The Environmental Working Group (EWG)'s Tap Water Database reveals the states with the highest numbers of contaminants across their drinking water systems, with some states having more than 150 contaminants in the water residents consume.

Some of these contaminants were found at extremely low levels, below both legal limits and the EWG's health guidelines, while other contaminants, some of which are widely associated with adverse impacts on human health, far exceeded even the Environmental Protection Agency (EPA)'s legal limits in some states.

The quality of U.S. drinking water has been increasingly brought into the spotlight. Various studies highlight the harm of contaminants widely found in drinking water systems, even at levels lower than the EPA deems safe. Additionally, the EPA's own data shows that millions of Americans are drinking water polluted with carcinogenic chemicals.

Although more and more data identify the harm of contaminants found in drinking water, the EPA has been adjusting a number of its regulatory measures, sparking concern among environmental associations and experts.

The EPA imposes maximum contaminant levels (MCLs) on a number of contaminants it deems harmful to human health in the National Primary Drinking Water Regulations. Cost, feasibility and public health considerations are all weighed to determine what the EPA believes are safe, acceptable levels.

The states with the highest numbers of contaminants across their drinking water systems included Texas with 207 contaminants, New York (197), California (175) and North Carolina (133), as shown in the map above.

In Texas, out of the 207 contaminants found in drinking water served by its 4,673 utilities, 14 of those exceeded legal limits, according to EWG.

Almost 100,000 residents in the state are drinking water with levels of total trihalomethanes (TTHMs) higher than deemed safe, which are formed during the water treatment process and are a human carcinogen.

More than 65,000 residents are drinking water in the state with unsafe levels of arsenic, an element that is also a known human carcinogen.

In New York, out of the 197 contaminants found in drinking water served by its 2,263 utilities, 11 exceeded legal limits, per the EWG database. While arsenic and TTHMs surpassed legal limits in some water utilities in the state, it was unlawful levels of haloacetic acids (HAA5) and trichloroethylene that affected the most residents -- more than 40,000 combined.

HAA5 are formed when disinfectants such as chlorine are added to water. High-level exposure to the acid is associated with birth defects and cancer, while trichloroethylene is an industrial solvent that can damage the immune system and has been associated with cancer as well, according to EWG.

Meanwhile, the states with the lowest number of contaminants were North Dakota and South Dakota, which had 46 and 44, respectively.

None of the contaminants in North Dakota exceeded legal limits, although a number exceeded the EWG's health guidelines. For South Dakota, two contaminants exceeded legal limits: radium, a radioactive element that may cause bone and other cancers according to EWG, and arsenic. However, this was only the case for one water utility each, where both served a much smaller number of people.

The reasons California, Texas and New York all have higher numbers of contaminants in their water is largely due to their higher populations and "substantial industry, including a lot of petroleum activity," Holly Michael, director of the Delaware Environmental Institute and professor of Earth Sciences and Civil, Construction and Environmental Engineering at the University of Delaware, told Newsweek.

She said that the main sources of contaminants are people, particularly their household waste and sewage; industry, such as chemical contaminants and petroleum byproducts; agriculture, due to fertilizers and pesticides; and geology, where contaminants are naturally occurring.

"The more people, industry, and agriculture we have, the more contamination we are likely to see," she said, adding that it also depends on local geology and where water is sourced from.

Another contributor is that different-sized utilities also have different sampling frequencies, Paul Westerhoff, a professor of sustainable engineering and the built environment at Arizona State University, told Newsweek.

He said that, for example, there are nine compounds of haloacetic acids. While the EPA currently regulates only five of them, some utilities and states report all nine, whereas others only report the five on which the EPA has implemented regulations.

This means that states "can be more (but not less) stringent than federal EPA regulations," resulting in slightly different reporting requirements between states, Westerhoff said.

Holly Michael, director of the Delaware Environmental Institute and professor of Earth Sciences and Civil, Construction and Environmental Engineering at the University of Delaware, told Newsweek: "What we should be most concerned about is contaminants above the legal limit, and what the concentrations are. The number of contaminants overall is one thing, but many different constituents can be considered contaminants, and some are harmful at all levels, while some are not harmful at lower levels. So the actual contaminant, its concentration in drinking water, and its potential health effects are the more critical things to consider."

She added, "We should be concerned about contaminants in our drinking water, and it's important that we all know where our water comes from and consider reports on its quality. For the most part in the U.S., tap water is just as safe as bottled water. But if we know what's in our water, we can take additional measures if needed, like using a simple activated carbon filter, for example. We should also make sure that laws and guidelines are being established and enforced to protect us, because the vast majority of these contaminants cannot be seen or tasted, and because once our water supply is contaminated, it can be difficult or impossible to clean it up."

While drinking water contamination is widespread, the EPA is looking to implement a number of changes to drinking water regulations, particularly in regards to PFAS chemicals.

PFAS chemicals, a group of thousands of different substances, are classified as a Group 1 carcinogen by the International Agency for Research on Cancer and have been widely found in U.S. drinking water systems.

Michael said that PFAS chemicals are now being recognized as a carcinogen, even at "extremely low levels," and that they're "everywhere" because they're so widely used.

MCLs for these chemicals were previously established, with water utilities instructed to meet new guidelines by 2029, but the EPA has delayed that deadline and is also seeking to remove the MCL regulations on three PFAS substances.

Drinking water map shows states with most contaminants