It’s summer. It’s hot. And you want to cool off. If you’d like a swim, it’s okay to take a dip in the Potomac River.

Unless you’re in the District.

In that case, don’t ever swim. Every inch of the District’s 45 miles of Potomac watershed streams and rivers is so tainted with bacteria from sewer overflows and other pollutants from stormwater runoff that the city prohibits swimming. If you jump into the river from any spot in the District, don’t be surprised if the Harbor Patrol swings by and orders you out.

Alexandria doesn’t want you in the water, either. Entering the Potomac for swimming from any public location there is a violation of a city ordinance, and it can lead to a fine. Swimming off private land is okay. But Alexandria’s health officials strongly advise against it for the same reason the District does: bad water quality.

Elsewhere, you can wade in for a swim.

Unless it rains heavily.

In that case, Maryland, which has technical jurisdiction over the river in the Washington region and controls policy decisions, advises all bathers to stay out of the water for at least two days. Bacteria often exceed acceptable levels after rains. Warnings for Maryland state parks and beaches are posted on the Web sites of the state’s departments of the environment, natural resources and health. The Virginia Department of Health also has a Web site with tips and points of contact for swimming.

Knowing when conditions are right for swimming can get complicated, says Clifford Mitchell, assistant director for environmental health and food protection at Maryland’s Department of Health and Mental Hygiene.

When rains wash motor oil, fertilizers, animal waste and city and suburban chemicals into waterways, the rule of thumb for bathing is to pick another day.

“We encourage people to stay out of the water after heavy rains,” Mitchell says. Bacteria lurk in the water waiting for a host. Any opening in the skin, the smallest cut or scratch, is a welcome mat for disease.

Mitchell says swimmers should search the state’s “healthy beaches” site for safety updates.

“It will tell you where there are perpetual algae blooms and other areas of concern,” says Mitchell. Green algae fed by pollutants such as nitrogen and phosphorus often cause skin irritation, vomiting and diarrhea. From the shore, it looks like a coat of thick green paint.

Otherwise, Mitchell and other experts say, jump on in. That is, unless the combined sewers that DC Water operates in the District overflow. And unless the Washington Suburban Sanitary Commission, which serves Montgomery and Prince George’s counties, experiences a problem with the pumps that control excess storm water.

Long story short: Some of DC Water’s pipes are inadequate for handling storm water along with the stuff that goes down sinks and toilets. Likewise, WSSC’s pumping stations sometimes get overwhelmed.

Rather than allow a mess to bubble back up into kitchen and bathroom drains, DC Water releases untreated wastewater mixed with chemically laced storm water into the Potomac and Anacostia rivers and Rock Creek.

During the summer, the Interstate Commission on the Potomac River Basin can field several calls a week from people wanting to know if a favorite spot on the water is safe.

“With few exceptions, we are unable to provide a definitive answer,” the agency’s Web site says.

So just to be safe, the commission advises:

Don’t enter the water for several days after a big rainstorm. Don’t swallow water. Don’t get in if you have a cut or open sore: It’s like a patio door for germs. Don’t swim if you have an immunosuppressive disease or general poor health.

And, swimmers, don’t forget to wash when you get out.