and heart diseases are more likely to experience adverse reactions to the pollution.

“Warnings are not sent out lightly,” he said.

The particles are mostly made by humans. Moffitt said the greatest contributor in Minnesota is automobile emissions.

Mark Sulzbach, spokesman for the Minnesota Pollution Control Agency, said people can do a lot to limit the amount of fine particles entering the air. He said he encourages everyone to decrease their driving, limit recreational fires and reduce electrical usage.

He said heating also adds a significant number of fine particles.

Andrew Phelan, assistant director of the University’s Department of Environmental Health and Safety, said the department is working to limit emissions on campus.

The University has entered voluntary partnerships, such as Clean Air Minnesota, which matches business and environmental concerns in an effort to improve air quality.

A new heating facility built in 1998 has helped the University lower its heating emissions by 29 percent over six years, Phelan said.

Moffitt said Minnesota has relatively clean air when compared to other states.

However, on Monday the Twin Cities had the worst air-quality rating in the nation, according to airnow.gov. Air quality was expected be slightly better today.