Pool Chlorine Hypothesis Remains Unproven

Can pool swimming promote the development of asthma in children or help alleviate its symptoms? This question has been debated among researchers ever since Belgian Professor Alfred Bernard published a 2006 study supporting the “pool chlorine hypothesis”. That hypothesis suggests that the increasing exposure of children to pool chlorine could be contributing to the rise of childhood asthma in the developed world. Other studies have found swimming improves asthma symptoms; Welsh et al., for example, reviewed the relevant scientific literature and found “positive effects of swim training on fitness as measured by improved aerobic efficiency, physical working performance, and recovery heart rates.”

Swimming researcher Dr. Joel Stager says swimming is “the only activity we know of where you can say that if that’s all you do for exercise, you can be almost perfectly fit.
Swimming researcher Dr. Joel Stager says swimming is “the only activity we know of where you can say that if that’s all you do for exercise, you can be almost perfectly fit.”

Since the Bernard study appeared, there has been much discussion of the pool chlorine hypothesis in the popular and scientific press. No research has confirmed Bernard’s findings. An extensive review of the literature conducted by Goodman and Hays in 2008 found asthma to be more likely among elite swimmers than among similar high-level participants in other competitive sports. Interestingly, it is unclear whether asthma in high-performance swimmers developed as a result of swimming or whether many high-performance swimmers are asthmatics who took up the sport when encouraged by their doctors to swim for therapeutic exercise (a true chicken and egg question–which came first?). That question aside, researchers could find no consistent association between asthma development and swimming pool use during childhood.

The Belgian Minister of Public Health in 2009 requested detailed scientific advice on this matter from her advisory body, the Superior Health Council. The Council responded in February, 2011 with a report stating that although a relationship between swimming pool attendance and childhood asthma has not been confirmed, it cannot yet be excluded. The Council concluded that, for now, the available evidence does not support advising children against swimming in chlorinated pools. That’s important because there are significant benefits to swimming.

How is a child’s life improved by learning to swim?

  • According to the Centers for Disease Control and Prevention (CDC) participation in formal swimming lessons can reduce the risk of drowning by 88 percent among children aged one to four years old.
  • Swimming can yield a broad range of benefits throughout a person’s lifetime. According to information on the National Swimming Pool Foundation website, swimming provides even resistance and a full-body workout, improving cardiovascular health, muscle tone and overall flexibility.
  • As many children and adults would agree, swimming is fun!

Keeping Swimming Healthy

A well-maintained pool is critical to healthy swimming, according to CDC. Proper pool chemistry, including adequate chlorine and pH, helps destroy waterborne pathogens that can cause everything from swimmer’s ear to gastrointestinal illness. Swimmers must also play a role in keeping swimming healthy by showering before swimming and not peeing in the pool. When pool operators and swimmers do their part to maintain healthy pools, there are fewer irritating disinfection byproducts generated in pools.

As with a lot of issues in life, everyone needs to balance known and suspected risks with known and suspected benefits. Early in my career as a physician, I was interested to learn that even bed rest has known risks. If swimming is important to your family, it’s important to stay informed on the latest health research. For now, the conclusion of the Belgian Superior Health Council is positive for parents who want their children to reap the considerable benefits of pool swimming.

Ralph Morris, MD, MPH, is a Physician and Preventive Medicine and Public Health official living in Bemidji, MN.