Tobacco smoke, besides being an irritant and significant indoor air pollutant, is known to cause lung cancer, heart disease, chronic obstructive pulmonary disease (COPD), emphysema, and other serious diseases in smokers (and in non-smokers as well). The actual mechanisms by which smoking can cause so many diseases remain largely unknown. Many attempts have been made to produce lung cancer in animals exposed to tobacco smoke by the inhalation route, without success. It is only by collecting the "tar" and repeatedly painting this on to mice that tumors are produced, and these tumors are very different from those tumors exhibited by smokers. Tobacco smoke is associated with an increased risk of developing respiratory conditions such as bronchitis, pneumonia, and asthma. Tobacco smoke aerosols generated at temperatures below 400 ℃ did not test positive in the Ames assay.