Monday, March 17, 2008

The WHO Study (Part 1)

A comprehensive study on ETS was designed in 1998 by a WHO subgroup called the International Agency on Research on Cancer (IARC). It compared 650 lung-cancer patients with 1,542 healthy people in seven European countries. The results were expressed as "risk ratios," where the normal risk for a non-smoker of contracting lung cancer is set at one. Exposure to secondhand smoke in the home raised the risk to 1.16 and in the workplace to 1.17. This supposedly represents a 16% or 17% increase. But the admitted margin of error is so wide (0.93 to 1.44) that the true risk ratio could be less than one, making second-hand smoke a health benefit.”

To fully appreciate what the study says, think about those polls that keep cropping up from Angus-Reid, Gallop and others. They all include a disclaimer to the effect that, “This poll is correct within a margin of error of plus or minus four basis points in 19 out of 20 cases.”

In the world of epidemiology, the margin of error becomes a “confidence interval” and is expressed as a range of values. In the case of the WHO study, the confidence interval for spouses exposed to second hand smoke is .93 to 1.44. The confidence level of the study was 95% (which compares to a poll’s 19 out of 20 cases). The WHO study refers to an OR (odds ratio), which is comparable to the relative risk factors used in North American studies.

This means that the relative risk factor is not set definitively at 1.16, but rather that it will fall somewhere between .93 and 1.44 (the confidence interval), 95% of the time. A relative risk of at least 2.0 is usually required to indicate a cause and effect relationship, and a relative risk of 3.0 is preferred.

Marcia Angell, editor of the New England Journal of Medicine says, “As a general rule of thumb, we are looking for a relative risk of 3.0 or more before accepting a paper for publication." And this from the (US) National Cancer Institute “Relative risks of less than 2.0 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." Or this from Robert Temple, director of drug evaluation at the (US) Food and Drug Administration "My basic rule is if the relative risk isn't at least 3.0 or 4.0, forget it."

The abstract from the study speaks for itself:

  • ETS exposure during childhood was not associated with an increased risk of lung cancer (odds ratio [OR] for ever exposure = 0.78; 95% confidence interval [CI] = 0.64-0 - 96).
  • The OR for ever exposure to spousal ETS was 1.16 (95% CI = 0.93 - 1.44). No clear dose-response relationship could be demonstrated for cumulative spousal ETS exposure.
  • The OR for ever exposure to workplace ETS was 1.17 (95% CI = 0.94 - 1.45), with possible evidence of increasing risk for increasing duration of exposure.
  • No increase in risk was detected in subjects whose exposure to spousal or workplace ETS ended more than 15 years earlier.
  • Ever exposure to ETS from other sources was not associated with lung cancer risk.
  • Risks from combined exposure to spousal and workplace ETS were higher for squamous cell carcinoma and small-cell carcinoma than for adenocarcinoma, but the differences were not statistically significant.

CONCLUSIONS: Our results indicate no association between childhood exposure to ETS and lung cancer risk. We did find weak evidence of a dose-response relationship between risk of lung cancer and exposure to spousal and workplace ETS. There was no detectable risk after cessation of exposure.

In simpler terms, there was no additional risk detected from lung cancer for children exposed to secondhand smoke and the increased risk for spouses exposed to secondhand smoke was “weak”, meaning it was suggestive of an increased risk, but not conclusive. In fact, the results of the WHO study were “statistically insignificant.”

For more information on the WHO Study:The Hittman Chronicles

No comments: