A NBC4 Washington article announced the start of an annual ban on open burning for the District of Columbia and nearby Maryland and Virginia Counties. From the article:

Frederick County Health Department spokeswoman Karen Amoss says the annual ban begins Wednesday and continues through Aug. 31 under Maryland law. It doesn’t apply to small recreational fires such as campfires, barbeques or gas grills.

She says the ban extends throughout the Metropolitan Washington Air Quality Region, encompassing the District of Columbia; the Maryland counties of Frederick, Montgomery, Prince George’s, Calvert and Charles; and the Virginia counties of Loudoun, Arlington, Fairfax, Prince William and Stafford.