The answer is simple, don’t mix cleaners if you’re not sure what they create. Make sure to research different solutions, understand how the ingredients interact, and find out whether or not harmful off-gassing will occur. Remember to always, always make sure that the rooms being cleaned are well-ventilated. In fact, there are a few specific combinations that should be avoided at all costs.
Bleach + Vinegar
Don’t mix chlorine bleach with any type of acid. Vinegar, being acidic, produces a toxic chlorine vapor when mixed with bleach.
Bleach + Ammonia
Many toxic, potentially lethal vapors are produced when bleach and ammonia interact. The main danger comes from chloramines vapor, which can cause severe respiratory problems such as coughing, shortness of breath, chest pain, wheezing, nausea, watery eyes, pneumonia, and fluid in the lungs.
Different Brands of the Same Type of Product
Every company could (and often does) have different formulas for the same type of cleaning product. Certain ingredients could react violently, produce toxins, or simply make the cleaner ineffective.
Highly Alkaline Products with Highly Acidic Products
Acids and bases, especially those on either extreme of the pH scale, can cause aggressive reactions that make liquid solutions bubble and splatter. On their own, things that are highly acidic or alkaline are caustic and may cause chemical burns. Remember to wear thick gloves and protect your face when using potentially dangerous products.
Certain Disinfectants with Detergents
Some detergents utilize “quaternary ammonia” and will have it listed on the ingredients section of their container. This will neutralize the effects of the disinfectant if you mix the two together – it’s not necessarily harmful, but makes both cleaning products useless.
Still don’t believe in the harms of mixing cleaners? In June 2002, a part-time caretaker in a Toronto school was rushed to the hospital after complaining to a co-worker that she was unable to breathe. She later died in the hospital. Investigators looking into the incident discovered that she had mixed a sanitizing product containing sodium hypochlorite with an acid toilet bowl cleaner. The two ingredients had reacted together to form deadly chlorine gas, which led to her death. It may be an extreme case, but it's a good example of the health hazards that can be associated with the uneducated mixing of chemical cleaners.