Benzalkonium Chloride

Benzalkonium chloride, usually shortened to BAK, is a quaternary-ammonium compound that kills microbes, so it preserves cosmetics and hand sanitizers and is the standard preservative in eye drops. The reason to skip it is skin reactivity in ordinary use. BAK is a recognized contact allergen and a well-known skin irritant: it was added to the standard North American patch-test panel, and dermatology clinics report allergic reactions to it of rising clinical importance, showing up as redness or eczema from leave-on and rinse-off products, sanitizers, and wipes. Overall allergy stays uncommon and its allergen-versus-irritant line is debated, but both routes land on the same caution for reactive skin. The US cosmetic-ingredient panel that cleared it at up to about 0.1% named that irritation and sensitization potential directly, and Europe holds it to the same 0.1% limit. A permitted low percentage does not settle it, because you cannot read the level off a label, and gentler preservatives, from the mild alcohols to the organic acids, do the same job without the same allergy record. The eye tells the same story more sharply and supports the call: BAK is in most eye drops, and ophthalmology research finds that repeated exposure damages the surface cells of the cornea and conjunctiva and brings on ocular surface disease, clearest in people using preserved drops daily for years and improving when they switch to preservative-free drops. Quaternary-ammonium compounds are also known triggers of occupational asthma. Put together, this is a sensitizing, avoidable preservative, which is why it lands at avoid rather than something to manage.