1,4-dioxane. The byproduct of ethoxylation, used to soften harsh ingredients. A March 2009 study by the Campaign for Safe Cosmetics found 67 percent of baby bath products tested contain 1,4-dioxane. The Food and Drug Administration says the levels are not hazardous. The Environmental Working Group ranks it a 10 out of 10 on its list of hazardous PC ingredients, linking it to cancer, reproductive problems and allergies. Products that include sodium myreth sulfate, or ingredient names that contain “xynol,” “ceteareth” and “oleth,” should be considered suspect.

Bismuth oxychloride. An inorganic pigment used in some mineral makeups to ease application and provide sheen. It has been shown to cause skin irritation and be harmful when ingested.

Dimethicone. A silicone-based polymer that adds a silky feel to lotions, cream and conditioners. A 2003 research review found that it is not absorbed when applied to human skin. However, it is a skin irritant, and some studies associate it with weight loss and decreased testicle size in rodents.

Formaldehyde. A toxic compound released inside the container as preservatives age. Ingredients likely to contaminate products with formaldehyde include quaternium-15, DMDM hydantoin, imidazolidinyl urea and diazolidinyl urea. The International Agency for Research on Carcinogens classifies formaldehyde as a carcinogen to humans. The Campaign for Safe Cosmetics found it in 82 percent of baby bath products tested.

Hydroquinone. A skin-bleaching agent often found in conventional antiaging products and other beauty creams. According to the Teens for Safe Cosmetics campaign, which lists hydroquinone among its “Dirty Thirty” cosmetics ingredients to avoid, it can be an immune system and respiratory toxicant, probable neurotoxin and possible carcinogen. Animal studies have shown it disrupts the endocrine system.

Nanotechnology. A science that engineers tiny particles—between 50,000 and 100,000 times smaller than the width of a human hair—with the intention of improving product performance. Some studies suggest nanomaterials in sunscreen may produce free radicals and damage DNA. One 2009 rodent study associated nanomaterials with nerve damage and reduced sperm counts.

Parabens. Some studies have linked antimicrobial parabens to low sperm counts and decreased testosterone levels, as well as breast cancer and allergic reactions.

Phthalates. These industrial chemicals are common in plastics, and also appear in shampoos, lotions and nail polishes. Studies link phthalates to problems with reproductive, endocrine and respiratory systems. A 2009 study of 261 Korean children, ages 8 to 11, found the higher the concentration of phthalate metabolites in the urine, the worse their attention-deficit hyperactivity disorder.

Propylene glycol. Used as a synthetic preservative in creams, hair gels and dyes, and makeup, it can cause allergic reactions, including shortness of breath, rashes and inflammation. Some animal studies show adverse effects on brain and reproductive systems. It may increase the ability of other toxins to penetrate into the bloodstream.

Synthetic surfactants. Chemical emulsifiers that make soaps and shampoos sudsy. They are often derived from petroleum, and during their chemical processing, unwanted byproducts, such as 1,4-dioxane, are often produced. –L.M.