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.