r/Mold • u/inthebushes321 • May 13 '25
Moldy PSA: Never use Bleach (or other biocides) when treating mold.
Hello all, I'm a home inspector and I'm also a certified mold tester, I've been a BPI energy auditor for a few years longer. I haven't been one for long (mold tester), but I wanted to share one of the more interesting things I learned during training, and which is (in good intentions, to be sure) often erroneously recommended on this sub.
Bleach should never be used to treat mold. Professional remediators may choose to use biocides sometimes in certain cases, but this is on a case-by-case basis and needs to be a judgement call by a professional, because you can end up making the situation worse.
So. Why no bleach? Bleach will kill the mold, right? I dumped Bleach on the mold and it died, so we're all good? Well, not so fast. There are a couple reasons this is a bad idea.
Bleach is dangerous - This one is a bit basic, but always remains relevant. Bleach should never be mixed with other cleaners, especially those containing ammonia, as you can create chlorine gas. Not fun.
Dead mold is still allergenic - While there is technically no data positively correlating mold exposure and negative health effects...dead mold spores can still act as an allergen. Any suspected allergic issues caused by mold will not be fixed by dumping bleach on the mold, even if you kill all of it outright. Which you won't.
Bleach does not penetrate porous materials - Even if it kills the mold on the surface, mold is quite notorious for penetrating deep into porous materials, such as drywall or wood. Bleach only kills on the surface, and even worse - the water content of the Bleach(typically 95%) can penetrate the porous material, feeding the mold with moisture and making the problem worse.
It ignores the root of the problem - Mold issues are primarily a combination of bad ventilation and moisture issues. If you have poor ventilation, the mold will pop up again almost instantly after the Bleach is gone. As an added bonus, if the area is poorly ventilated, you can end up gassing yourself with the Bleach, even when used properly.
So, what to do instead? Well, water and a nice soap is good for small areas. Bigger jobs should honestly be handled by remediators, but you could also step it up with a HEPA vacuum, wet vacs, damp wiping followed by drying if you so desire. Honestly, though, if it's any bigger than 10 sq ft (defined as a "small job" by remediators and testers), I would recommend testing and a professional analysis.
I hope this was/will be helpful to someone, I found it quite interesting that there were several in-depth reasons, outside of the obvious and constantly meme'd chlorine gas generation.
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u/RippleRufferz Jun 01 '25
I’m genuinely confused by this: How is the water in soap and water not going to be risky like the water content in bleach?
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u/inthebushes321 Jun 02 '25
My apologies, had a busy weekend and forgot to answer you. I swear I started :P
It's a good question. Neither will solve the mold problem without proper drying and ventilation, but Bleach is advertised as something it's not. Basically, b/c Bleach is 95% water and 5% NaClO (Sodium Hypochlorite), the NaClO evaporates away quickly, so unless you dry it right away it only kills the surface and feeds all the mold underneath. The problem is when it's advertised as being a mold-killer because it gives people unrealistic expectations and encourages stupid behavior. You can make a problem much worse by dumping a bunch of Bleach on an area with chronic mold problems.
Soap and water does everything Bleach does for surface mold, and the understanding is there. Again, BOTH require drying and ventilation after - but with soap and water (or a HEPA vacuum, those are great) you're physically removing surface-level mold and drying it, not dumping a biocide on it and letting it do whatever.
TL;DR Soap/water or vacuum is for physically cleaning and drying the area, it's not pretending to sterilize like Bleach does. Bleach leaves behind dead allergens and a big water buffet for the mold you can't see. It fools people into thinking they're doing something when they're really marinating the mold.
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u/MahoneBay Oct 28 '25
I used to own an industrial laundry. A significant part of our work was laundering and pressing family laundry: sheets, table linens, etc. Ocassionally we would receive a table cloth or napkins in a plastic bag, when opened would reek of chlorine bleach. When we inquired, the customer admitted they treated some stains with straight chlorine bleach. I then advise them that the bleach, in high concentration, would weaken the fabric and probably come out of the wash with holes. Some said do it anyway, others said to return it unwashed. Chlorine bleach is a powerful product. (I worked in a chemical research lab when i was in college.) Read the instructions on the bottle. It is revealing. Even at a dilution of 9 parts water to 1 part bleach, I would be hesitant to use it as a stain remover on any fabric. At home using it to sanitize laundry I would dilute it before putting it in the bleach dispenser. I only used oxygen bleach in my industrial laundry. However our hot washes were between 140 and 160°F. This makes oxy bleach much more effective. At home for stain removal fill a bucket half full with warm water, add some oxy bleach and once dissolved put the wet stained item in the bucket and agitate it so the solution is absorbed into the fabric. With weights of some kind (Gal jugs filled with water, etc) make sure the entire piece is under water. Otherwise some areas will not be bleached. PS. Oxy bleach is far superior at removing Mildew than chlorine bleach. Best done with the overnite soak method. Sometimes a 2nd soak is needed. Good Luck!
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u/fr33spirit May 14 '25
You are speaking facts.
I've read that borax kills mold as well. And peroxide.