The EPA doesn’t require safety tests for most chemicals, including the three that spilled into a Delaware River tributary

The three chemicals that spilled into a Delaware River tributary on March 24 have been known to science for more than a century. Yet as with most other chemicals in widespread use, the U.S. government has not set limits on their presence in drinking water.

So when area water utilities scrambled to protect residents from potential hazards in the city’s tap water, federal health officials could offer only rough guidance on what amounts would cause concern — including one estimate developed just weeks earlier, in response to the train derailment in East Palestine, Ohio.

Ultimately, testing did not find any of the three substances in drinking water at levels thought to pose a risk — and in fact, they were not detected at all. But the episode illustrates how environmental regulators can be hard-pressed to keep up with evaluating the safety of all the chemicals in production.

Evidence suggests that most of them almost certainly are safe, especially at the levels people are exposed to in their everyday lives. But when the government tries to evaluate — and place restrictions on — substances of concern, it often is hampered by outdated statutes, industry pushback, and the challenge of conducting science on chemicals that are widely present in the real world.

Protecting drinking water is just one challenge. The U.S. Environmental Protection Agency has barely begun to evaluate how thousands of chemicals might pose other kinds of environmental and health risks. That’s because when the agency first got the authority to regulate toxic substances in 1976, manufacturers were allowed to keep making most of them — including the three that spilled from the Trinseo plant in Bristol — without testing them for safety.

Seven years ago, the efforts of then-New Jersey Sen. Frank Lautenberg were pivotal in updating the law to allow the EPA to start prioritizing these older chemicals for evaluation. But so far just a few dozen, including well-known hazards such as asbestos and formaldehyde, have undergone initial review.

That leaves consumers largely in the dark, said Maria J. Doa, senior director for chemicals policy at Environmental Defense Fund, a Washington-based nonprofit.

“It’s important for the EPA to review these chemicals and see if they pose a risk,” she said. “They’ve just started.”

» READ MORE: Companies have discharged millions of pounds of chemicals into the Delaware, records show

Chemical safety is governed through a dizzying array of regulations and agencies in the U.S.

The FDA regulates food, drugs, and cosmetics. The Occupational Safety and Health Administration (OSHA) handles the workplace. The EPA oversees pesticides through one set of regulations, and other chemicals (including the three in the March 24 spill) through a different patchwork of rules.

In most cases, chemicals are not directly tested on people before they are approved for use, said physician Robert Laumbach, an associate professor at Rutgers University School of Public Health. The exception is medications, because they are intended for human consumption.

The safety of other chemicals is evaluated through a variety of analytical methods and animal studies, as well as an assessment of its physical properties. Is it volatile? Does it accumulate in the environment? Is its structure similar to that of other chemicals that are known to be harmful?

Common tests in laboratory mice include one called LD50 — determining the amount of a substance that kills 50% of a group of animals. Other mouse studies take place over a longer term, determining what levels of exposure may lead to cancer, reproductive issues, or neurological deficits.

A proper evaluation takes into account that we are not exposed to single chemicals in isolation, said Jack Vanden Heuvel, a professor of molecular toxicology at Pennsylvania State University.

“You can be exposed to multiple chemicals that may have similar activities,” he said. “The effects would be additive.”

Also key is the old adage that “the dose makes the poison,” said Virginia Tech chemist Joshua Worch. That is, some substances may be toxic in high amounts, yet have a neutral or even benign effect at lower concentrations.

Once a substance is approved for use, real-world evidence of harm may emerge after the fact, through epidemiology — measuring populations of exposed people for potential health effects.

To protect the safety of drinking water, the EPA has set limits for more than 90 harmful contaminants — including microorganisms, organic chemicals such as benzene and vinyl chloride, and inorganic chemicals such as lead, arsenic, and fluoride. On March 14, the agency proposed adding six more to the list, all from a class of “forever chemicals” abbreviated as PFAS.

But for thousands of other substances, the agency has set no standards. In one case, a component of rocket fuel called perchlorate, the EPA failed to set limits despite clear evidence of harm, said Emma Bast, an attorney for the environmental group PennFuture. Still other substances have not received enough study.

“Our understanding as a society about what chemicals are dangerous is incomplete,” she said.

Chris Crockett, chief environmental, safety and sustainability officer for Aqua Pennsylvania, which provides water to hundreds of thousands of customers in suburban Philadelphia, countered that the EPA’s approach is largely sound — keeping tabs on substances that are likely to find their way into drinking water.

“There’s no way to develop standards for all the chemicals, nor is there a way to test for everything,” he said. “What we really need to know is what’s the health risk, and which chemicals are of concern.”

Still, sometimes there are surprises, as in the March 24 spill of butyl acrylate, ethyl acrylate, and methyl methacrylate.

Federal health officials used the best available evidence from animal and lab studies to estimate a maximum safe level for each of the three, Crockett said.

An estimate for butyl acrylate had been set just a few weeks earlier, at 560 parts per billion, in response to the East Palestine derailment, by the U.S. Agency for Toxic Substances and Disease Registry (that is, no more than 560 molecules of the chemical for every billion molecules of water). Safe limits for the other two chemicals also were estimated in the hundreds of parts per billion, Philadelphia Water Department officials said.

Even though the guidance for these chemicals was in the hundreds of parts per billion, water companies went well beyond that, using equipment that could detect them down to 1 part per billion. That was more for the sake of public perception, as people can smell these chemicals at those trace amounts, well below any level that is thought to be a health risk, Crockett said.

“Would you want to risk it being in water that your customer tells you smells like paint, and you turn around and tell them ‘Don’t worry’?” he asked.

Staff writer Frank Kummer contributed to this article.