LEWISTON - Just as it feels warm enough to go outside, health and weather experts advise it may not be safe for some to do so.

Environmental experts on Monday were cautioning that air pollution could reach unhealthy levels over the next few days.

The reasons are mostly meteorological. Weather experts say a lingering air mass is keeping warm air aloft while cool air lingers near the surface where it traps particles.

The unusual weather event also comes with robust southwest winds that move pollution from emissions as well as smoke from wood stoves and fireplaces over populated areas.

"It's bad this year because of the air mass we're in," said meteorologist Tom Downs. "We usually have more winds from the north and the west. If we had a real good January thaw (with westerly winds), we wouldn't have all this pollution."

Technically, it's called an inversion. Though it felt warmer in the past few days, the really warm air - 50 degrees or greater - has been higher in the atmosphere. Cool air and pollution with it is trapped closer to the ground where it can be breathed in, possibly causing problems for the young, the old and those with respiratory or heart problems.

The Maine Department of Environmental Protection on Monday released a statement warning of the pollution problem. It advised that people susceptible to breathing problems limit outdoor activity over the next few days.

The MDEP also advised people against strenuous outdoor activity such as jogging, and suggested that use of aerosol products be delayed.

Ground-level particle pollution concentrations are expected to reach unhealthy levels in the western and eastern interior section of the state as well as southwest coastal areas today. Dirty air reached the southwestern corner of Maine on Monday.

According to Downs, the problem will likely begin to ease a day later as a new weather system moves in. For once, cold air and sloppy conditions will provide a measure of relief along with the misery it brings.

"I expect when the rains come on Wednesday, we'll get a better mixture," Downs said.

In its statement, the MDEP said people who suffer from asthma should particularly heed the warnings. For more information about it, visit MDEP's Web site at: epa.gov/asthma.