Homes heated by forced air have a fan in the furnace which circulates the heated air throughout the house. A filter is usually present between the fan and the heat-exchanger. This filter can be coated with a plastic which reacts with pollutants in the air of the house and removes them as the air is circulated by the furnace fan. The report shows that formaldehyde can be removed from the air when the fiberglass filters are coated with the polymer polyethylenimine mixed with glycerol to keep it from hardening. By increasing the amount of polymer (plastic) applied to the filter it has been possible to extend the life of the coating to almost four months during which time the level of formaldehyde in the house is well within acceptable levels. The same coating has previously been shown to be effective in removing acidic gases such as oxides of nitrogen, sulphur dioxide, hydrogren sulphide and complex hydrocarbons from air streams where they were present at relatively high concentrations. The researchers showed that for NO2 at about 1 part per million in air, the coating is still effective, but at lower concentrations, as found in a home where the gas is produced by open fires such as a kitchen range or space heater, the coated filter was marginally effective. It was also shown that in the process of air-conditioning a house or dehumidifying the air the formation of condensation will remove part of the water soluble pollutants, such as formaldehyde, from the air.