Where you may be using it: Hair conditioners and styling products
What to look for on the label: Benzalkonium chloride, benzethonium chloride, quaternium-15, guar hydroxypropyltrimonium chloride, centrimonium bromide, or polyquaternium followed by any number
Why it's bad for you: It's a formaldehyde releaser, which sounds liberating, except in the case of formaldehyde. Any ingredient that's also found in latex paint just can’t be good to lather, rinse, repeat with. With all its complex, cunning names, this is a preservative known for causing allergic reactions like dermatitis.
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Barbara Woolsey is a Berlin-based writer who begrudgingly threw away nail polishes after writing this. Find more from her on Facebook and Twitter.