The New York Post‘s headline screams

Chemical found in US drinking water is linked to 15% higher risk of colorectal cancer, 33% for bladder cancer

The chemical in question is a collection of trihalomethanes (THM)—chloroform, bromodichloromethane, dibromochloromethane, and bromoform—that can form when chlorine reacts with water. Chlorine is routinely added to our nation’s potable water by water producers in order to eliminate bacteria and viruses from our drinking water. Chlorine is very effective at that.

The second paragraph in the Post‘s article lays out the…fear:

A new analysis out of Sweden reports that disinfecting water with chlorine creates chemical byproducts that can increase the risk of bladder cancer by 33% and colorectal cancer by 15%.

What’s the baseline rate of those cancers in the US, though? How big an increase are those 15% and 33% bumps? Those bits of context, critical to understanding the true nature of these risks, are elided by the news outlet.

Here are some data with which to fill in those gaps. According to the American Cancer Society,

[T]he lifetime risk of developing colorectal cancer is about 1 in 24 for men and 1 in 26 for women.

According to the National Cancer Institute,

Approximately 2.2% of men and women will be diagnosed with bladder cancer at some point during their lifetime

Applying the increased risk values to these really quite low baseline probabilities, we get

• likelihood of men getting colorectal cancer rises from 4.2% to 4.8%
• likelihood of women getting colorectal cancer rises from 3.8% to 4.4%
• likelihood of men or women getting bladder cancer rises from 2.2% to 4.2%

Keep in mind the lifetime nature of those risks, which is nicely matched by the lifetime practice we have of drinking chlorinated tap water. Thus, it’s certainly true that these increases are worth watching, especially the bladder cancer risk increase, but these already very low baseline risks remain quite low even with chlorinated water.

Further balancing those risk increases are the enormously reduced risk of infection by serious and debilitating water-borne disease.