Why does mixing bleach and ammonia become so dangerous?

When bleach (sodium hypochlorite) and ammonia are combined, they react to form chloramine gases, primarily monochloramine, dichloramine, and nitrogen trichloride. These compounds are highly toxic and volatile, meaning they quickly become airborne and can be inhaled. Exposure to chloramine vapors causes severe respiratory irritation, chest pain, shortness of breath, and fluid buildup in the lungs. At high concentrations the gases can be fatal. The reaction can also produce hydrazine, which is a known carcinogen. The danger is compounded by how common both chemicals are in household products like glass cleaners, floor cleaners, and disinfectants, making accidental mixing relatively easy if people are unaware of the risk.