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Harvey victims face long-term health dangers following the storm's historic flooding

Hurricane Harvey has dumped historic levels of rainfall on southern and eastern areas of Texas, causing catastrophic flooding. Harvey was the single-greatest rainstorm in the history of the continental United States, with rainfall up to 51.88 inches.

The storm is responsible for at least 40 fatalities, and officials fear that the death toll will rise dramatically now that floodwaters are receding.

A 2012 systematic review published in Environment International assessed the impacts of flooding events on human health. The study covered the longer-term health effects and found that the long-term impacts are not well understood. However, the study found that mortality rates were to increase by up to 50 percent in the first year post-flood.

Flooding events place major immediate stresses on human health ranging from floodwater injuries and drowning, to the effects of exposure to contaminated water. But there are many long-term mental and physical health impacts that threaten flood-affected areas.

Harvey by the numbers AP

Moses Juarez, left, and Anselmo Padilla wade through floodwaters from Tropical Storm Harvey on Sunday, Aug. 27, 2017, in Houston, Texas. (AP Photo/David J. Phillip)

Contaminated water poses both immediate and potential long-term threats. Industrial and hazardous waste sites leak human waste and other dangerous toxins into floodwaters, increasing the risk of infectious diseases.

Waterborne and communicable respiratory and gastrointestinal diseases often spread as a result of the contaminated water. While some of these diseases appear immediately, others will linger post-flood.

The level of water contamination from Harvey is currently unknown. The flood-affected regions are home to dozens of Environmental Protection Agency Superfund sites that hold dangerous toxins and chemicals.

"There is going to be a level of contamination that we have never seen before in floodwater because of the proliferation of chemical plants and oil refineries," Dr. Irwin Redlener, director of the National Center for Disaster Preparedness at Columbia University, said to NBC News.

The risk of the spread of infectious diseases is further increased by the rise in mosquito populations. While mosquito populations were initially wiped out during the storm, populations will surge following the flood. The remaining floodwater will serve as an optimal breeding ground for mosquitoes.

A year after Hurricane Katrina in 2006, a study found that the number of cases of West Nile infection increased by two-fold in affected areas. The study suggested that exposure was widely the cause of the increase, as evacuees often spent days outside waiting for rescue.

Flooding and water damage in buildings from Hurricane Harvey will likely contribute to the growth of mold, as demonstrated in previous natural disasters.

Many molds reproduce by forming spores that are released into the air. Spores will land and begin to grow on a suitable moist surface. Mold penetrates porous materials and release chemicals, according to Occupational Safety and Health Administration (OSHA).

"Most molds are harmless but some can cause infections, allergy symptoms and produce toxins. Infections are rare in healthy individuals and the effect of toxins is still not well understood," according to OSHA.

Mold Superstorm

In this Jan. 24, 2013 photo, Victor Pena of Flag Enterprises steam-cleans a floor in a home in Massapequa, N.Y. Homeowners in New York and New Jersey are struggling to combat outbreaks of mold from water-logged homes in the wake of Superstorm Sandy (AP Photo/Frank Eltman, File).

There is evidence that breathing in mold can also worsen existing respiratory problems like asthma, exacerbate allergies and sicken those with weakened immune systems, according to the World Health Organization.

The increase in mold following Hurricane Sandy led to serious debates among officials. In 2013, the New York congressional representatives asked the federal government to help resolve what they called, “this emerging crisis.”

Similarly, mold also caused panic following Hurricane Katrina. Some studies found that mold was present in about half of the flooded homes. Fear arose among relief workers and residents, who felt they were not adequately protecting themselves from breathing in mold particles.

The long-term health effects of Hurricane Sandy and Hurricane Katrina are still being monitored. The potential long-term health threats posed by Hurricane Harvey will further be assessed as the floodwater continues to recede.