Complex sentence: GEV is inefficient and costly as compared to local exhaust ventilation, and given the lack of established exposure limits for most nanomaterials, they should not be relied upon for controlling exposure, although they can provide negative room pressure to prevent contaminants from exiting the room.
Split into simpler sentences: GEV is inefficient and costly as compared to local exhaust ventilation, and given the lack of established exposure limits for most nanomaterials, they are not recommended be relied upon for controlling exposure. However, GEV can provide negative room pressure to prevent contaminants from exiting the room.