YAKIMA, Wash. — A burn ban will remain in place across Yakima County at least through the weekend, the Yakima Regional Clean Air Agency said Friday.

The current ban prohibits agricultural burning and use of all wood-burning appliances unless they are a home’s sole source of heat. The stricter Stage 2 ban has been in effect since Thursday.

Agency spokesman Dave Caprile said an inversion continues to trap air pollutants near the surface of the ground. Air quality is deemed unhealthy in both the Upper and Lower valleys, especially for susceptible populations such as infants, the elderly and those with respiratory problems.

The federal Environmental Protection Agency imposed a burn ban Friday on the Yakama Nation reservation. The agency, which enforces the federal Clean Air Act on Indian reservations, said the ban will remain in place until at least Monday, when air quality will be examined again.

Reservation residents are being asked to reduce all sources of air pollution by refraining from excess driving and not using wood stoves or fireplaces unless they are the only source of heat.

Conditions were nominally better in Kittitas County, where the state Department of Ecology announced a Stage 1 burn ban for the county on Friday.

That ban also was expected to apply through at least 10 a.m. Monday, the agency said.

Use of uncertified fireplaces, wood stoves and inserts is prohibited unless they are a home’s sole source of heat. However, use of certified devices is allowed. No outdoor burning is allowed.