Scientists are trained to express themselves rationally. They avoid personal attacks when they disagree. But some scientific arguments become so polarized that tempers fray. There may even be shouting.

Such is the current state of affairs between two camps of scientists: health effects researchers and regulatory toxicologists. Both groups study the effects of chemical exposures in humans. Both groups have publicly used terms like “irrelevant,” “arbitrary,” “unfounded” and “contrary to all accumulated physiological understanding” to describe the other’s work. Privately, the language becomes even harsher, with phrases such as “a pseudoscience,” “a religion” and “rigged.”

The rift centers around the best way to measure the health effects of chemical exposures. The regulatory toxicologists typically rely on computer simulations called “physiologically based pharmacokinetic” (PBPK) modeling. The health effects researchers—endocrinologists, developmental biologists and epidemiologists, among others—draw their conclusions from direct observations of how chemicals actually affect living things.

The debate may sound arcane, but the outcome could directly affect your health. It will shape how government agencies regulate chemicals for decades to come: how toxic waste sites are cleaned up, how pesticides are regulated, how workers are protected from toxic exposure and what chemicals are permitted in household items. Those decisions will profoundly affect public health: the rates at which we suffer cancer, diabetes, obesity, infertility, and neurological problems like attention disorders and lowered IQ.

The link from certain chemicals to these health effects is real. In a paper published earlier this year, a group of leading endocrinologists concluded with 99 percent certainty that environmental exposure to hormone-disrupting chemicals causes health problems. They estimate that this costs the European Union healthcare system about $175 billion a year.

Closer to home, Americans are routinely sickened by toxic chemicals whose health effects have been long known. To cite one infamous example, people exposed to the known carcinogen formaldehyde in FEMA trailers after Hurricane Katrina suffered headaches, nosebleeds and difficulty breathing. Dozens of cancer cases were later reported. Then there are workplace exposures, which federal government estimates link to as many as 20,000 cancer deaths a year and hundreds of thousands of illnesses.

“We are drowning our world in untested and unsafe chemicals, and the price we are paying in terms of our reproductive health is of serious concern,” wrote the International Federation of Gynecology and Obstetrics in a statement released on October 1.

Yet chemical regulation in the United States has proceeded at a glacial pace. And corporate profit is at the heart of the story.

That the chemical industry exerts political influence is well documented. What our investigation reveals is that, 30 years ago, corporate interests began to control not just the political process but the science itself. Industry not only funds research to cast doubt on known environmental health hazards; it has also shaped an entire field of science—regulatory toxicology—to downplay the risk of toxic chemicals.