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I have a wall covered in mold. I have used various cleaning wipes attempting to clean it, including a vinegar mixture. I now want to spray the wall with a bleach cleaner. I am concerned for fumes from what may be considered mixing. For how long is the previous cleaning considered active enough to mix with the current cleaning ingredients? In other words, if I cleaned with vinegar a week ago, has enough time elapsed to safely use the bleach?

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    $\begingroup$ Vinegar and bleach are not a problem. The problem mixing is ammonia and bleach which creates chlorine gas. On a wall bleach is definitely the product to kill mold. $\endgroup$ – MaxW Feb 1 '16 at 21:29
  • $\begingroup$ Bleach reacts with acids to produce chlorine gas as well. I use this reaction in the lab to make chlorine (under controlled situations) for a particular reaction. $\endgroup$ – Ben Norris Feb 1 '16 at 22:36
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This answer is a bit hand-wavy because there are a lot of factors that we can't easily know in advance. You are unlikely to make very much in the way of toxic fumes from the small residues of vinegar left after it is dry. However if the wall is covered with mould, the room is probably poorly ventilated, so it is more reasonable to be somewhat concerned.

When mixing bleach with other cleaning products, the biggest concern is the reaction with ammonia, which produces extremely poisonous nitrogen chlorides. However chlorine bleach (hypochlorite ion) is a very reactive substance that will react with a wide range of things, including acids such as vinegar.

Acetic acid -- the "active ingredient" in vinegar -- is less volatile than water so it could easily remain a long time in a damp, poorly ventilated room. On the other hand, it will also slowly react with the wallpaper, the sheetrock, etc etc... Fortunately, since acetic acid warns us of food going bad, the human nose is very sensitive to it. If the wall doesn't smell vinegary, then there's not much left.

The main reaction product between acetic acid and chlorine bleach will be free chlorine gas plus smaller amounts of chlorine monoxide gas. This is a very simple reaction: hypochlorous acid is a very weak acid, so some hypochlorite ion is shifted back to acid form by the acetic acid (or any other acid); but hypochlorous acid is also very unstable, and decomposes to those materials plus chloride ion.

You will easily tell if this occurs: it will stink even more chlorine-y than the bleach, and probably make your eyes and the back of your throat sting. If that does happens, leave the room immediately and do not reoccupy it until it has had a long time to air out. Get in the shower to rinse your eyes and nose.

(A small fraction of the acetate will also be converted to chloroacetates. These are not volatile, but are extremely toxic. More of a concern for cleaning food-handling areas. In this case not a big deal unless you plan to eat your wallpaper.)

I would suggest that whatever you do, you first need to get good ventilation in there if you don't already have it. It is a common fallacy that "natural" things are harmless and evil "chemicals" will poison you. In fact, in a poorly ventilated space almost anything except fresh air is sub-optimal for your health. Apart from anything else, the mould spores can cause serious health issues. Furthermore, it is quite likely that ventilation is the root cause of the problem and you will not permanently fix the problem until that is addressed.

Once you have good ventilation, the quite small amounts of toxic gas that the bleach may produce from the quite small residue of acid, is not so concerning. Personally, I would probably use an oxygen (peroxide) bleach, but it isn't so much of a concern, and it doesn't fade the stains as much. You may not need bleach at all. Once you have had a while for the moisture level in the wall to drop, a lot of things will kill your mould relatively easily, although removing the unsightly blotches is another question.

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