CONCORD, N.H. (AP) - The New Hampshire Department of Environmental Services says a three-day period of unhealthy air pollution levels in the southwest part of the state is coming to an end.

The department forecast concentrations of fine particle pollution to be unhealthy for sensitive individuals in populated valley areas, especially in Keene. The forecast started Monday and ends late Wednesday morning.

The unhealthy particle pollution levels are the result of locally emitted pollution stagnating under certain temperature conditions. During calm, cold nights, warm air above can trap cold air below. Such events prevent pollution near the ground from mixing with cleaner air.

Much of the pollution comes from heating devices, especially residential wood-burning stoves and boilers.

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