More than 5.6 million Americans have possibly unsafe levels of nitrates in their drinking water, and Hispanic residents are the most affected.

A new study found that for every percent increase in the proportion of Hispanic residents, the public drinking water water was 2 percent more likely to have nitrate levels over 5 ppm.

“In some ways, these seem like small percent increases, but it does suggest overall that Hispanic residents do have a disproportionately high exposure to elevated nitrate levels,” said Laurel Schaider, a research scientist at Silent Spring Institute.

The most common source of nitrate contamination is agricultural fertilizer, and 57 percent of all farmworkers in the U.S. are Hispanic, according to the U.S. Department of Agriculture.

However, Schaider’s research accounted for nearby farmland.

“When we did our statistical models, we found that even when we took into account local agricultural land use, we still saw an association between nitrate levels and the proportion of Hispanic residents,” she said.

Her study also looked at the relationship between poverty, homeownership and nitrate levels in nearby drinking-water sources.

“We did not see an association between more poverty and higher nitrate levels,” she said. “So even accounting for economics, we still saw this association with proportion of Hispanic residents.”

The current federal nitrate standard of 10 ppm was set after it was found that infants who ingested formula made with water that had over 10 ppm of the contaminant can become seriously ill or even die from a condition called blue baby syndrome.

But new research suggests the federal standard may need to be updated.

Studies from the National Cancer Institute consistently find that nitrate levels over 5 ppm increase the risk of colon, kidney, ovarian and bladder cancers.  

“The vast majority, over 99 percent, of the systems that we studied do meet the federal drinking-water standard of 10 ppm,” Schaider said. “But 5 ppm was chosen due to questions about whether the current drinking-water standard is adequately protective to everyone’s health.”

According to the Environmental Working Group, 85 utilities in Arizona  have drinking water with nitrate levels above 5 ppm, affecting 403,910 Arizonans.

“Nitrate can be a marker for other contaminants,” Schaider said. “We’ve found public and private drinking-water wells with high levels of nitrates also have high levels of unregulated wastewater contaminants like pharmaceuticals and consumer product chemicals.”

This isn't the first time it’s been revealed that Hispanics may be exposed to dangerous drinking water at higher rates than other groups.

A 2005 study found Hispanic residents in Arizona were more likely to get their water from a system that exceeded safe arsenic levels.

“People who rely on a public water source can find out if their drinking water has high levels of contaminants by contacting their water supplier or by looking for their annual consumer confidence report,” Schaider said.

For the roughly 4 percent of Arizonans who get their water from a private well, Schaider recommends vigilance and frequent testing.