The ABC's of SHS: The Story of Exposure to Secondhand Smoke

Smoke Plumes Rising 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.]

The Cigarette is a Major Source of Pollution

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.

Particulate matter consists of millions of tiny particles of diverse chemical composition. Particulate matter from tobacco smoke includes many particles in the size range that reflects light, which explains why tobacco smoke is easily seen by the eye. In contrast to smoke particles, gases emitted by the cigarette such as benzene and carbon monoxide (CO) are invisible to the eye. Particles smaller than 2.5 micrometers (PM2.5) are major components of cigarette smoke and can enter deep into the lung where they can cause serious health problems. To illustrate how small a PM 2.5 particle is, consider that 25,000 particles of this diameter, when placed side to side, can fit into 1 inch on a ruler.

Although a single cigarette is small in size and typically weighs less than 1 gram, a cigarette typically emits between 7 and 23 milligrams (mg) of PM2.5 when it is smoked, depending on the manner of smoking and the brand (see References 1 and 2 on the reference citation list). When people congregate in an airport baggage area or enter a smoking lounge where many brands are smoked, the average amount of PM2.5 mass emitted per cigarette is about 14 mg (see Reference 3). Although 14 mg may not seem like a lot of mass emitted, each cigarette weighs only about 0.9 grams total, making it an extremely potent source of air pollution for its weight.

As we shall see in subsequent chapters of this booklet, the 14 mg of particles emitted by each cigarette is really a large amount of particulate matter mass, causing extremely high indoor air pollutant concentrations when a cigarette is smoked at home or in a car. The chapter "Where does the smoke go?" presents calculations that you can do yourself to illustrate that a single cigarette smoked indoors is a potent source of exposure to toxic pollutants, causing concentrations indoors that are often higher than the federal air quality standards designed to protect public health in ambient air outdoors.

What Happens When a Person Smokes?

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. Elevated CO in the blood may persist for many hours after the cigarette has been smoked, and it is possible to determine if a person has smoked a cigarette simply by measuring the elevated CO in a sample of the person’s blood. Some portion of this inhaled smoke is exhaled by the smoker as processed mainstream smoke. More than half the pollutants emitted by a cigarette come not from the smoked end of the cigarette but from its other end – the cigarette’s burning end – and are called sidestream smoke. Secondhand smoke, sometimes denoted as SHS, is a combination of both the exhaled mainstream smoke and the sidestream smoke emitted by the burning end of the cigarette, as well as any other smoke emitted from the end of the cigarette held by the smoker. Thus, secondhand smoke (SHS) is the total amount of pollution that leaves the immediate surroundings of the smoker.

Since each cigarette emits a large amount of fine particulate matter (7-23 mg) as SHS, and since this particulate matter comprises the visible part of the emitted smoke, some scientists believe the act of “smoking” should really be called "particling," a less romantic but scientifically accurate description of the activity of smoking. Tobacco company advertisements would be less exciting to young people and less likely to cause people to start smoking if these ads described the act of smoking as “particling.”

Pollutants in Secondhand Smoke

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). In addition to 3 standard criteria air pollutants (carbon monoxide, particulate matter, and lead) and 33 hazardous air pollutants (HAPs) regulated in ambient air under the federal Clean Air Act, secondhand smoke contains 47 pollutants that are classified as hazardous wastes whose disposal in solid or liquid form is regulated by the Resource Conservation and Recovery Act, 67 pollutants known to be human or animal carcinogens, and 3 industrial chemicals regulated under the Occupational Health and Safety Act.1

Where Does the Smoke Go?

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. For example, if the particulate matter generated from smoking a single cigarette is emitted into a bedroom with a physical volume of 41 cubic meters (m3), and the mass is uniformly dispersed over the bedroom’s air volume, then the resulting PM2.5 concentration in the room is calculated by dividing the total amount emitted (14 mg) by the total room volume (41 m3):

Maximum Concentration = (14 mg)/(41 m3) = 0.341 mg/m3

Normally, pollutant concentrations are expressed in micrograms per cubic meter (μg/m3), and since 1 mg = 1,000 micrograms, the maximum PM2.5 concentration associated with a single smoked cigarette will be (0.341 mg/m3)(1,000 micrograms/mg) = 341 μg/m3, a relatively high pollutant concentration.

Thus, because of the large amount of smoke particle mass emitted by a single cigarette, the cigarette in a bedroom wil cause a very high initial concentration of 341 μg/m3. This initial high concentration occurs soon after the cigarette stops burning, which then is followed by a slow decrease in the concentration in the room with time5. This slow decrease in concentration happens during the pollutant decay period, during which fresh air infiltrates into the room through cracks, gaps, and windows, gradually replacing the room’s air while at the same time some particles from the smoke deposit on the walls and furniture.

To gain insight into this phenomenon from measurements in a real home, a single Marlboro regular filter cigarette was smoked in a 41 m3 bedroom with the door closed5, and the particle concentration was measured over the next 3 h with an instrument that measures the particle mass concentration:

This graph shows that a maximum particle concentration over 320 μg/m3 occurred almost immediately after the cigarette ended at 2:38 PM, which is close to the the predicted concentration using the equation above. For the next hour and a half, the concentration in the room gradually decreased, whereupon the bedroom door was opened by one of the occupants, causing the bedroom’s particle concentration to decay more rapidly as the pollutant passed through the door into the rest of the house.

To summarize, while the bedroom door was closed, the smoke was gradually removed from the air in the bedroom both by deposition (sticking) on surfaces and by exchange with external air (infiltration), but the removal process took several hours after the cigarette was smoked for the smoke to fully clear. Many toxic air pollutants were released in the emissions of the cigarette, some in gaseous and some in particulate form. The particles in particulate matter are themselves composed of many toxic compounds, such as polycyclic aromatic hydrocarbons (PAHs).

The horizontal dashed line in the figure above shows the US federal National Ambient Air Quality Standard for particulate matter (PM2.5), which specifies that outdoor air concentrations in the U.S. should not exceed 35 μg/m3 averaged over 24 h. These regulatory standards are based on extensive research on the effects of daily variations in ambient concentrations on human health and on other health-related studies, and this research is summarized in EPA’s research compendium on particulate matter6. For example, a recent study of women in Seattle7 found that deaths from heart attacks, coronary disease, strokes, and clogged arteries were 24% more likely for every rise in the ambient PM2.5 concentrations of 10 μg/m3.

Even though high concentrations will persist in the bedroom for hours after the cigarette ends, just one cigarette will not necessarily cause the federal air quality standard of 35 μg/m3 to be exceeded in this bedroom. However, in this bedroom, it turns out that 2 or more cigarettes smoked can this health-based ambient standard to be violated indoors, depending on the ventilation rate in the room (see next section).

Daily Incremental Exposure

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. An EPA measurement study of 178 homes in Riverside, CA, found that homes with smokers had indoor concentrations that averaged 30 μg/m3 higher than homes without smokers 5-7, an important number that has been confirmed in other measurement studies in U.S. homes with smokers8. Of course, some homes with smokers have lower indoor concentrations than the average and some homes have higher indoor concentrations than the average, but on average smoking in a home adds about 30 μg/m3 to the indoor fine particle concentration.

In the single bedroom example in the figure shown in the previous section, how does each additional cigarette smoked per day change the 24-h average concentration? A useful formula exists for the average Daily Incremental Exposure (DIE) contributed by one cigarette:

      DIE = \frac{x_o}{24D} (1 - e^{-24D}) ~ \frac{x_o}{24D}

where DIE = 24-h Average Concentration Contributed by the Source, x_o = Initial Maximum Concentration (μg/m3), D = Decay Rate (h-1)

Notice that the volume of the room does not appear in this formula, because the volume already is reflected by the initial maximum concentration x_o caused by smoking the cigarette in this room. Each 24-hour average concentration predicted by this equation for every source can be added together, a useful property of the incremental daily exposure.

To apply this formula to the single cigarette smoked in the bedroom, we need first to find the decay rate for the main portion of the particle concentration curve that is decreasing with the door closed. The method for finding this important parameter is discussed elsewhere in the literature9, and it requires performing regression analysis on the logarithm of the concentration versus time, which gives the result of D = 0.5 h-1. Substituting the initial concentration of x_o = 341 μg/m3 and this decay rate into the above equation for the daily average concentration gives:

      DIE = \frac{x_o}{(24)(0.5)} (1 - e^{-(24)(0.5)}) ~ \frac{341}{12} = 28.4 μg/m3

Thus, a single cigarette smoked in this bedroom raises the average 24-h concentration by 28.4 μg/m3, which is quite high but is less than the federal 24-h health-based standard of 35 μg/m3. Smoking just one additional cigarette in this bedroom, however, will bring the average for 2 cigarettes in 24 h to (2)(28.4) = 56.8 μg/m3, which is well above the EPA health-based standard. Violating the federal health-based standards happens easily because of the incredibly high particle emissions of each cigarette. In summary, even though a single cigarette lasts for only about 8.5 min, its aftermath of elevated concentrations in the room can persist for several hours, and smoking just 2 cigarettes per day in this bedroom will cause the indoor concentration to exceed the 24 h federal standard of 35 μg/m3 designed to protect health outdoors under the US Clean Air Act.

It is clear that smoking many additional cigarettes per day in this bedroom would raise the 24-h average particle concentration to extremely high levels. At first glance, these high concentrations in the bedroom may seem inconsistent with the known average incremental concentration of 30 μg/m3 for smoking homes reported in the EPA field studies mentioned above. A smoking parent and a small child may spend time together in a bedroom with the door closed, but usually the smoker does not stay in one room but instead smokes in different parts of the house, and the volume of the entire house then becomes important. If a person smokes a half-pack per day (10 cigarettes) in a typical house with a volume of 300 m3 and the house has this same rate of decay for particles, then the 24-h concentration in the home will be 38.8 μg/m3, causing the indoor air of the entire home to violate the federal 24-hour ambient air quality standard for PM2.5.

[To be extended by referring to the California house volume data in the literature, illustrating that each cigarette is a potent source of fine particulate matter, pushing 24-h averages close to and often exceeding the NAAQS for PM-2.5.]

Smoking Multiple Cigarettes in a Home

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. When this happens, the concentration from the first cigarette does not have enough time to drop appreciably before the second cigarette begins, and the overall concentrations in the room can remain high for long periods or can even increase over time.

Several studies of smoking in homes show high intensity SHS particulate pollution in rooms where smoking of one or more cigarettes takes place, and substantial peak and average levels in separate rooms. Particle levels from a single cigarette can persist at relatively high levels above 50 μg/m3 for several hours.

For example, the top 5 panels of the figure above show simulated real-time SHS particle levels in the different zones of a typical house -- with labels giving the 24-hour average level (Source: Klepeis & Nazaroff, 200612). Cigarette activity, depicted by vertical bars in the bottom panel, leads to peaks in particle concentration in a given room as high as 300 μg/m3. Peak particle levels reach 100-200 μg/m3 in adjacent rooms to where smoking occurs, indicating that levels measured in any room of the house reflect elevated pollutant levels in other rooms. Average levels in adjacent rooms are also elevated. The 24-hour average particle levels of 35-51 μg/m3 in rooms with the least smoking (bedroom, hallway) are about 50% to 90% of the average in rooms with the most smoking (kitchen, living room).

Note that in the example shown above, the 24-hour average particle concentration due to smoking equals or exceeds 35 μg/m3, which is the 24-hour USEPA ambient air standard, for every room in the house except the bathroom.

How Well Does Ventilation Control Concentrations?

In theory, using a high enough ventilation rate in a room can help reduce the concentrations of SHS produced from smoking. However, ventilation will not necessarily reduce levels of SHS to safe levels or eliminate SHS exposure. If one is in the same room as an active smoker, then during smoking one will be exposed to smoke that is being mixed in the room -- even if there is a high ventilation rate. Furthermore, if one is fairly close to the active smoker (within 3 to 6 feet), one's exposure is likely to exceed by several times that of a person positioned farther away from the smoker. This "proximity effect" will occur regardless of the amount of ventilation in the room.

Outdoor Exposure to Secondhand Smoke

There are currently few studies focused on air pollution resulting from outdoor smoking activity. The handful of studies that have been completed are uniform in their assessment that exposure to outdoor tobacco smoke can sometimes rival exposure to secondhand smoke occurring indoors. As one moves closer to an outdoor smoker and spends more time downwind from the smoker, one's exposure goes up. Of course, as the number of outdoor smokers increases, one's risk of exposure also increases.

A recent study by Klepeis et al. (2007)13 shows that thin streams, or plumes, of tobacco smoke near an outdoor smoker can reach levels over 1000 μg/m3. For reference: On what the USEPA considers a relatively clean day, background air pollution levels are under 20 μg/m3. Visit the following link to learn more about this study:

If one were to spend time outdoors near multiple smokers over a day, say as a worker at an outdoor pub or as a child accompanying a smoker, it would be possible to receive an outdoor exposure to particles that exceeds the current USEPA health-based standard for PM-2.5, which is currently 35 μg/m3. By sitting at an outdoor table for an hour with a smoker, who consumes 2 cigarettes over the hour, one could be exposed to a level of PM-2.5 greater than that caused by being in a smoky tavern for an hour.

Summary and Conclusions

From a scientific standpoint, the story of exposure to secondhand smoke has been muddled by pro-smoking advocates. These critics make attempts at poking holes in the previous epidemiological research studies linking smoke exposure to long-term adverse health effects, such as cancer and heart disease.

What the pro-smoking critics seem to forget is that the health effects of most of the pollutants that make up secondhand smoke – such as fine particles, carbon monoxide, benzene, PAHs – already are well-known in environmental science.

These critics also tend to completely ignore various acute, but still significant health effects caused by the toxic pollution in secondhand smoke, such as an increased severity of asthma, respiratory infection, and simple irritation of the eyes or throat.

Prior to the enactment by cities, counties, and states of recent environmental laws designed to protect public health, the adverse effects of these pollutants already were documented and their effects were well-understood.

Because of the enormous mass of pollutants generated by a single cigarette, persons close to a smoker or living in the same house will be exposed to pollutant concentrations that are many times higher than those permitted outdoors under existing environmental laws. Any reasonable person must ask, "Does it make sense to set uniform national standards under the Clean Air Act for the maximum pollutant concentrations allowed in the ambient air, while at the same time exposing children and adults in homes, automobiles, and restaurants to pollutant levels many times higher than these standards?"

The potency of the cigarette in violating clean air standards has been misrepresented by pro-smoking advocates. This fact needs needs greater attention and understanding by the public. The potency of the cigarette as a generator of toxic pollutants that can cause cancer and other long and short-term health effects, argues strongly for restricting smoking in public places as a matter of prudent public policy.


  1. Repace, James L. (2007) “Exposure to Secondhand Smoke,” Chapter 9 in Ott, Steinemann, and Wallace, eds., Exposure Analysis, CRC-Press, Taylor & Francis Group, Boca Raton, FL.
  2. Martin, P., Heavner, D.L., Nelson, P.R., Maiolo, K.C., Risner, C.H., Simmons, P.S., Morgan, W.T., and Ogden, M.W. (1997) “Environmental Tobacco Smoke (ETS): A Market Cigarette Study, Environment International, Vol. 23, pp. 75-90.
  3. Klepeis, N.E., Ott, W.R., and Switzer, P. (1996) “A Multiple-Smoker Model for Predicting Indoor Air Quality in Public Lounges,” Environmental Science and Technology, Vol. 30, No. 9, pp. 2813-2820.
  4. Hoffmann, D. and Hoffmann, I. (1998) “Chemistry and Toxicity” in Cigars: Health Effects and Trends, Smoking and Tobacco Control Monograph 9, National Institute of Health, National Cancer Institute, Bethesda, MD.
  5. Ott WR, Klepeis NE, Switzer P (2003) Analytical solutions to compartmental indoor air quality models with application to environmental tobacco smoke concentrations measured in a house. Journal of the Air and Waste Management Association 53:918-936.
  6. USEPA (2004) Air Quality Criteria Document for Particulate Matter, Volume I, EPA/600/P-99/002aF, and Volume II, EPA/600/P-99/002bF, Office of Health and Environmental Assessment, Environmental Criteria and Assessment Office, U.S. Environmental Protection Agency, Research Triangle Park, NC.
  7. Wallace, Lance A. and Smith, Kirk R. (2007) “Exposure to Particles,” Chapter 8 in in Ott, Steinemann, and Wallace, eds., Exposure Analysis, CRC-Press, Taylor & Francis Group, Boca Raton, FL.
  8. Clayton, C.A., Perritt, R.L., Pellizzari, E.D., Thomas, K.W., Whitmore, R.W., Wallace, L.A., Özkaynak, H., and Spengler, J.D. (1993) “Particle Total Exposure Assessment Methodology (PTEAM) Study: Distributions of Aerosol and Elemental Concentrations in Personal, Indoor, and Outdoor Air Samples in a Southern California Community, Journal of Exposure Analysis and Environmental Epidemiology, Vol. 3, pp. 227-250.
  9. Özkaynak, H., Xue, J., Spengler, J.D., Wallace, L.A., Pellizzari, E.D., and Jenkins, P. (1996) Personal Exposure to Airborne Particles and Metals: Results from the Particle TEAM Study in Riverside, CA, Journal of Exposure Science and Environmental Epidemiology, Volume 6, pp. 57-78.
  10. Miller, S.L. and Nazaroff, W.W. (2001) “Environmental Tobacco Smoke Particles in Multizone Indoor Environments,” Atmospheric Environment, Vol. 35, pp. 2053-2067.
  11. Ott, W. (2007) “Mathematical Modeling of Indoor Air Quality,” Chapter 18 in Ott, Steinemann, and Wallace, eds., Exposure Analysis, CRC-Press, Taylor & Francis Group, Boca Raton, FL.
  12. Klepeis N.E. and Nazaroff W.W. (2006) Modeling Residential Exposure to Secondhand Tobacco Smoke. Atmospheric Environment. 40(23):4393-4407.
  13. Klepeis NE, Ott WR, Switzer P (2007) Real-time measurement of outdoor tobacco smoke particles. Journal of the Air and Waste Management Association, 57:522-534.