Hurricane Harvey, a storm described as “catastrophic” has flooded the streets and neighborhoods of Houston. “The breadth and intensity of this rainfall are beyond anything experienced before,” the National Weather Service said in a statement Sunday. “Catastrophic flooding is now underway and expected to continue for days.”

Rising flood water not only displaces people from their homes but poses significant risks to health and safety. The water itself can be full of contaminants, and even when the water subsides, what remains can be risky.

“Flood water mixes with everything below it,” says Dr. Richard Bradley, chief of the division of emergency medical services and disaster medicine at McGovern Medical School at the University of Texas Health Science Center at Houston. “If it covers a field with pesticides, it picks up the pesticides. It can also carry animal waste from fields and forests.”

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Health and environmental experts recommend people avoid intentional contact with flood water due to potentially high levels of contamination. The United States Environmental Protection Agency (EPA) says substances like raw sewage and other chemicals can get into the water, as storage containers of industrial chemicals or solvents can be disturbed or moved by surging flood waters.

Exposure to contaminated flood water is linked to health issues like intestinal problems, upset stomach, headache, and flu symptoms. “Anyone experiencing these and any other problems should immediately seek medical attention,” says the EPA.

“The bacterial count in floodwater is extremely high,” says Bradley. “The chance of getting a skin infection is really quite serious.” Bradley says exposure to chemicals can cause short term poisonings or increase a person’s risk for long term health complications.

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But as footage of the flood areas reveal, many people in affected regions cannot avoid wading through water to reach safer ground. Anyone who is exposed to flood water should frequently wash their hands, inform doctors of any wounds, and make sure kids do not play in flood water or with toys that have been in flood water. Being up to date on their vaccinations is also an important way to lower the risk of getting sick during or after a flood.

Mosquitoes and the diseases they carry, such as Zika and West Nile Virus, are also a health hazard, even after the rains subside. Mosquitoes thrive in standing water, and after Hurricane Katrina, the number of cases of West Nile Virus doubled in affected areas in Louisiana and Mississippi. Bradley says Texas often has a mosquito bloom after bad weather, and while he doesn’t expect the rise in mosquitoes to be worse than after other serious storms, he does recommend residents use mosquito repellent.

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The moisture from standing water after a flood can also spur mold growth, which is why proper clean up is critical for homes and buildings. After Hurricane Katrina, the U.S. Centers for Disease Control and Prevention (CDC) determined that about 46% of inspected homes had visible mold growth and that residents and workers did not consistently use appropriate protection, which put them at risk for developing respiratory problems. “We recommend that when anyone who is cleaning a home after flood wear equipment like shoes, gloves, and a surgical mask of some type,” says Bradley.

When returning home, health experts recommend people remove items that can promote mold, including things like carpeting, wall paper, and clothing. Making sure pets are also checked for flood-related conditions is important. After Hurricane Katrina, a study of 414 rescued dogs and 56 rescued cats found that many had evidence of multiple infectious diseases.

With so many residents forced to walk through high flood water, Bradley recommends that anyone exposed get out of wet clothes quickly, rinse their bodies entirely, and seek medical attention immediately for any infections or unusual symptoms such as upset stomach or diarrhea.

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