Health Canada has been monitoring tobacco smoke of cigarettes sold in Canada for over 30 years. Tobacco smoke is a combination of poisonous gases, liquids, and breathable particles that are harmful to health. It contains over 4,000 chemicals, including at least 70 that cause, initiate or promote cancer. Most of these chemicals are formed during the combustion of the tobacco; others are found naturally in the tobacco and are released as the tobacco burns or as it is chewed. Since 2000, the level of monitoring increased with the addition of almost 40 chemicals to the original list of tar, nicotine and carbon monoxide.