However, GEV can provide negative room pressure to prevent contaminants from exiting the room. The use of supply and exhaust air throughout the facility can provide pressurization schemes that reduce the number of workers exposed to potentially hazardous materials, for example keeping production areas at a negative pressure with respect to nearby areas.[1]:11–12 For general exhaust ventilation in laboratories, a nonrecirculating system is used with 4–12 air changes per hour when used in tandem with local exhaust ventilation, and sources of contamination are placed close to the air exhaust and downwind of workers, and away from windows or doors that may cause air drafts.[2]:13