There are several varieties of "no-see-ums" and they are seasonal. The sand fleas of the southeastern coastal and Piedmont regions are terrible! Many of the southeastern no-see-ums I've encountered can get through regular screening, too - we keep the windows and doors closed and use the ACs. When FT Stewart, GA, was the 24th Infantry Division, they used to go to the field smelling like a French bordello because Avon Skin-So-Soft was the only thing they found (at the time) that would keep them off. Troops must have bought the stuff by the gallon. There may be better repellents now.
Rob