The danger occurs due to any of various chemical reactions between the ingredients which result in dangerous substances being created.
For example, hat we most commonly refer to as “bleach” is chlorine based chemical mixes, such as sodium hypochlorite (NaClO) usually in a 3-6% solution with water. Sodium hypochlorite will natural decompose with water into ~~NaOH~~ HCl, hydrochloric acid. When you mix it with ammonia (NH3) you get chloramine gasses, which are bad on their own, but if enough excess ammonia is present you can then get liquid hydrazine which is potentially explosive too.
The primary source of the danger in this situations is the chlorine, which is an extremely reactive element and thus can form many compounds which are dangerous to humans in sufficient quantities. They are also extremely useful for purposes such as disinfection and cleaning and thus we also use them frequently. Its a risk vs. reward type of situation.
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