Due to popular demand (well a question from Asya in response to our post on Five Best shampoos), I thought I'd pull together a list of things that you'd prefer to wash right out of your hair. We know to avoid irritating sulfates and parabens, but what else is the enemy of safe shampoo? This list isn't comprehensive, but I have tried to include ingredients that tend to show up frequently, particularly in drugstore sudsers.

Acrylates copolymer are more likely to be in hairspray, but they do crop up in shampoo too. Not to be ingested or swallowed. Animal tests have also shown it to be harmful if come into contact with the skin and/or eyes.

Behentrimonium chloride. Acts as an anti-static and conditioning agent. Its waxy texture also makes it a great defrizzer and emulsifier. Primarily used in hair care products such as conditioner, dye and mousse. Behentrimonium chloride is a toxic compound, and concentrations of .1% and higher have been shown to damage the eyes by causing tissue death of the mucous membranes. It’s also highly flammable, and irritating to the skin. Don't confuse it with behentrimonium methosulfate, which is perfectly safe.

Cetrimonium bromide. A surfactant. According to the EWG, one or more animal studies show brain and nervous system effects at low doses, there is strong evidence that it is a human skin toxicant, and animal studies show reproductive effects at low doses.

Cocamide DEA. Studies on rats have shown that it can lead to the formation of nitrosamines, which are carcinogenic. Research has also demonstrated it to be a mild dermal irritant and sensitizer. It is increasingly being replaced with the gentler coco betaine. This is generally considered safe, although there have been reports of allergic reactions (thought to be due to it being contaminated with amidoamine and dimethylaminopropylamine, two impurities commonly associated with skin sensitization and irritation.

Methylchloroisothiazolinone is a preservative most often seen in shampoos and other rinse-off products. It is a member of a group of chemicals known as isothiazolinones, the most potent allergens on the consumer market. In high concentrations Methylchloroisothiazolinone can cause chemical burns and it is a skin and membrane irritant. It was largely removed from most cosmetic products except for those with only short duration skin contact such as rinse-offs, although its inclusion in certain forms makes it more acceptable to sensitive users.

PEG-150 Distearate. According to a study published in the International Journal of Toxicology, PEG 150 Distearate can contain harmful impurities, including: Ethylene Oxide, known to increase the incidences of uterine and breast cancers and of leukemia and brain cancer, according to experimental results reported by the National Toxicology Program; 1,4-dioxane, a known carcinogen; PAHs, known to increase the risk of breast cancer; lead; iron; and arsenic (Source). Products and formulas containing PEGs should not be used on broken or irritated skin.

Sodium Cocoyl Isethionate. Like all surfactants it can be an irritant at higher concentrations. Sodium Cocoyl Isethionate has been specifically shown to cause some eye irritation at higher concentrations. According to the Cosmetics Database, it is also linked to organ-system toxicity (the American College of Toxicology reported that one or more animal studies showed broad systemic effects at high doses).