Chlorine is a yellow-green gas with a strong odor that ranks among the top 10 most common chemicals produced in the United States, as of 2011. Used during World War I as a chemical weapon, today its uses include disinfecting everything from tap water to industrial waste and sewage. In swimming pools, chlorine kills potentially hazardous bacteria, but it also adversely affects some swimmers.
The human body absorbs chlorine primarily through inhalation but also via the skin. At low levels, chlorine inhalation causes irritation to the skin and eyes along with a sore throat or a cough. At higher levels, inhalation causes symptoms of asthma such as wheezing and tightness of the chest. Indoor pools without proper ventilation place swimmers at a higher risk of chlorine exposure and associated compounds such as chloroform. Chlorine combines with compounds in sweat and urine to form even more potent irritants called chloramines, which directly cause asthma when inhaled.
Younger swimmers are more susceptible to the effects of chlorine because their bodies absorb it more readily than adults. More chlorine ends up in the blood of children than older swimmers. Children who frequently visit indoor pools have a greater likelihood of developing asthma. Scientists in a 2006 study found an association between the increase in asthma in industrialized countries and increased exposure to the chlorine in indoor pools, as outlined in the journal Occupational & Environmental Medicine. Improved ventilation may not solve the problem, as chlorine is absorbed both through the skin and the airways.
An increased level of activity -- such as swimming -- accelerates the body's absorption of chlorine. Professional swimmers who spend a lot of time in indoor pools may breathe in dangerous amounts of chlorine through their high levels of exertion. Elite athletes often swim multiple times a day, which does not give the body time to purge chlorine from the system before more is absorbed. This could lead to toxic buildup in the body. Competitive swimmers are more likely to suffer from asthma and other respiratory diseases.
Over-chlorinating pools turns the water acidic, which can wear away the tooth enamel of frequent swimmers, a condition called "swimmer's erosion." Joseph G. Hattersley, writing in the Journal of Orthomolecular Medicine in 2000, cites numerous studies linking exposure to high levels of chlorine to a higher risk of diseases such as melanoma, bladder and rectal cancer and asthma. The term "swimmer's asthma" is becoming common as more cases of asthma are linked to competitive swimming in indoor pools. Reducing the disease risks entails reducing the amount of chlorine in swimming pools or switching to an alternative water-treatment method.