Heading to the beach? You probably slap on sunscreen, slip into a bathing suit and check the park district's Swim Report.

Green flag, OK to swim; red flag, dirty water…right? The city shutters the lakefront when bacteria counts are high-but what, exactly, does a closed beach tell swimmers about the water?

Well … not much.

The thousands who flock to Chicago's beaches each season rely on the park district to tell them when contaminated lake water poses a health risk. The city, following U.S. Environmental Protection Agency rules, tests the water for E. coli bacteria and issues advisories or a swim ban when bacteria levels cross a certain threshold.

But scientists say this system has several major flaws. Not only do test results lag behind changing water conditions, but the test itself can't determine if disease-causing pathogens are actually present.

Bottom line: A closed beach doesn't necessarily mean the water is dangerous, and an open beach doesn't necessarily mean the water is clean.

And there's not much you can do to find out the truth.

How much don't you know about Chicago's beaches?

Myth #1: E. coli

Chicago, like every other waterfront town, is on the lookout for E. coli. But what many beachgoers don't realize is that the type of E. coli that officials test for is not the same strand that sickens some people who eat undercooked beef or unwashed vegetables. That type of E. coli - O157:H7 - causes violent illness and even death, but other types of the bacteria won't hurt humans.

"Ninety-nine-point-nine-nine percent is not a threat," says Dr. Richard Whitman, chief scientist at the U.S. Geological Survey's Lake Michigan Ecological Research Station in Porter, Ind. "It's not a dangerous organism. The O157:H7 'hamburger E. coli' that you read about is very rare."

So why test for it? "E. coli is omnipresent in the human gut," says USGS aquatic biologist Dr. Meredith Nevers, "so anytime there's human sewage, there should be E. coli."

The E. coli that naturally thrives in the intestines of warm-blooded animals is only a signal of the real problem: the potential presence of human waste and the nasty bugs it carries.

"There's a lot of stuff in human sewage," Whitman notes-like clostridium, shigella and hepatitis, to name a few possible pathogens. Swim in water that's been contaminated with sewage and you might get sick-but the chances are overwhelming that E. coli won't be to blame.

Myth #2: If the beach is closed, E. coli counts are high that day

The park district tests water from the city's beaches for E. coli at least five days each week, says spokeswoman Zvez Kubat. If tests find bacteria levels of 235 colonies or more per 1,000 milliliters of water, the city issues a swimming advisory. If levels reach 1,000 colonies or more, the beach closes.

But in reality, even frequent monitoring doesn't do much good. It takes 18 hours for the test results to come back-so beach managers open or close the beaches based on yesterday's water sample.

That's a problem because E. coli levels change quickly as weather, wind and sunlight patterns shift. "There's not much evidence that I'm aware of that yesterday's information is predictive," says Dr. Al Dufour, senior research microbiologist at the EPA's National Exposure Research Lab in Cincinnati.

Which means, says Nevers, "You tend to close the beach too late or leave it open when it should be closed."

There is a better way: sophisticated computer models that crunch the numbers on environmental factors like wave height, wind speed, amount of sunlight and rainfall.

Lake County, Ill., uses a model called SwimCast on three of its beaches. Mark Pfister, associate director of environmental health at the Lake County Health Department, says sensors placed in the water and on land look for conditions that correlate with high E. coli levels. SwimCast uses the data to predict how much bacteria will be found-and Lake County issues advisories or bans based on that prediction.

"The mathematical modeling is an order of magnitude-at least an order of magnitude-more efficient than testing yesterday's water," Whitman says.

But Chicago Park District officials, who could not be reached for comment, still count on tests that lag behind real water conditions when it comes to deciding whether to swim-and we're not alone. "I can tell you scientifically that the system that is being utilized throughout most of the United States, datawise, is not effective," Whitman says. "Across the nation they're more often wrong than right."

Today's beach status doesn't tell you what today's water is really like, but don't despair. A high E. coli count doesn't equal a trip to the emergency room.

Myth #3: High E. coli counts mean there's definitely something nasty in the water

Current testing methods in Chicago detect high bacteria levels a day late, but remember, beach managers are testing for a type of E. coli that isn't harmful to humans. The presence of E. coli suggests that human sewage is also present-but doesn't guarantee it. In fact, E. coli can come from almost anywhere in the beach environment.

According to Whitman, E. coli grows naturally in the sand and on algae in Lake Michigan. It's also found in animal and bird waste-so dogs, cats, pigeons and seagulls can all contribute to a high E. coli count.

And don't forget that we carry loads of the bacteria ourselves. Dufour estimates that one person swimming for 15 minutes will shed 100 million E. coli into the water.

"Even as a surrogate indicator, [E. coli] is ineffective," Whitman says. "It doesn't tell you that it's human. It doesn't differentiate human from seagulls or from sand or naturally occurring E. coli. I don't think E. coli, the word, means a whole lot."

Though the bacteria are found on the beach naturally, heavy rains make water E. coli levels spike. As this storm water, called runoff, washes from the city's streets to the lake, it picks up pet waste and garbage; rain also flushes naturally growing bacteria off the beach and into the water. "The water and sand is one large soup of E. coli," Whitman notes.

Seagull waste and city runoff might sound gross, but scientists say it's not likely to hurt you-even though they are the primary reasons the park district closes beaches. "It is difficult to translate the finding of E. coli [that] has an animal source into a health effect in a human," Dufour says. With only a few exceptions, viruses, parasites and bacteria from animal waste can't be passed to people.

So why don't beach managers skip the E. coli and look for the really bad stuff? The answer is simple, experts say: It's too expensive, time-consuming and difficult to test for each individual pathogen. However imperfect, E. coli is the most accepted method of warning swimmers about the possible presence of something truly threatening: human sewage.

At least Chicagoans don't have to worry too much about untreated sewage getting onto the beaches. In other Midwestern cities, like Detroit and Milwaukee, heavy rains can overwhelm the municipal sewer systems and cause large sewage outfalls directly into Lake Michigan.

But in Chicago, overwhelmed sewers discharge into the Chicago River and other area waterways-which now flow away from the lake. The Metropolitan Water Reclamation District of Greater Chicago monitors more than 200 such points of discharge.

But Chicago's lake isn't completely immune to outfalls. When massive rains overwhelm even the rivers, the city opens any of the three locks that discharge into Lake Michigan. The last time that happened, in August of 2002, 1.75 billion gallons of untreated sewage were dumped directly into the lake.

Human waste may also seep into the water through faulty septic systems or even dirty diapers that find their way onto the beach.

Testing yesterday's water makes it possible for a beach to be open when E. coli counts are high, but the more likely problem is the so-called false closing. "The current approach to keeping swimming waters safe is probably overprotective," Dufour says.

A system so prone to false closings-shutting down the beach when there's no real threat-is bound to leave beach lovers frustrated. Dr. Sabina Shaikh, a senior lecturer in environmental studies at the University of Chicago, even put a dollar amount on how much Chicagoans value their waterfront experiences: $34.94 per beach trip, to be exact. That number, published in a 2004 study, is based on factors like time and money invested in getting to the lake.

Unfortunately for beachgoers left high and dry, scientists say there's not much the average person can do to be sure the water is safe for swimming under the current testing system.

"You live with the system you have," Dufour acknowledges. Whitman adds that concerned swimmers can avoid the beach right after storms that have probably washed extra E. coli into the water.

But researchers are working on ways to make the beach-closing system more accurate nationwide. The EPA is now trying to develop new monitoring techniques and find a better indicator of human waste than E. coli. Research is scheduled through 2010, with new beach monitoring criteria expected by 2012. One piece of technology under consideration is a machine that bumps the wait time for E. coli tests from 18 to two hours.

Experts say that's an improvement, but it's not enough given E. coli's weaknesses as an indicator. "Ultimately we're going to have to measure the pathogens, or groups of pathogens, themselves," Whitman says. "If we can't measure pathogens, then we can measure something that is uniquely human." The EPA is now looking at a number of other possible indicators of human sewage, including caffeine and codeine.

Until the beach monitoring methods are revised, though, Chicago beachgoers must deal with an imperfect system. "That's the best we have to warn people," says Whitman.

There's no need to swear off Chicago beaches-just don't rely on the Swim Report to tell you what's in that water.