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Relative Risk

Fact: The goal of an epidemiological study is to determine Relative Risk (RR).

Relative risk is determined by first establishing a baseline, an accounting of how common a disease (or condition) is in the general population. This general rate is given a Relative Risk of 1.0, no risk at all. An increase in risk would result in a number larger than 1.0. A decrease in risk would result in a lower number, and indicates a protective effect.

For instance, if a researcher wants to find out how coffee drinking effects foot fungus, he first has to find out how common foot fungus is in the general population. In this fictional example, let's say he determines that 20 out every 1,000 people have foot fungus. That's the baseline, a RR of 1.0. If he discovers that 30 out of 1,000 coffee drinkers have foot fungus, he's discovered a fifty percent increase, which would be expressed as a RR of 1.50.

If he were to find the rate was 40 out of 1,000, it would give him a RR of 2.0.

He might find foot fungus was less common among coffee drinkers. A rate of 15 out of 1,000 would be expressed as a RR of 0.75, indicating that drinking coffee has a protective effect against foot fungus.

The media usually reports RRs as percentages. An RR of 1.40 is usually reported as a 40% increase, while an RR of .90 is reported as a 10% decrease. (In theory, at least. In practice, negative RRs are seldom reported.)

Note: Some studies calculate an Odds Ratio (OR) instead of an RR. The formulas for determining the two numbers are different, but when studying rare diseases the results are approximately the same. When studying more common diseases ORs tend to overstate the RR.

Fact: As a rule of thumb, an RR of at least 2.0 is necessary to indicate a cause and effect relationship, and a RR of 3.0 is preferred.

"As a general rule of thumb, we are looking for a relative risk of 3 or more before accepting a paper for publication." - Marcia Angell, editor of the New England Journal of Medicine"

"My basic rule is if the relative risk isn't at least 3 or 4, forget it." - Robert Temple, director of drug evaluation at the Food and Drug Administration.

"Relative risks of less than 2 are considered small and are usually difficult to interpret. Such increases may be due to chance, statistical bias, or the effect of confounding factors that are sometimes not evident." - The National Cancer Institute

"An association is generally considered weak if the odds ratio [relative risk] is under 3.0 and particularly when it is under 2.0, as is the case in the relationship of ETS and lung cancer." - Dr. Kabat, IAQC epidemiologist

This requirement is ignored in almost all studies of ETS.

You want studies? Here you go

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While it's important to know the RR, it's also very important to find the actual numbers. When dealing with the mass media, beware of the phrase "times more likely."

"In general, there was no elevated lung cancer risk associated with passive smoke exposure in the workplace. ..." Brownson et. al., 1992 "Passive Smoking and Lung Cancer in Nonsmoking Women" American Journal of Public Health, November 1992, Vol. 82, No. 11

"... an odds ratio of 0.91 ... indicating no evidence of an adverse effect of environmental tobacco smoke in the workplace." Janerich et al., 1990 "Lung Cancer and Exposure to Tobacco Smoke in the Household" New England Journal of Medicine, Sept. 6, 1990

"... the association with exposure to passive smoking at work was small and not statistically significant." Kalandidi et al., 1990 "Passive Smoking and Diet in the Etiology of Lung Cancer Among Non- Smokers" Cancer Causes and Control, 1, 15-21, 1990

"Among women exposed only at work, the multivariate relative risks of total CHD were 1.49 ... among those occasionally exposed and 1.92 ... among those regularly exposed to secondhand smoke, neither of which is statistically significant according to commonly accepted scientific standards." Kawachi et al., 1997 "A Prospective Study of Passive Smoking and Coronary Heart Disease" Circulation, Vol. 95, No. 10, May 20, 1997

"No association was observed between the risk of lung cancer and smoking of husband or passive smoke exposure at work." Shimizu et al., 1988 "A Case-Control Study of Lung Cancer in Nonsmoking Women" Tohoku J. Exp. Med., 154:389-397, 1988

"We did not generally find an increase in CHD [coronary heart disease] risk associated with ETS exposure at work or in other settings." Steenland et al., 1996 "Environmental Tobacco Smoke and Coronary Heart Disease in the American Cancer Society CPS-II Cohort" Circulation, Vol. 94, No. 4, August 15, 1996

"... no statistically significant increase in risk associated with exposure to environmental tobacco smoke at work or during social activities...." Stockwell et al., 1992

"Environmental Tobacco Smoke and Lung Cancer Risk in Nonsmoking Women" Journal of the National Cancer Institute, 84:1417-1422, 1992

"There was no association between exposure to ETS at the workplace and risk of lung cancer." Zaridze et al., 1998 "Exposure to Environmental Tobacco Smoke and Risk of Lung Cancer in Non- Smoking Women from Moscow, Russia" International Journal of Cancer, 1998, 75, 335-338

"Field studies of environmental tobacco smoke indicate that, under normal conditions, the components in tobacco smoke are diluted below existing Permissible Exposure Limits (PELs), as referenced in the Air Contaminant Standard (29 CFR 1910.1000)."--OSHA, July 8, 1997

"I assumed tobacco smoke and pollution were the problem (the rising asthma and respiratory diseases in children) -- this was the politically correct way to think. But these factors turned out not to play a major role."--Dr. Fernando Martinez, co-author, EPA Report, director of respiratory sciences at the University of Arizona--Atlantic Monthly, May 2000

"The data are insufficient to claim that secondhand smoke causes asthma. Further research is needed to determine if a causal link...can be established."--EPA's Dr. Morton Lippmann, Congressional Testimony