In a nutshell, winter indoor air is often even more toxic than summer indoor air because there is typically less ventilation from opening windows or doors; very low humidity, which lowers the body's ability to cope with indoor toxins; and more use of appliances that contribute their own toxic by-products into the air. Here is a list of the most commonly cited indoor airborne toxins, courtesy of the Canadian Lung Association : Fumes and gases from wood-burning stoves and fireplaces Tobacco smoke, fumes, by-products Household cleaning products, air fresheners, and scents, including candles and          perfumes Carbon monoxide, particulate matter, and nitrogen oxides from gas appliances Formaldehyde from pressed wood furnishings and carpeting Volatile organic compounds (VOCs) from use of indoor pesticides, home improvement products, and craft glues Radon seeping up from cracks in foundation and walls Mould and mildew growth in damp areas such as ba...