Serious hazards occur when dryer vents do not exhaust directly to the outside. Faulty installations can vent dryer exhaust into the attic, crawl space, chimney, or interior walls, which can cause indoor air deterioration and mold buildup. If heavy clothes, such as blue jeans or towels, are taking a long time to dry or clothes feel hotter than usual at the end of the cycle, a clogged dryer vent exhaust is likely the problem.
There have also been various fires reported where the person had overloaded the dryer with too much clothing or bedding materials. The overloaded dryer could not move air through the drum causing it to overheat and begin to burn.
The last improper use is placing cotton towels contaminated with cooking oils or gasoline in the dryer and leaving them. Tests have been run where the contaminated cotton towels have been dried and left in a pile where they spontaneously combusted. Spontaneous combustion occurs when a flammable or combustible substance is slowly heated to its ignition point through oxidation and many substances begin to release heat as they oxidize. Use the dryer as it is meant to be used, not abused. Some materials are not to be dried in a clothes dryer. Look at the product labels and instructions for washing and drying. Certain plastics, rubbers, and synthetic foam materials should not be dried in the dryer.
The process is sped up through heating in the dryer. Once the towels are removed and folded or placed in a pile, the heat has no way to escape, and the temperature will rise to a level high enough to ignite the oil and cause a fire in the towels or cloth.