You can feel the bad odor coming from the air vents of your house or office. In some cases, the smell is persistent. In other cases, it comes and goes and its intensity changes over time. Even though the symptoms may vary, they are a sign of a serious problem which must be identified and dealt with quickly. Find out more about the main causes of foul smell coming from the air ducts and how to resolve them. Less often, the bad odor can come from gasses which have been released into the air ducts. If the heat exchanger of the HVAC system has a leakage, it will release combustion gasses such as carbon monoxide into the air ducts. These gasses can pose a serious health threat. Action for resolving the problem must be taken immediately. If the structural components and finishes of a building release chemicals in gas form, they can be carried by the HVAC system and their bad smell can come from the air vents. In this case, the dangerous materials must be removed and all surfaces including the walls of the air ducts must be properly cleaned. When there is bad odor coming from the air vents, you must take the necessary steps to deal with the problem without delay.