1. Bleach & Ammonia
You should never mix cleaning products. Period. But you should especially never work with an ammonia-based and a bleach-based cleaner at the same time. This is most likely to occur in a bathroom or kitchen, so carefully check labels before getting to work. When vapors from ammonia and bleach mix together, they form a toxic gas that is extremely dangerous. If you discover that you, or someone in your home, has been exposed to these fumes, leave the area immediately and call 911.
Why: Inhaling the vapors could cause severe respiratory damage and burn your throat.
The worst that could happen: If ammonia is present in excess, toxic and potentially explosive liquid hydrazine may be formed.
2. bleach & Vinegar
My grandmother loves bleach and I love vinegar. While the combination does create a good disinfectant, these two common agents should never be used at the same time, no way no how.
Why: Adding any weak acid to bleach will release toxic chlorine and chloramine vapors.
The worst that could happen: You can get a nasty chemical burn, especially of your eyes and lungs. Ouch.
3.Bleach & rubbing Alcohol
Why: Bleach contains sodium hypochlorite, which reacts with ethanol/ isopropyl alcohol to produce chloroform, hydrochloric acid, and other compounds, such as chloroacetone or dichloroacetone.
The worst that could happen: You could damage your nervous system, eyes, lungs, skin, liver, and kidneys. Trust me, no stain is worth this boo boo. Call 911 immediately if you come in contact with this.
4. Vinegar & hydro-Peroxide
These two ingredients are commonly touted as natural cleaning methods, but combining the two in the same container could result in a corrosive acid.
Why: You can use them on the same surface separately while cleaning, but don't mix them in the same container. You'll create paracetic acid which, despite being an effective sanitizer, is potentially corrosive and irritating.
The worst that could happen: The health risks aren't well-known, but in high enough concentrations, paracetic acid "is very irritating to the skin, eyes, nose, throat, and lungs, with the potential for causing permanent scarring of the skin, cornea, and throat."