Exposure to TCE occurs mainly through contaminated drinking water. With a specific gravity greater than 1, trichloroethylene can be present as a dense non-aqueous phase liquid (DNAPL) if sufficient quantities are spilled in the environment. Another significant source of vapor exposure in Superfund sites that had contaminated groundwater, such as the Twin Cities Army Ammunition Plant, was by showering. TCE readily volatilizes out of hot water and into the air. Long, hot showers would then volatilize more TCE into the air. In a home closed tightly to conserve the cost of heating and cooling, these vapors would then recirculate.