According to researchers in Flint, Michigan, blood tests have revealed a distressing conclusion—the tap water residents are drinking is causing elevated lead levels in the city’s children.

A new study headed by Dr. Mona Hanna-Attisha, a researcher and director of the pediatric residency program at Hurley Medical Center, compared blood tests taken over a span of months in 2013 with samples taken over a similar span this year. During the time between, the city of Flint changed the source of its drinking water from Lake Huron to treated water from the Flint River.

What they found by comparing the samples is that the percentage of children with lead levels over the acceptable limit nearly doubled after the change. In 2013, about 2 percent of tests had results over 5 micrograms per deciliter (the current “safe” cut-off level, above which steps to identify and mitigate the source of lead exposure are recommended). About 4 percent of the later samples revealed elevated levels. Areas that yielded the highest blood levels had an increase in abnormal samples from 2.5 percent to 6.3 percent.

The consequences of increased lead exposure, especially for younger children, can be dire. Symptoms can include developmental delays, irritability and sluggishness, as well as gastrointestinal problems like poor appetite, abdominal pain, and vomiting. Newborns can experience not only slowed development, but slowed growth.

The new study recommends that Flint city water not be used to mix infant formula, or consumed by pregnant women.

The reason the new water source may be contributing to these elevated lead levels does not have to do with the lead content of the river water itself, but rather its effects on the lead pipes that approximately 15,000 homes in the city are reported to have.

A group of volunteer researchers from Virginia Tech University have been doing ongoing work on the safety of Flint drinking water. Their lead investigator, 2007 MacArthur fellow Dr. Marc Edwards, has noted that treated Flint River water is 19 times as corrosive as water from Lake Huron. This causes the treated water to leach lead from pipes and soldering.

In a separate report, the Virginia Tech team analyzed Flint city water directly and found that 42 percent of the 120 samples collected had lead levels greater than 5 parts per billion, and 20 percent had levels about 15 ppd, the cut-off at which the Environmental Protection Agency requires action.

What is to be done about this problem remains to be seen. Dr. Hanna-Attisha has said that city officials told her a return to Lake Huron water is not financially feasible for Flint. The mayor is asking the state for $10 million in funds to replace lead water service lines.