Why must bleach and ammonia never be mixed?

Because the two react with each other to make a new family of toxic compounds called chloramines. Household bleach contains hypochlorite, which releases chlorine, a powerful oxidizer that works by breaking the chemical bonds that give pigments their colour. Ammonia is the other half of the story. When the two meet, they react to produce chloramines, the family that includes nitrogen trichloride, a yellow, oily, explosive liquid.

Here is the twist: most people believe the danger is chlorine gas. That one is real, but it comes from mixing bleach with acids, such as limescale removers. With ammonia the story is different. A common misconception says it also releases chlorine, when in reality the two produce chloramines that are still dangerously toxic. The result is the same, a hazardous gas in a small, closed room, but the culprit has a different name.

You already know the smell of one chloramine without ever having mixed anything. The distinctive chlorine smell of a swimming pool is not chlorine: it is chloramines, formed when chlorine in the water reacts with biological substances such as the urea in sweat and urine. Used in low concentrations, chloramine is even a water disinfectant, less aggressive than chlorine itself.

That is why the rule is so simple. Dilute bleach already irritates the skin and respiratory tract on its own, and an estimate from 2002 put the accidents needing hospital treatment caused by it at about 3,300 a year in British homes. Use each product alone, never together.