The burning of tobacco causes the emission of over 4,500 compounds, including 50 suspected carcinogens. These include NO2, NO, hydrogen cyanide (HCN) and N-nitrosamines, nicotine, CO, nitrogen oxides, aromatic hydrocarbons, acrolein, acetone, benzene, nitrogen oxides, and aromatic and respirable suspended particles. Particulate matter originating from tobacco smoke, in rooms where smoking is permitted, can range from 100 to over 1,000 ug/m3. For these reasons, tobacco smoking has been banned from public indoor areas in many countries throughout the world. Where tobacco smoking is allowed in public indoor areas, filtration devices are often employed to remove contaminants and ensure the comfort and safety of non-smokers in the same room or building.