Most commonly, fumigation refers to the process of disinfecting a material or an area by using some type of toxic material in gaseous form. The term has a more specialized meaning in environmental science, where it refers to the process by which pollutants are mixed in the atmosphere. Under certain conditions, emitted pollutants rise above a stable layer of air near the ground. These pollutants remain aloft until convective currents develop, often in the morning, at which time the cooler pollutants "trade places" with air at ground level as it is warmed by the sun and rises. The resulting damage to ecosystems from the pollutants is most obvious around metal smelters.