Clearing the Air: What Does Science Really Say About Secondhand Smoke?

Secondhand smoke is a bit like abortion and global warming: it invokes such strong emotions that it is difficult to view the evidence objectively. In his analysis of the subject (see preceding article) Sidney Zion is right about one thing: anti-smoking activists have overstated the evidence against secondhand smoke. He, on the other hand, understates the evidence. He’s guilty of the same kind of distortion he criticizes, only in the opposite direction.

No one believes secondhand smoke is totally harmless. If anyone really believed that, they would have no compunction about exhaling cigarette smoke directly in a newborn baby’s face or blowing cigar smoke at a child having an asthma attack. It’s smoke, for goodness sake!

What do you do when you’re sitting by a campfire and the wind shifts, blowing the smoke towards you? You move. Tobacco smoke is polluted air, with particles, carbon monoxide, 69 known carcinogens, radioactive Polonium 210, and 4,000 other chemical compounds.

If you accept the overwhelming evidence that smoking is a health hazard, you have already accepted the fact that secondhand smoke is a health hazard in large enough doses. If you lived in a telephone booth with a chain smoker, you could expect the same effects as if you smoked yourself. In fact, there is some evidence that sidestream smoke from cigarettes is more toxic than the smoke inhaled by the smoker.

The cardinal rule of toxicology is that the effect of poison depends on the dose. The real question is whether secondhand smoke in public places provides a high enough dose to produce detectable harmful effects in nonsmokers. Since any effect is bound to be small, it is difficult to detect and quantify; hence the disagreements.

We can’t perform a definitive experiment with humans as guinea pigs: we are limited to epidemiologic and other indirect evidence. The evidence about smoking has been accumulated over years of epidemiologic studies, animal studies, and laboratory studies. Those revealed a plausible mechanism, a dose/response correlation, and the evidence from all sources combined to make an unassailable case. It is impossible to say that any given patient’s lung cancer was caused by smoking, because nonsmokers also get lung cancer. Over the entire population, we can say that thousands of people are dying prematurely because they smoked—we just don’t know which ones. We are necessarily dealing with probability and statistics.

It is easier to study firsthand smoking, because people can report fairly accurately how many cigarettes they smoked a day for how many years. Secondhand smoke is much harder to quantify. There is no good way to measure how much a nonsmoker actually inhales. Many of the epidemiological studies have used marriage to a smoker as an indication of secondhand smoke exposure, but it makes a difference whether you are cooped up in an unventilated room with a smoking spouse or whether he considerately steps outside to puff on his stogie.

Biomarkers like cotinine levels have been used in some studies, but these markers are problematic and at best they only represent one point in time. We simply don’t have any really reliable long term data, so we have to go by what we do have. Meta-analyses have pooled data from different studies, but if the original studies were poor, we may have a garbage in/garbage out situation.

The uncertainty is analogous to the uncertainty over alcohol and pregnancy. We know regular consumption of alcohol leads to fetal alcohol syndrome. We’re pretty sure one glass of wine during a pregnancy is harmless. What about two glasses? 20 glasses? 200 glasses? We don’t know where to draw the line, so current medical advice is to avoid alcohol entirely during pregnancy.

Another analogy is with diagnostic x-rays. We know large doses of radiation can cause cancer. The amount of radiation from a single chest x-ray is much smaller than the natural variation in background radiation, so that we can never do adequate studies to show whether the radiation from one chest x-ray is statistically harmful. We can’t guarantee that any level of radiation exposure is absolutely safe. So we try to minimize exposure to radiation. We don’t know what the threshold is, or even if there is a threshold, so we err on the side of caution.

The Surgeon General’s 2006 report (“The Health Consequences of Involuntary Exposure to Tobacco Smoke,” points out that we don’t know if there is a threshold below which secondhand smoke is absolutely innocuous. Critics say that’s nonsense because if all you get is one whiff in a lifetime from a passing smoker, there’s no way that amount is going to hurt you. Since the poison is in the dose, we can assume there should be a safe threshold; we just don’t know what it is. Hysterical anti-smoking activists have gone overboard, using this uncertainty alone to support their demands for total bans. Hysterical anti-anti-smoking activists have gone overboard in the other direction, treating uncertainty as if it amounted to complete exoneration. Common sense tells us that a low level of exposure is innocuous enough for all practical purposes.

Common sense tells us that judges should not be taking children away from parents just because the parents smoke. Common sense also tells us that minimizing exposure is a good idea.

The estimate of 63,000 American deaths each year from secondhand smoke is a best guess based on extrapolation of relative risk from pooled studies. Some of the studies show a trend but do not reach significance. When those studies are pooled, a significant effect emerges.

Pooling the data in that way may not be justified, and 63,000 may be a gross overestimate. That doesn’t mean the true number is zero. Those who reject the 63,000 figure also acknowledge that there is reason to believe secondhand smoke does some harm.

Zion selectively quotes the IARC: “The observations on nonsmokers that have been made so far are compatible with either an increased risk from ‘passive’ smoking or an absence of risk.” He omits the sentence that follows: “Knowledge of the nature of sidestream and mainstream smoke, of the materials absorbed during ‘passive’ smoking, and of the quantitative relationships between dose and effect that are commonly observed from exposure to carcinogens, however, leads to the conclusion that passive smoking gives rise to some risk of cancer.”

Zion focuses on deaths from heart disease and lung cancer. If secondhand smoke did not cause any deaths from either of these diseases, it would still be implicated in a number of other health hazards. He gives the impression that Elizabeth Whelan of the ACSH (American Council on Science and Health) doesn’t believe secondhand smoke is harmful. She does. All she is objecting to is exaggeration and extrapolation from uncertain data. Here is the ACSH position statement in full (

  • Irritation of the eyes, nose, and respiratory tract is the most common and firmly established adverse health effect associated with exposure to ETS [Environmental Tobacco Smoke].
  • Exposed infants and children, in particular, are at increased risk of respiratory infections, middle-ear effusion (fluid inside the eardrum), and the exacerbation of asthma and other respiratory symptoms.
  • Exposed adults are at increased risk for respiratory ailments; ETS may aggravate the symptoms of preexisting asthma and emphysema.
  • Extensive epidemiological evidence indicates that ETS exposure is a weak risk factor in the development of lung cancer in nonsmokers regularly exposed to ETS in the workplace and/or at home.
  • Epidemiological evidence also suggests that ETS is a weak risk factor for heart disease in nonsmoking spouses of smokers and in nonsmokers regularly exposed to ETS in the workplace and/or at home.
  • Other reported links between ETS and chronic disease (breast cancer, cervical cancer, and leukemia, for example) have not been scientifically established and are not addressed in this report.

Zion dismisses the Surgeon General’s 700 page report as reflecting “not one fresh study.” That’s a bit disingenuous. The report’s purpose was to analyze all the evidence from all the published studies to date. It was designed to include all the fresh studies since the previous report in 1986, and it shows that the evidence is stronger today than it was then. It even lists the most recent studies that could not be included in the report itself. It was written by 22 experts and reviewed and revised by 40 peer reviewers. Each chapter provides a comprehensive review of the evidence, with a rigorous analysis of possible sources of error or bias. It establishes a 4-level hierarchy of strength of evidence, and comes to conclusions such as the following on page 13:

The evidence is inadequate to infer the presence or absence of a causal relationship between maternal exposure to secondhand smoke during pregnancy and spontaneous abortion…

The evidence is sufficient to infer a causal relationship between exposure to secondhand smoke and sudden infant death syndrome.

The evidence is suggestive but not sufficient to infer a causal relationship between maternal exposure to secondhand smoke during pregnancy and preterm delivery.

The report discusses the problems associated with methodology and meta-analysis, but comments on page 22: “However, the availability of consistent evidence from heterogenous [sic] designs can strengthen the meta-analytic findings by making it unlikely that a common bias could persist across different study designs and populations.”

On page 29 the report discusses how tobacco industry activities have tried to undermine the scientific consensus, noting, “A full range of scientific evidence, extending from the molecular level to whole populations, supports the conclusion that secondhand smoke causes disease….”

The guidelines for causal inference include coherence, which is defined as the extent to which all lines of scientific evidence converge in support of a causal conclusion.

Zion misrepresents the 2003 British Medical Journal study as a definitive argument against the received wisdom that passive smoke kills. In fact, the study is flawed and has generated a great deal of controversy. It was based on long-term follow-up of only 7% of a group of original subjects who were classified by their spouse’s smoking status in 1959. Its methodology is suspect, and its conclusions may be unwarranted. The BMJ was roundly criticized for publishing it, and received a huge number of letters to the editor, most criticizing the study on scientific grounds, with some even calling for retraction. There is an extensive critique of the study at http://www.ash.orguk/html/passive/html/BMJ0503critique.html. Even if you accept the study, its own conclusion states:

The results do not support a causal relation between environmental tobacco smoke and tobacco related mortality, although they do not rule out a small effect. The association between exposure to environmental tobacco smoke and coronary heart disease and lung cancer may be considerably weaker than generally believed.

Note that they do not rule out a small effect, and they acknowledge that there is an association, although they think it is weaker than generally believed. Although they didn’t find a causal relation to mortality, the data did support a causal relation between secondhand smoke and COPD (Chronic Obstructive Pulmonary Disease, or emphysema).

Care to guess who said the following?

We also believe that the conclusions of public health officials concerning environmental tobacco smoke are sufficient to warrant measures that regulate smoking in public places. We also believe that where smoking is permitted, the government should require the posting of warning notices that communicate public health officials’ conclusions that secondhand smoke causes disease in non-smokers.

Was this some hysterical anti-smoking zealot?

No, I took this from the Philip Morris website (on a page that has links to all the studies Zion criticizes) at http://www.philipmorrisusa. com/en/health_issues/secondhand_smoke.asp. There is a similar statement on the R.J. Reynolds website. Even the tobacco industry agrees that secondhand smoke is harmful!

Should there be no-smoking laws? The answer to that question really doesn’t hinge on whether secondhand smoke kills people. Society has seen fit to pass laws against public nudity, and as far as I know public nudity never killed anyone. Nudity is restricted because the majority of people find it offensive. I think the current campaign against smoking is really for a similar reason: the majority of the population agrees that breathing smoky air is aesthetically offensive as well as irritating to the respiratory tract of nonsmokers. Zion is worried about the infringement of smokers’ civil liberties. What about the civil liberties of the nudist who is forced to wear clothes in public?

And think about this: if you have breathed enough secondhand smoke to notice it, that means you have already inhaled its complex mix of smoke particles, toxins and carcinogens; and that means a foreign substance has been introduced into your body without your permission. What if someone introduced a prescription drug into your body against your will? Most of us would object strongly, even if it were only a few molecules, and even if we couldn’t demonstrate that it could do us any harm. We would object on principle.

What the Science Really Says

Secondhand smoke is a health hazard but we don’t really have a firm handle on how much smoke is how much of a hazard. Lawmakers should base their decisions on what the science really says, not on hysterical overor under-reactions to the data. In the face of uncertainty, it seems prudent to lean gently to the side of caution without going overboard.

This article was originally published in Skeptic magazine.

Dr. Hall is a contributing editor to both Skeptic magazine and the Skeptical Inquirer. She is a weekly contributor to the Science-Based Medicine blog and is one of its editors. She has also contributed to Quackwatch and to a number of other respected journals and publications. She is the author of Women Aren’t Supposed to Fly: The Memoirs of a Female Flight Surgeon and co-author of the textbook, Consumer Health: A Guide to Intelligent Decisions.