Abstract: A floor drain for hazardous environments comprises a grating, a housing, and an elastic one-way valve clamped by a flange between the grating and the housing. The valve has a tubular body with contacting parallel wall portions which open when a liquid enters the valve. The valve closes when there is no pressure at the inlet. The valve closes more tightly in the event of elevated down stream pressure, positively preventing the escape of gas. An existing, unprotected drain can be retrofitted with the one-way valve.