Wednesday, July 05, 2006

Surgeon General's Report Casts Doubt on Surgeon General's Statements Regarding the Acute Effects of Secondhand Smoke

Last week, I reported that the Surgeon General's press release accompanying the report on health effects of "involuntary smoking" stated that: "Even brief exposure to secondhand smoke ... increases risk for heart disease and lung cancer..." and I commented that this statement is not supported by the data in the Surgeon General's report itself, which do not document any increased risk of atherosclerosis from a brief tobacco smoke exposure.

The best evidence as to why the Surgeon General's assertion is misleading (if not inaccurate) is the Surgeon Generals' reports on smoking themselves. Most important to consider is the 1990 report on the health benefits of smoking cessation. In this report, the Surgeon General concludes that the increased risk of heart disease among smokers drops dramatically following smoking cessation. Within just one year, the risk of heart disease drops by at least one-half. The increased risk continues to drop, although a bit more gradually, and it disappears completely after about 10 years.

Among light cigarette smokers, the decrease in risk appears to be even more rapid and more dramatic. In fact, among all ex-smokers of less than one-half pack per day, there was no increased risk of heart disease in one large cohort study.

As the Surgeon General concluded: "The data are compatible with a rapid, partial decline in risk, followed by a more gradual decline reaching levels of never smokers after a prolonged period. The initial decline appears to occur within 1 year of cessation or perhaps even less and constitutes a reduction of about one-half or more of the excess risk associated with current smoking. The remaining decline in excess risk is more gradual, with the risks reaching those of never smokers only after a number of years of smoking abstinence. This pattern of decline in excess risk is compatible with multiple effects of smoking on the process of developing CHD, including both short-term influences on platelets and other factors relating to thrombosis which may be more rapidly reversible and long-term increases in atherosclerosis which are only slowly reversible."

The Rest of the Story

The 1990 Surgeon General's report demonstrates that many of the effects of even chronic active smoking on the cardiovascular system are dramatically and immediately (within one year) reversible. This means that in order for active smoking to present a risk of developing heart disease, exposure must not only be chronic, but it must also be continued. As soon as you discontinue the exposure, many of the adverse cardiovascular effects are reversed. This is particularly notable among former light smokers, who cannot be distinguished from lifetime neversmokers in terms of overall heart disease risk in the population.

As the Surgeon General explained, these findings are explained by two types of effects of tobacco smoke: (1) short-term effects on platelets and other factors related to thrombosis (blood clotting) which are rapidly reversible; and (2) longer-term effects of the actual development of atherosclerosis, which are not so rapidly reversible.

The effects of brief secondhand smoke exposure which Surgeon General Carmona is referring to are in the first category: short-term effects on platelets, endothelial function, lipid metabolism, and other factors related to thrombosis and coronary artery elasticity and dilatability. But as Surgeon General Sullivan noted, these effects are rapidly reversible. In order to present a sustained risk of heart disease in persons without existing atherosclerosis, the exposure to tobacco smoke needs to be not only prolonged, but also continued. As soon as the exposure is removed, the risk is going to rapidly return to baseline.

So for an individual who is exposed briefly to secondhand smoke, the exposure is not continued and thus an increased risk of heart disease will not be realized.

If people who have actively smoked 2-3 packs per day for 40-50 years need to continue smoking in order for their risk of heart disease to be maintained, how could it possibly be that a nonsmoker with a single brief exposure to secondhand smoke is at a substantial risk of getting heart disease even if his or her exposure is not continued?

If it were true that people with brief exposure to secondhand smoke were at significantly increased risk of heart disease, then one would certainly observe that such individuals in the population had a higher risk of heart disease than those never exposed. However, this cannot possibly be the case, since people who have actively smoked a quarter pack of cigarettes a day for years but have quit are not at increased risk of heart disease.

For Surgeon General Carmona's statement to be true, it would necessarily have to be the case that brief exposure to secondhand smoke was more hazardous to the heart than actively smoking a small amount (less than 1/2 pack per day) for years. This is obviously a preposterous notion.

The rest of the story is that the documented dramatic and rather immediate reversibility of some of the effects of even chronic active smoking on heart disease risk argue against the validity of an assertion that the effects of a single brief exposure to secondhand smoke are somehow not reversible.

For the Surgeon General's statement that a brief exposure to secondhand smoke is sufficient to cause heart disease to be true, secondhand smoke exposure would have to be more hazardous than active smoking.

It just is not the case.

The conclusion disseminated widely by the Surgeon General concerning the cardiovascular disease risk associated with a brief secondhand smoke exposure is not a valid one. It is not only unsupported by any scientific data, but I have now shown that it is actually inconsistent with existing scientific data and is highly implausible.

Hopefully this misrepresentation of the science of secondhand smoke will be corrected so that the public will not be misled about the risks. As I will discuss in a subsequent post, this could lead to adverse consequences for protecting the public's health.

(Thanks to Dr. Lewis Siegel for the tip).

No comments: