Why does mixing bleach and ammonia create dangerous fumes?

When bleach (sodium hypochlorite) and ammonia are mixed, they react chemically to produce chloramine gases (primarily monochloramine, dichloramine, and nitrogen trichloride). This happens because the hypochlorite ion reacts with ammonia molecules, substituting chlorine atoms onto the nitrogen. The resulting chloramine compounds are volatile and toxic. Inhaling these fumes irritates and damages the respiratory tract, eyes, and lungs. At high concentrations or with prolonged exposure, the gases can cause fluid buildup in the lungs, chemical burns to airways, and potentially death. The reaction can also produce small amounts of hydrazine, another toxic compound. This is why these two common household chemicals should never be combined, even in small quantities or in poorly ventilated spaces.