The Humane Society and Safer Chemicals, Healthy Families seem like two groups that could, conceivably, coexist in adjoining booths at your local Earth Day festival. But these organizations have instead ended up on opposite sides in a debate over how the Environmental Protection Agency should go about regulating thousands of potentially harmful chemicals that Americans come into contact with every day.

This disagreement between the people who would like to help you adopt a puppy and the people who would like to help you avoid hormone-altering deodorant — and conversely, a spot of agreement between the Humane Society and trade associations for the petrochemical industry — goes beyond the details of a specific EPA proposal. Instead, experts say, it reflects a long-standing (and maybe impossible-to-solve) difference of opinion about risk, data collection and the role that animal sacrifice plays in keeping humans safe.

From early 20th century factory workers poisoned by radioactive paint to growing concerns that flame-retardant chemicals used on sofas and pillows could be linked to birth defects and cancer, the history of American industry is rife with cases of workers and consumers learning that a product they thought was safe actually wasn’t. In some instances, the effects of a toxic exposure play out in quick and obvious ways — those factory workers who painted watch dials with glow-in-the-dark paint made from radium began to sicken and die in gruesome ways after just a couple of years on the job. But some health impacts — an increase in lifetime risk of cancer, say — are harder to spot. And the links between those effects and exposure to a specific amount of a specific chemical are harder to prove.

That difficulty is compounded by the fact that, much of the time, we don’t even know what chemicals we’re being exposed to, at what amount, or what risks those chemicals are associated with. The EPA has had the authority to regulate the chemicals used in household products since 1976, but it had no control over chemicals that were already being manufactured and sold before that year: a collection that includes 62,000 substances. Moreover, the EPA isn’t even always aware of what all’s in the stuff you buy. The trade publication Chemical and Engineering News recently wrote about a study in which EPA researchers ground up 100 consumer products, including cereal and baby toys, and found 3,800 chemicals — only 200 of which were chemicals the EPA had expected to turn up.

Last summer, Congress took the first step toward increasing public knowledge about, and regulatory oversight of, toxic chemicals, passing the bipartisan Frank R. Lautenberg Chemical Safety for the 21st Century Act. Among other things, the act made it mandatory for the EPA to evaluate all the chemicals used in consumer products and to apply a risk-based safety standard, rather than one that has to balance a chemical’s risks against its industrial usefulness. It’s such a popular change that even the proposed 2018 EPA budget, which would reduce the agency’s funding by 31 percent and eliminate 50 programs and subprograms, provides increased funding to implement the Lautenberg Act. But putting the plan into action means filling in the holes in our knowledge, which means doing a lot of toxicology research — and this is where groups that might otherwise join hands and sing “Kumbaya” around the peace pole start to disagree.

The EPA published its proposed plan for implementing the new act Jan. 17 in the Federal Register. In public comments submitted to the agency, Safer Chemicals, Healthy Families, an advocacy network that works in education and policy for consumer environmental health, supported what the EPA wants to do. And the Humane Society and other animal-rights organizations didn’t. And that’s because of the tools used by the field of toxicology. This research, aimed at understanding the effects of chemical substances on the human body and where to draw the line between a safe amount and a dangerous exposure, relies on data collected from laboratory experiments. Some of those experiments are “in vitro” — they’re done in a test tube (or, more accurately, in one of dozens of tubes packed together like a honeycomb) using human or animal cells. Others, though, are “in vivo,” conducted in the bodies of living creatures such as rats or zebra fish. Representatives from both the Humane Society and Safer Chemicals, Healthy Families said they want to see the Lautenberg Act implemented. But the Humane Society is concerned that the EPA’s specific plan will increase the number of animals who die on our behalf.

In particular, the Humane Society is worried about an extra step that’s been added to the process. As legislated, the Lautenberg Act was a two-step system meant to help the EPA move quickly through tens of thousands of substances: First, prioritize chemicals based on potential for risk, then evaluate the safety of those designated “high priority.” And the EPA’s proposed plan retains this framework. Once a chemical enters the prioritization phase, the EPA has nine to 12 months to figure out whether it should be low priority or high priority. Low-priority chemicals are effectively set aside, as far as the evaluation process is concerned, and the industry can keep using them as before. Meanwhile, high-priority chemicals enter the evaluation phase, where the EPA has three years to figure out what risk the substance poses to human health and how it should be regulated. But the EPA has added a step: The pre-prioritization phase, a time for figuring out what data exists for a given chemical and whether the agency has enough information to truly know how the chemical should be prioritized. And, unlike the other steps, there’s no ticking clock for pre-prioritization. It could last a month. It could last years. “It creates this new sort of black box where they’ll ask for all the info they think they’ll need for the entire risk-assessment process,” said Kate Willett, director of regulatory toxicology, risk assessment and alternatives for the Humane Society. Willett thinks there’s a lack of transparency here that could result in companies feeling pressured to conduct new animal research for chemicals that might end up classified as low priority anyway, a scenario that, she said, would make those animals’ deaths a waste.

The EPA did not respond to a request for comment on this added phase, but Holly Davies, senior toxicologist in the Washington State Department of Ecology and a member of the EPA’s scientific advisory board on chemical safety testing, said a pre-prioritization step made sense to her because it helps the agency ensure that it doesn’t end up with more chemicals in the prioritization and evaluation phases than it can process in the allotted time frame. She pointed out that there’s no minimum amount of data that a company has to provide as part of pre-prioritization. And, she said, even if a company chose to do more tests at that phase, the information would be valuable no matter what priority the chemical was later assigned. “For me, to do a test in animals and find there is no effect, it’s not wasted,” she said. “Because now you know there is no effect. If testing puts something in the low-priority category, then that’s useful.”

Useful? Or a waste? What’s really being debated here isn’t just the presence or absence of a this phase in the EPA’s assessment of chemicals. It’s something more fundamental: If we want to keep people safe, is it necessary to experiment on animals?

That is a far-reaching question without easy answers. “It’s been going on for years,” said Andy Igrejas, director of Safer Chemicals, Healthy Families. He believes animal testing is a crucial part of protecting humans.

But concerns about animal testing — what animals it’s being done on and how well it works — aren’t confined to animal-rights groups. In 2015, the National Institutes of Health stopped funding research on chimpanzees, the animal most closely related to humans, after a National Academies report found that the research being done on them was unnecessary and unethical. Other research has found considerable problems associated with translating the results of animal studies to humans, both because of differences in biology and because of research problems created by the humans themselves. A 2009 paper examining why animal models weren’t terribly predictive of human responses to chemical exposure noted that animal studies were significantly less likely than human studies to be systematically reviewed, a process in which scientists compile and analyze multiple published studies on the same topic to see if they are pointing toward a common outcome. The same paper found that it was more common for animal studies not to follow basic scientific methodological rules, like blinding to prevent the researchers from knowing which animals have been exposed and which haven’t.

That convergence of practical, ethical and biological problems has created a movement within toxicology to get more research done, faster, and with less reliance on animals. The National Toxicology Program, a branch of the National Institutes of Health, even has a lab that’s dedicated to developing and testing alternative toxicological methods. Primarily, that means using new high-speed testing systems that allow researchers to run the same experiment quickly on hundreds of cells at once and then using mathematical modeling to take the results of many kinds of these studies — research on human cells, research on purified human hormone receptors, research on fish embryos — and combine them into a single prediction of a chemical’s impact on a full-scale human.

Davies still thinks traditional animal research is important, though. Almost every method we have of studying the human biological impacts of chemical exposure is flawed, she said. Nothing, not even a human, is a perfect proxy for what will happen to humans, broadly, when they come into contact with a given chemical. Instead, she said, animal testing is part of a suite of testing strategies that, taken together, can help us understand potential risks. “It’s building up a picture of what we know,” Davies told me. “You have data that’s in vitro, data in non-mammals, data in mice. And they all contribute to the story in a way that makes sense.”

From Willett’s perspective, groups primarily focused on animal rights and groups primarily focused on environmental health agree on more of this than they disagree on. But faced with imperfect methodology, deeply important questions for human health and big philosophical and ethical implications, different people are likely to come to diffferent conclusions about what to prioritize and how to proceed. That’s part of what the EPA is considering now, as it reviews public comments and puts together a finalized plan for the future of chemical exposure research.

And those differences will continue to have big implications for the process of turning science into policy, at the EPA and elsewhere. “If you aren’t increasing the information base, then you aren’t solving one of the main problems,” Igrejas said about the EPA’s toxicology testing. Willett, meanwhile, questioned whether that was true. “From our standpoint, based on experience over the past 30 years or so, simply adding more information to the mix doesn’t necessarily make you make better decisions.”