Rodents get a bathtub full of this possible carcinogen; humans get a raindrop.
The issue revolves around a chemical called 1,4-dioxane that is found in a wide variety of products ranging from shampoos, cosmetics, detergents & some laxatives & traces of it in some sunscreens. It is created as a byproduct when we make surfactants. The things that create the luxurious foam in your shampoo are surfactants, & it's the chemistry involved that strips away dirt, grease & grime from your clothes. But it's not the surfactants per se that some worry about; a manufacturing byproduct in making them involves fats or oils reacting with ethylene oxide to make them foam & feel smoother. It is during this reaction that 1,4-dioxane forms, which is a small ring-shaped solvent molecule. The concern is that if this molecule is not removed, it ends up going down the drain & eventually ends up in our drinking water. But it's hard to filter out because it's very small, very water-soluble, doesn't stick to sludge & doesn't break down easily. So water treatment plants can't catch it.
The concern stems from the International Agency for Research on Cancer's (IARC) classification of 1,4-dioxane as "possibly carcinogenic to humans," & the U.S. National Toxicology Program views it as "reasonably anticipated to be a human carcinogen." But these evaluations were based on rodent studies that were fed huge amounts—5,000 parts per million (ppm), or 5,000 mg per liter. That's 0.5% pure 1,4-dioxane, equivalent to 1/2 a tsp of chemical in every cup of water. In reality, no one is exposed to anything close to this amount. That's why Health Canada set limits at a maximum of 0.05 ppm (0.05 mg per liter), which is 50 micrograms per liter, equivalent to 1 drop in 400 gallons of water. (400 gallons = 6 full bathtubs). 0.05 ppm is 100,000x lower than what rodents received. There are two measurements used in toxicology: risk, which is the amount we would typically encounter that could have a negative effect; and hazard, which is what the IARC & the U.S. National Toxicology Program base their conclusions on. The real-world human exposure risk is negligible.
Furthermore, most drinking water doesn't even reach the max levels determined by Health Canada. Typical levels are 0.0005-0.005 ppm. That's 10,000 to 100,000x lower than rodent doses & 10-100x lower than the regulatory dose. In shampoos & cosmetics, they are even less of a concern because manufacturers remove it by using steam distillation or vacuum stripping, where they pull it out under reduced pressure. These methods reduce 1,4-dioxane to 1 ppm or lower. Since a shampoo bottle is 300 ml (10 oz), that's just 0.3 mg total, so only a tiny fraction touches skin & almost none gets absorbed. Even if every household dumped shampoo down the drain, the final water concentration would be far below the regulatory limits. 1,4-dioxane is also found in some laxatives (not Metamucil) allowed at up to 10 ppm, but the actual amount you'd consume is in micrograms & your body rapidly removes it. Toxicologists consider this amount inconsequential.
The last point to make is what the term "possibly carcinogenic" really means. This category also includes pickled vegetables, aloe vera extract, working as a hairdresser, engine exhaust & very hot beverages. It means there is some evidence in animals, but human evidence is weak or absent. Remember the golden rule of toxicology: "Dosis sola facit venenum," written by the Swiss physician Paracelsus (1493-1541), which means "Only the dose makes the poison." Risk depends entirely on dose. Rodents get massive doses; humans get trace amounts. Also, one member of the public thought that 1,4-dioxane was the same thing as dioxin. Dioxin, I would be worried about; 1,2,4-dioxane, not so much.
