As schools across the country begin reopening after months of closure, some districts are learning that hazardous material accumulated in their plumbing systems, and experts say stagnant water poses health risks to students and staff.

In Michigan and North Carolina, reports of elevated lead levels and bacteria in the water are scaring school officials. Their worry is that students and staff might be exposed to dangerous drinking water.

Lead can enter water when plumbing materials that contain it begin to corrode, according to the Environmental Protection Agency. Flushing school plumbing systems with fresh water is one way to ensure hazardous material does not form or linger in the water. But because of concerns related to the pandemic, schools across the United States have generally been closed for months.

According to the Environmental Protection Agency, lead is a toxic metal "linked to damage to the central and peripheral nervous system, learning disabilities, shorter stature, impaired hearing, and impaired formation and function of blood cells" in children.

There is no safe level of lead in drinking water, Elin Betanzo, a water engineer and the founder of Safe Water Engineering, a Michigan consulting firm that works to improve access to safe drinking water, told Insider.

"The longer we let it sit there, potentially, the greater concentration of chemicals are in the water," said Andrew Whelton, a water quality researcher and professor of civil engineering and environmental and ecological engineering at Purdue University.

In addition to harmful metals that seep into stagnant water, Whelton said that the lack of running water can allow a "slime" layer of bacteria and other organisms known as a biofilm to grow, allowing harmful bacteria, like Legionella — the cause of Legionnaires' disease — to enter the water.

"When water is moving, you have water flowing, and because of that, you have shear forces that don't allow the slime layer to get that thick," he said. "And the thicker of the slime layer, potentially the easier it is for pathogens and disease-causing organisms to hide in it. And when the water stops moving, then the bacteria and other organisms will come out into the water."

Read the full article about stagnant water by Connor Perrett and Yelena Dzhanova at Business Insider.