Ask the Doctors

Dear Doctor: With cooler weather here in Minnesota, our kids, who love swimming, are switching from the lake to the pool at our local Y. Just how dangerous is it to swim in a public pool? I keep hearing there's a bug that chlorine doesn't kill.

Dear Reader: Any time you share a common space, whether with one other person or a crowd, you run the risk of being exposed to whatever germs, viruses, parasites, bacteria, molds or other environmental hitchhikers they may have brought along. Unsurprisingly, this applies to the public swimming pool, where diarrhea is the most common recreational water illness. And while chlorine significantly reduces the potential health risks posed by public swimming pools, it's not a guarantee.

When used in the proper amounts needed to maintain a consistent pH, chlorine eliminates all manner of nasty bugs. Chlorinated pool water kills E. coli in less than a minute. Norovirus and Hepatitis A perish after 15 minutes. Giardia lasts for only about 45 minutes in properly chlorinated water.

However, Cryptosporidium, a microscopic parasite that causes a diarrheal disease that can last two to three weeks, is different. Due to a protective coating during part of its life cycle (we'll get to that in a minute), Cryptosporidium, commonly known as Crypto, can survive for up to 10 days in the pool, even one that is properly maintained. According to the Centers for Disease Control and Prevention, in recent years Cryptosporidium has become the leading cause of swimming pool-related outbreaks of diarrheal illness in the United States.

Any time an infected person passes any amount of feces into the water, they're also releasing the parasite. At this point in its life cycle, Crypto is encased in a hardy, thick-walled shell, a form known as an oocyst. When a swimmer somehow swallows or inhales water that contains an oocyst, the parasite enters the body. It then moves through the digestive system and into the cells lining the gut, where it begins to multiply rapidly. Up to 100 million oocysts can be released with a single bowel movement.

It's not just swimming pools that are at risk when used by people infected with Crypto. Water parks, hot tubs and recreational fountains can become contaminated as well. So can lakes and the ocean, although there, with the vastly larger volume of water, the odds of contact are somewhat reduced.

Ironically, the very chemicals used to keep swimming pool water clean can themselves become health hazards. The scent of chlorine, particularly in indoor pools where adequate ventilation can be difficult, is a sign that chloramines, a compound present in chlorinated water, has turned to gas. Chloramines can cause nasal irritation, coughing or wheezing, and can even trigger asthma attacks.

Your best defense is insisting on good pool etiquette.

-- Choose a pool that insists patrons take a pre-swim shower.

-- Make sure that anyone with diarrhea never goes into the pool.

-- If you have young children, take them on frequent bathroom breaks.

-- Remember: Diapers don't belong in swimming pools.

-- Any time you see feces in the water, or when the smell of chlorine becomes strong, tell pool management.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

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