Researchers report that drinking chlorinated water is associated with a statistically significant 60 percent increase in bladder cancer among a population of 3,000 residents of Iowa, including all bladder cancer cases identified in Iowa during 1986-1989.
But the study is not convincing for a number of basic reasons:
- The reported 60 percent increase is too small an increase to be reliably detected through this type of study (case-control epidemiology)
- The results are not based on verified exposures. Study subjects were asked to recall (as far back as 60 years) their usage of chlorinated water. No records of actual usage were available.
- The reported 60 percent increase in bladder cancer was observed only among those classified as using chlorinated drinking water for more than 60 years. But this group was small (i.e., less than 100 cases) and statistically weak. No other usage group was associated with a statistically significant increase in bladder cancer.
- Reported increases in bladder cancer rates were observed only in smokers -- a demographic group with higher bladder cancer rates to start with. Non-smokers were reported to have a decreasing bladder cancer rate with increasing consumption of chlorinated water.
- Inexplicably, no increase in bladder cancer was observed among women.
- What happens to the reported results if data from other than Iowa during 1986-1989 is considered? Does even this weak association vanish?
In the end, chlorine is the most effective and inexpensive drinking water disinfectant. It's only too bad chlorine can't disinfect epidemiology.
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Copyright © 1997 Steven J. Milloy. All rights reserved. Site developed and hosted by WestLake Solutions, Inc.