Cigarette smoke contains tiny particles, which can be collected on a filter pad in a laboratory. Tar is usually defined as the particulate matter collected in this way, after water and nicotine are extracted. Tar is identified by many health authorities as being associated with the cancer risks from smoking. They vary with the tobacco blend and with the cigarette design, which can, for example, include tiny air holes to reduce the amount of smoke.
Nicotine occurs naturally in the tobacco plant and is a constituent of tobacco smoke.
It can also be found in other plants, but at much lower levels than in tobacco. Many public health authorities believe that nicotine is responsible for promoting and maintaining an addiction to smoking through its effects on the central nervous system. However, the science suggests that, while important, nicotine is not the only reason for smoking. Carbon monoxide is a gas formed when organic substances burn. It is formed, for example, when wood burns, and in cigarettes when tobacco is burned.