In the summertime, there's nothing more inviting than refreshing pools of water. Unfortunately, pools, ponds, rivers and oceans are also contaminated with microbes that could make you sick.
At Gunpowder Falls State Park in Maryland, it's easy to think everything is fine. The park is beautiful, with a lush green lawn, a row of shade trees and picnic benches. There's a clean, broad beach and a sparkling, blue river a mile wide. Several dozen people splash about under the watchful eye of a lifeguard.
But Thaddeus Graczyk knows there is at least the potential for problems. In his day job, he's a microbiologist at the Johns Hopkins Bloomberg School of Public Health. For the last three years, he has been coming here on weekends to play.
"This is very nice spot for recreation," Graczyk says. "I use it for windsurfing."
But right from the start, he noticed something: "When there are a lot of people in this water, the water is turbid."
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 some grad students together. They waded out into the water and collected samples on weekdays, when there were few people in the water, and on weekends, when there were a lot of people.
"More samples came up positive on the weekend," he says. "And concentrations of the pathogens in the weekend samples were higher than the weekday samples."
So how much should you worry about these and other microbes before diving in?
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, he says.
"We see 30 to 40 outbreaks per year occurring in recreational waters, but we think that's the tip of the iceberg," Beach says.
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 so should another group.
"People who have compromised or weakened immune systems should consider that swimming is a communal bathing activity," he says. "They need to consider whether swimming is an option for them."
At Gunpowder Falls State Park, Bill Howard and his wife and son are eating their lunch. They aren't fazed by talk about germs.
Though not all parks are 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 – the closings show officials are paying attention.
"Probably I'm more worried about sun overexposure than what you might get from the water," Howard says.
Another picnicker, Kristin Devaux, is here only because her father-in-law likes it. She prefers her water chlorinated — like in community pools, "because I know what's in them."
She's assuming chlorine takes care of the germs. But does it?
"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 disinfectants in swimming pools," says the CDC's Michael Beach.
And nobody checks pools for these chlorine-resistant protozoa. Still, chlorination takes care of most bugs.
Beach says 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.
Beach also advises people to protect your fellow swimmers and stay out of the water if you've had a recent gut-wrenching illness. And parents shouldn't rely on swim diapers for their kids.
"These are not a panacea," he says. "If your child is ill with diarrhea, swim diaper or not, they shouldn't be in the water."
Overall, your chances of getting sick are slim.
"There are hundreds of millions of visits to swimming venues every year," Beach says. "Most of those people are just fine."
Beach regularly swims in a lake — he just tries not to swallow the water.
As for Graczyk, he likes his spot on Maryland's Gunpowder River. You won't see him packing away his windsurfing gear, unless the wind is better somewhere else.