When it blows warm air, it may also be blowing dirt built up in the duct work. You may not see the dirt, but it's in there. Keep your home's gas furnace, water heater, dryer and fireplace in top condition. When furnaces and other appliances powered by a flame are not burning properly, they produce carbon monoxide. Low levels of this poisonous gas can cause headaches, drowsiness, dizziness, nausea and rapid breathing.
Higher levels can lead to coma and death.
Buy a carbon monoxide detector. There are many types of carbon monoxide detectors on the market. These detectors should not be placed in the furnace room, where they may give off a false reading. The number of detectors needed depends on the size and layout of your home. The level of humidity in a home can create indoor-air problems.