A liquid pool cover forms an invisible layer one to two molecules thick on the surface of a pool to reduce evaporation and heat loss. Tests showed heat bill savings ranging from 15.5% to 40% depending on pool use. The Professional Pool Operators of America tested a liquid cover against a standard blanket, finding them equally effective at conserving energy over 10 days. While the liquid cover is effective, its effects on chlorine gas levels during treatment have not been fully explained scientifically.