LINDA WERTHEIMER, host:

This is MORNING EDITION. I'm Linda Wertheimer.

STEVE INSKEEP, host:

And I'm Steve Inskeep.

Today in Your Health, we're reporting on microbes that you may encounter during swimming season. Ponds and pools and lakes and rivers and oceans are all very inviting this time of the year and inviting to some microorganisms too.

We've sent NPR's Joanne Silberner to find out how often people get sick from swimming and if there's any way to protect yourself.

JOANNE SILBERNER: A summer day just doesn't get any nicer than this - Gunpowder Falls State Park in Maryland. A lush green lawn, a row of shaded trees and picnic benches, a clean broad beach and a sparkling blue river a mile wide. Several dozen people splash about under the watchful eye of a lifeguard.

Babbish Graczyk(ph) sits on a picnic bench. He's also watching the water. In his day job he's a microbiologist at the Johns Hopkins Bloomberg School of Public Health. For the last three years he's been coming here on weekends to play.

Professor BABBISH GRACZYK (Johns Hopkins Bloomberg School of Public Health): This is very nice old recreation and I'm using one place for wind surfing.

SILBERNER: From the start, though, something worried him.

Prof. GRACZYK: What I noticed is that when there are a lot of people, the water is turbid.

SILBERNER: Turbid, murky. People were kicking up sediments. Sediments can carry organisms called protozoa, and some of those can make you sick. At swimming areas that are monitored, local officials routinely test the water for bacteria but not for protozoa.

Graczyk got to get his some grad students.

Prof. GRACZYK: When we tested samples collected on weekdays and weekends, more samples came up positive on the weekend, and also concentration of the pathogens in the weekend samples were higher in the weekday samples.

SILBERNER: So Graczyk found that protozoa were especially prevalent at busy times. But how much should you worry about these and other microbes that can cause problems like diarrhea.

That's a hard question to answer, says Michael Beach. He's head of the Healthy Swimming Program at the Centers for Disease Control and Prevention.

There are no national surveys. But...

Dr. MICHAEL BEACH (Centers for Disease Control and Prevention): We see 30 to 40 outbreaks per year occurring in recreational water. But we think that's the tip of the iceberg of what's actually occurring.

SILBERNER: Outbreaks can affect a few people or a few thousand. Usually the infections aren't serious. But Beach says people with open wounds should be extra careful and...

Dr. BEACH: People who have compromised or weakened immune systems should consider that swimming is a communal bathing activity. So many of the germs that are going to be in those people who are swimming can be transmitted easily to them.

SILBERNER: At Gunpowder Falls State Park, Bill Howard and his wife and his son are eating their lunch. They're unfazed by all this talk about germs.

Mr. BILL HOWARD: No, no. Probably we feel a little bit better after being here.

(Soundbite of laughter)

SILBERNER: Though not all parks are not monitored, this one is. Park officials have closed the beach a few times in the past few years because of bacteria contamination. So Howard figures he's safe. They're paying attention. Whatever Graczyk found, Howard has different concerns.

Mr. HOWARD: Probably I'm a little bit more worried about sun overexposure than what you might get from the water.

SILBERNER: Another picnicker, Kristin Devaux(ph), is here only because her father-in-law likes it. She prefers her water chlorinated like in community pools.

Ms. KRISTIN DEVAUX: Just because I know what's in them.

SILBERNER: She's assuming chlorine takes care of the germs. But maybe not.

Again, the CDC's Michael Beach.

Dr. BEACH: I think that was reasonable until about two decades ago, when we've seen the emergence of a parasite that's actually chlorine-resistant, so now we have a bug out there that can bypass the major barrier to disinfection in swimming pools.

SILBERNER: And nobody routinely checks pools for these chlorine-resistant protozoa. Still, chlorination does take care of most bugs. Michael Beach says busy pools should be chlorinated several times a day. As for rivers, ponds and oceans, think twice before jumping in after a heavy rain, when sewage runoff can contaminate the water.

To protect your fellow swimmers, stay out of the water if you've had a recent gut-wrenching illness, Beach says. And parents, don't rely on swim diapers for your kids.

Dr. BEACH: These are not a panacea. They're not containment vessels. And so if your child is ill with diarrhea, swim diaper or not, they shouldn't be in the water.

SILBERNER: Still, overall your chances of getting sick are pretty low.

Dr. BEACH: There are hundreds of millions of visits to swimming venues every year. Most of those people are just fine.

SILBERNER: Michael Beach regularly swims in a lake - he tries not to swallow the water.

As for Graczyk, who found the protozoa here, he likes this spot on the river. You won't see him packing away his windsurfing gear, unless the wind is better somewhere else.

Joanne Silberner, NPR News.

INSKEEP: If you dare to check on the water quality at your favorite swimming pool or beach, you can get suggestions on how to do that at npr.org.

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