The two main causes for the decrease in the secondhand smoke (SHS) particle concentration in a room with time – fresh air filtration through gaps and cracks and removal of particles due to deposition on surfaces – cause the particle concentration in the room to decrease with time, but the decrease for each cigarette occurs very slowly. Often cigarettes are smoked in a sequence, one after another. Each new cigarette adds to the pollution lingering from the previous cigarette.

As we have seen, a single cigarette can cause very high pollutant concentrations indoors due to the large quantity of particulate mass and other pollutants it emits. Furthermore, the pollutants emitted by a cigarette tend to linger indoors for many hours due to the relatively low air change rates found in most homes.

Tobacco smoke air pollution is a mixture of more than 4,000 chemical by-products of tobacco combustion, 500 of which are in the gas phase (References 1 and 4). Of these byproducts of secondhand smoke, 172 are known toxic substances, many of which are regulated under existing clean air laws (see list in Table 9.1 of Reference 1).

The exhaled mainstream smoke and the sidestream smoke enter the air surrounding the smoker. If the physical volume of the location in which smoking occurs is relatively small, as in a car or a bedroom, then the concentrations of gases and particulate pollutants in this volume will become extremely high.

When a person smokes a cigarette, the part of the smoke that is inhaled directly into the lungs is called mainstream smoke. Pollutants inhaled in the mainstream smoke enter the lung directly and can be absorbed by the blood stream and body tissue. For example, inhaled carbon monoxide (CO) gas enters the blood stream where it ties to the human blood molecule (hemoglobin), thereby depriving the brain of oxygen as the blood enters the brain.

The pollutants generated by the cigarette arise from the chemical process of burning organic matter, or combustion of tobacco and paper. Combustion processes, such as wood burning or waste incineration, emit thousands of pollutants, some of which are in the gas phase and some of which are in the form of small particles called particulate matter.

As professional environmental scientists, we conduct research and publish scientific findings that contribute to protecting public health. Collectively, our total research experience in the science of exposure analysis adds up to more than 100 years, and we have published numerous scientific papers that have been reviewed and accepted by our peers in major scientific and engineering journals.

This booklet is a primer on the nature of SHS, or "secondhand smoke" exposure, sometimes also called "environmental tobacco smoke" (ETS). If you are new to the topic of SHS, then you should start here.

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[Editor's note: Although mostly complete, a few sections of this booklet still need to be finalized. Thank you for your patience.]