A “shallow water blackout” is a silent killer

In Jacksonville, Florida, a 50-year-old woman was found at the bottom of her backyard swimming pool. She was an experienced scuba diver who “often stayed at the bottom of the 9-foot deep end without oxygen to increase [her] lung capacity for future dives.”

Despite receiving CPR from her son, she could not be revived.

The Associated Press story about this tragic incident did not explain why a swimmer with her background drowned.

It appears to be a classic case of “shallow water blackout.” This phenomenon occurs when people hyperventilate before diving.

An increasing level of carbon dioxide (CO2) is what triggers the urge to breathe. Hyperventilating causes hypocapnia, a reduced amount of CO2 in the blood. If a swimmer uses up enough oxygen to pass out before the CO2 trigger point for breathing is reached, drowning will occur without notice. Victims are usually found at the bottom of the pool.

Here’s what it looks like in a diagram from Wikipedia:

A physician who lost her son to this little-known phenomenon started a website to heighten awareness of the problem. The site contains more information and stories about other drownings caused by shallow water blackouts.

Here is a video of a woman swimming laps of a pool underwater. Advance to the 0:50 point and watch what happens as she begins to slow down. [Addendum 8/13/15 12:50 pm: Warning. The video is graphic. It shows the unconscious swimmer being pulled from the water.]
 

A shallow water blackout may have been responsible for the death of Natalia Molchanova, the world’s foremost freediver, who went missing a few days ago.

Hyperventilating prior to diving is not recommended. Tell your friends.

Swimming in Chlorinated Water Causes Bladder Cancer (Not)

Here is a great example of what can happen when a press release is published as news.

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.