Referring to a recent paper, several outlets came out with headlines such as “Swimming too often in chlorinated water 'could increase risk of developing bladder cancer', claim scientists” and “Swimming too often in chlorinated water 'ups bladder cancer risk.'” The paper, entitled “Socioeconomic status and exposure to disinfection by-products in drinking water in Spain,” is available in full text on line.
The researchers looked at data from a study of people in various regions of Spain. The 1271 subjects in the study had served as controls in a different study that was done 10 years ago. They concluded that those in higher socioeconomic groups drank less tap water but took longer showers and used swimming pools more often. It seems that by-products of chlorination, trihalomethanes (THM), have been linked to an increased incidence of bladder cancer. And THM can be absorbed through the skin, making a link of bladder cancer with swimming plausible.
Here is the catch and it can only be discovered by reading the full text of the paper. Participation in swimming was characterized as follows:
“swimming pool attendance, [was recorded] as a dichotomous variable: attending a swimming pool once or more than once per year contrasted with never (or less than once per year)”
In other words, a subject was asked whether he swam in a pool during a year. If he said “Yes,” he was listed as a swimmer, whether he swam once a year or once a day.
You need not be a scientist or a statistician to see the fallacy of this purported association of swimming and bladder cancer.
By the way, as far as I can tell, none of the 1271 subjects actually developed bladder cancer. The research wasn’t designed to look at that. It was taking a post hoc look at water usage by a group of people who were the subjects of a different research hypothesis.
Also, an author of the paper conceded in the press release that the risk of contracting bladder cancer from swimming was “small.”
Bottom line. Swimmers, please relax and ignore this overblown drivel.
And if you are worried about getting bladder cancer from drinking chlorinated water, here are two interesting points. One, a study from the New England Journal of Medicine a few years ago reported that drinking a lot of fluid decreases the risk of bladder cancer. Two, the incidence of bladder cancer has been remarkably stable over the last 30 years.