Indoor air quality is affected by many pollutants, including chemical compounds that are natural, such as radon. One of the most important sources of indoor air pollution is moisture; the high humidity that accompanies a house during cold months can cause mold or bacteria to grow on surfaces that have succumbed to mildew and mites. This causes poor indoor air quality. Tobacco smoke contains carbon monoxide and nicotine, both of which can be hazardous to health, especially for those who suffer from asthma.