Which two substances should never be mixed together for safety reasons?

Mixing ammonia and bleach is highly dangerous, as these two substances react chemically to produce toxic gases, specifically chloramine vapors and potentially even chlorine gas, which can cause respiratory issues and other health hazards when inhaled. The severity of the reaction creates a significant safety risk, which is why this combination is advised against in both home and professional environments.

While other combinations may have varying levels of reactivity or can create undesirable effects, they do not produce the same level of toxic fumes or immediate health risks that ammonia and bleach do. For instance, vinegar and baking soda create a harmless acid-base reaction that simply produces carbon dioxide gas, while water and oil, although they do not mix, are not reactive in a hazardous way. Alcohol and acetone can dissolve well together but do not pose the same danger as mixing ammonia with bleach.