A combination of chemical pollutants in US drinking water will result in an estimated 100,000 cancer cases, according to a new assessment by the non-profit organization Environmental Working Group (EWG).

In four states, levels paralleled the rates of cancer associated with air pollution.

One out of every million people exposed to this grouping of contaminants over the course of their lifetime – which for the sake of the study is 70 years – will develop cancer, or about 100,000 people in the US. In other words, the likelihood of you developing cancer from your tap water is about one-in-a-million.

Between 2010 and 2017, researchers used the same approach as the National Air Toxics Assessment that determined the likelihood of developing cancer from common air pollutants. Typically, government tests only examine the effects of one contaminant or a similar group of them at a time. This time around, EWG analyzed the combined health impacts of cancer-causing chemicals in almost 50,000 community water systems, excluding homes that use private well systems, or about 14 percent of the US population.

Altogether, over 100,000-lifetime cancer cases are expected to occur from exposure to carcinogenic chemicals found in tap water, such as arsenic, disinfection byproducts, and radioactive contaminants like uranium and radium. Highest risk water systems were those that serve smaller communities and rely on groundwater, which can be saturated with cross-contamination. However, large surface water systems also contribute to a significant share of overall risk because of the larger population being served. Writing in the journal Heliyon, the team notes that decreasing levels of chemical contaminants in drinking water “represents an important opportunity for protecting public health.”

"Drinking water contains complex mixtures of contaminants, yet government agencies currently assess the health hazards of tap water pollutants one by one," said Sydney Evans, lead author of the paper, in a statement. "In the real world, people are exposed to combinations of chemicals, so it is important that we start to assess health impacts by looking at the combined effects of multiple pollutants."

However, the researchers suggest that their findings are likely conservative estimates due to the fact that they do not include contaminants that are infrequently monitored or not watched at all. Their findings are based on cancer unit risks published by government agencies on certain chemicals and do not contain chemicals outside of that category. Furthermore, the statistics are based on an “evolving body of science” – the actual potency of these chemicals could be higher or lower than previously published data suggests and does not account for groups of people who may be more susceptible such as children or infants.

The authors suggest Americans take a deeper look at the quality of water being consumed every day.

"The vast majority of community water systems meet legal standards. Yet the latest research shows that contaminants present in the water at those concentrations – perfectly legal – can still harm human health," said study co-author Olga Naidenko. "We need to prioritize source water protection, to make sure that these contaminants don't get into the drinking water supplies, to begin with.”

In the meantime, experts recommend installing water filters specific for contaminants commonly found in tap water.