The state Department of Environmental Services has declared an "air quality action day" for southwestern New Hampshire.
The state said on Thursday, it expected air pollution concentrations to reach unhealthy levels for children, older adults, and anyone with heart or lung disease such as asthma, emphysema and bronchitis.
The air quality concern is the result of local pollution stagnating under calm winds and temperature inversion conditions. That happens during calm, cold nights when warm air above traps cold air below, and prevents pollution near the ground from mixing with cleaner air.
Much of the locally emitted pollution comes from heating devices such as wood-burning fireplaces, stoves and boilers.
Conditions were expected to improve by Friday morning as wind speeds increase.