Why does mixing bleach and ammonia create toxic gas?

When bleach (sodium hypochlorite) and ammonia are mixed, they react chemically to produce chloramine gases (primarily NH2Cl, NHCl2, and NCl3). The hypochlorite ion in bleach reacts with the nitrogen in ammonia, transferring chlorine atoms to form these compounds. The reaction also produces hydrazine as a byproduct in some conditions. Chloramine gases are toxic because they irritate and damage the mucous membranes of the respiratory tract, eyes, and lungs. Even at low concentrations they cause coughing, shortness of breath, chest pain, and fluid buildup in the lungs. At higher concentrations they can be fatal. The reaction happens quickly at room temperature, which is why mixing common household cleaners containing these chemicals is genuinely dangerous.