The state Department of Environmental Protection has forecast a Code Orange Air Quality Action Day Monday for Butler County and 19 other counties due to an increase in fine particulate matter caused by warming temperatures.

On air quality action days, the DEP recommends that young children, elderly and people with respiratory problems limit outdoor activity.

Levels of fine, inhalable particulate matter of 2.5 micrometers rose faster than expected over the weekend when calm, colder air was trapped below warmer air. The weather pattern is expected to remain until Tuesday morning when a cold front is expected to pass through Southwestern Pennsylvania.

The U.S. Environmental Protection Agency’s standardized air quality index uses colors to report daily air quality. Green signifies good; yellow means moderate; orange represents unhealthy pollution levels for sensitive people; and red warns of unhealthy pollution levels for all.

The DEP encourages residents and business to voluntarily restrict pollution-producing activities by refueling vehicles after dusk, reducing use of wood stoves and other sources of fine particulate matter, carpooling or using public transportation and combining errands to reduce trips.