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How Borax Kills Mold

by BrianReeves
types of mold

Borax – Molds alkali arch enemy!

You hear a lot about how to kill mold, or how to remove mold.  There are debates about whether you should use bleach or some natural alternative such as vinegar or grape seed extract.  One thing that really helps to know, however, is how something kills or removes mold.   In this blog post, we’ll cover the seemingly magical powers of borax.

Borax has a really cool name, maybe because it sounds really mean like anthrax, or Clorox, or maybe just because it has the phonic ‘ax’ in it.  At any rate, borax is interesting because it is a naturally occurring substance, and in addition to its mold-fighting powers, it is also used in making glass and ceramics.  The key superpower of borax, when it comes to fighting mold, is it’s extreme pH level.  Borax is found in very alkaline (basic) mineral deposits, and carries a pH level of about 9.3, which on a scale of 1 to 14, makes it extreme enough to be effective, yet not so extreme that it becomes harmful to humans.


You see, just like you and me, mold thrives in an environment where it feels comfortable.  For most types of mold, they like to live in an environment where the ph is somewhere between 3 and 7, which is more on the acidic side (a pH of 7 is considered neutral).  Some molds even like to live in pH environments below 3!   Enter borax, the alkaline archenemy of mold.  When you apply a borax solution to mold, it turns its environment into hostile territory.  It’s like taking a shade-loving, water sucking fern from Florida, and transplanting it into Death Valley, California, where it is extremely hot, dry, and no shade.  The plant cannot survive in such environment, so it shrivels up and dies.  And so it is with mold when you apply a borax solution.

So how do you use borax when removing mold?  Below is a quick reference guide for you to use.  Borax can be purchased at any grocery store, and it is relatively inexpensive.   You can safely use borax on any surface.  If, for some reason, you are unsure of what it might do to a bathroom sink, or a piece of clothing, simply test it on a small area first.  You shouldn’t worry though, because borax is commonly used in laundry detergents.

How to kill mold with Borax

Recipe: Combine 1 cup of borax to 1 gallon of water. (1 to 16 ratio of borax to water) You can usually find “20 mule team borax” in the laundry section of your grocery store.

Application:  Apply solution to the moldy area, be careful not to use too much on a porous material, like unfinished wood or drywall.  These materials absorb water, which will feed the mold by keeping it moist. Scrub the area with a rag or somewhat abrasive material until mold is removed.  Wipe area clean and dry the area.  You can lightly rinse the area, and lightly apply the solution again, but there is no need to completely rinse it off because borax inhibits any new mold from growing.  That’s it!

Cautions: Don’t eat it, otherwise it’s pretty safe.
So there you have it.  Not only do you know how to fight mold with borax, you now know how it wins!

David Green is a mold enthusiast.  Yeah, that’s kind of weird, but when he’s not learning about mold, he’s writing about it.  You can find more ways on how to kill mold, and other moldy tidbits at removemoldguide.com




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Kris C. May 28, 2014 - 7:06 pm

Interesting post on not just the “what” but the “how” on killing mold. I am thoroughly enjoying your blog, I hope you continue to post frequently.

mary crosby June 21, 2014 - 12:35 pm

Have a full house basement the rafters in the ceiling are unfinished wood. Had a leak and that is cleared up but have mold growing all over the ceiling.Had it sprayed with a mold product but it keeps coming back.You stated that borax could help but said dont saturate wood too much. Any hints for me?

avi July 5, 2014 - 8:37 pm

I have a similar problem to Mary’s although its in smaller sections throughout the house, on unfinished wood. Also interested in advice. Thank you!

shannon July 23, 2014 - 4:36 am

Please include warning: Borax is dangerous to mucus membranes such as eyes. Also dangerous if inhaled accidently, and is also dangerous to have near pets.

Don August 23, 2014 - 3:59 pm

Okay, so borax will kill mold, but will it kill the Black Mold that is dangerous to human health?

Lindy October 6, 2014 - 10:51 am


Thanks so much for this informative post! With regard to not soaking wood with too much water which will feed the mould – could I mix borax with medical alcohol used for antiseptic purposes?

If you find the time, I would really appreciate an email reply as I’m facing a serious health problem with an old side-board full of brown mould. Thanks!

sharon October 22, 2014 - 11:18 pm

I made a paste of borax and spread it with a brush on some wood with black mold. I just left it to dry. When I cleaned the residue off, no evidence mold ever existed dead and “bleached” away

elaine November 10, 2014 - 6:19 pm

Hi… Borax in grocery stores? are you referring to borax that is a laundry booster (20 mule team borax) or do i need to buy a pure product of “borax” from some chemical place? and, if not much mold is present… can the smell of a 40 year old musty basement be removed by a purifier or some other means? much much thanks for any and all help. Best, -elaine

Don November 12, 2014 - 10:27 am

You can find the borax powder you need at Amazon.com

SK-B November 23, 2014 - 1:17 pm

Does anyone know if Borax is the main ingredient in Concrobium? And if using borax would give as much benefit as Concrobium, without the high cost?

Thanks in advance for this information.


Brian February 20, 2015 - 1:16 am

Best explantion so far of use of Borax to kill mould. Now just have to order some online as you cant but it over the counter in the UK now.

Robert Butcher April 16, 2015 - 2:48 pm

I live in Florida and have to pressure clean my concrete patio every six months and then spray chlorine on it to try to eliminate my black mold but it keeps coming back. How can I eliminate the black mold permanently?

Ken Lilly April 30, 2015 - 9:48 pm

I am impressed with your site.
I never thought of this.
Now I know, and I have the link.
And I’ll be back for more.

How can I get a landlord to repair a roof that continually creates mold in the living spaces below.

Karen May 20, 2015 - 7:21 pm

I recently came across an interesting machine called the Air Reactor that according to testing results has been very effective at eradicating mold in the air.

elaine May 21, 2015 - 10:10 am

Hi Karen, I read about some of these machines but ratings were low when you searched consumer reports

Karen May 22, 2015 - 8:49 pm

Hi Elaine,

Do you have a link to what you’re referring to?


Tracey h June 20, 2015 - 11:15 am

Does 21 mule borax for laundry kill black mold spores when washed with 1 cup per load in scalding hot water than dryer in a hot dryer on 2 cycles

Betty Haston July 11, 2015 - 10:29 am

I have small strips of black mold (roughly a foot long and inch wide) in three different portions of my home as a result of a leaking roof. If I treat the mold I can see, will that kill any mold that might be ABOVE the ceiling in the attic? How would I be able to find out if ther IS mold in the attIc?

Carol July 21, 2015 - 4:29 pm

The only borax I can find in stores is 20 mule team borax laundry booster. Will this kill mold or do I need some other kind of borax

Ray O. July 22, 2015 - 8:57 pm

I suffer from mold allergy for about 6 years after going to many doctors which did not know anything about mold, I found a medicine at at Sprouse supper market for Allergy treatment called BioAllers and now I don’t suffer from
mold allergy anymore.

Paige Smith July 24, 2015 - 6:22 am

Using borax is a really neat idea for removing mold. I have been told before that borax is a necessary ingredient in making your own laundry soap. Borax also works as a great natural alternative.

Dave August 29, 2015 - 10:02 pm

Hi Carol, Yes 20 Mule Team is borax and is typically what you would use for these applications.

Brian September 1, 2015 - 8:23 am

Had a leak in the basement through a basement window. Replaced the windows and now I would like to get rid of the mold in behind the molding.

Christine Preble September 7, 2015 - 1:23 pm

For 4 months I lived in an apartment with serious mold/black mold problems, developing major health problems. It began with skin rashes, large boils, then hair loss culminating in ulcerative/lymphocytic colitis, losing 27 lbs. (going from 115lbs to 88lbs.). Finally I put everything in storage until I found a ‘mold free’ home. Now I don’t know if I should throw everything away. I would hate to part with some of my lovely antique furniture, however, I am still suffering same symptoms, from which I had a respite. I have sprayed antique wicher sofa with vinegar, done the same with all my lamp shades. Does mold live on? How about frozen food? Should all frozen food be thrown away; also how about salt/pepper – condiments like that? Should I throw it all away? I really do not want to get ‘really’ sick again. One Dr. is committed to helping me but of course she has other patients. I thank you in advance.

Marie September 10, 2015 - 12:32 pm

My question….what causes the mold in my closets? Do I need a dehumidifier in my house to reduce the growth of mold?

Brenda September 13, 2015 - 7:28 am

I used this borax mixture several times in my shower and there’s still mold present. Help!!

jl October 17, 2015 - 1:56 pm

Where are the answers to all these questions?

Jerry Emery November 5, 2015 - 12:11 pm

Don, black mold is not necessarily dangerous. Stachybotris is a type of black mold that is associated with a number of health issues, but not nearly all black mold is Stachy. The most common type of black mold in most areas are Aspergillus. This is a close relative of Penicyllin. The hazards of mold depend on the age, general health, and allergy response of the inhabitants.

Karen R November 16, 2015 - 5:05 pm

Mold is a type of fungus. Search: kill fungus effective. I found a site confirming the effectiveness of Tea Tree Oil, Grape Seed Extract, vinegar, bleach, baking soda. They work by changing the pH, removing oxygen, moisture, or the food source which the fungus needs to live.
I arrived at this site to find out the proportions for mixing borax to kill the brown mold which causes wood rot, aka dry rot. Borax seeks out moisture and is deposited in the soft tissue of the wood (which I believe is the soft spring/summer growth) and the result is that it hardens the wood … Not all the way but enough if it decorative wood and is not structural (I.e. The rotted pine around my windows is decorative, it just holds the glass in place … It’s not wood supports load such as my roof).
Minwax has a wood hardener. I used PC Petrifier … (Searching these products will get you more info.). And now am wanting to mix up my own. Neat idea about the paste.
They say to apply the hardener to the wood (remove any paint first of course). And a couple of applications may be necessary.
I drilled holes through the solid wood to see how far the rot had spread. And squirted the borax hardener into the holes with a very small straw (the one that comes with WD-40) and a kiddy-syringe the pharmacist gave me for free. Some utube videos show simpler applications, too. My surface is vertical, so I’m thinking about how gravity will help spread the hardener. And now that I’ve done a few applications, I’m going to drill some more holes even higher up to make sure any remaining fungus in the wood is killed.

I added some Tea Tree oil … As it apparently will kill all types of fungus. Will do this my next few windows too. And may try the other chemicals too (bleach, vinegar, NaOH).

About people saying the mould came back especially in Florida which doesn’t surprise … Think about all the pesticides that are put on say the peanut crops to keep the fungus at bay, (and may be the real cause of peanut butter allergies). Well, it is in the air, the spore babies. These little life forms break down the leaves in autumn and spring, renewing the soil … Circle of life stuff. I understand that humans support 13 pounds of critters which don’t have our own DNA, … We are part of each other’s ecosystems. Live with it.
But like the spiders and flies that get into my house, I won’t support these fungus critters either if they are going to be eating at my windows. So I’ll change the pH of their environment, take away the oxygen or moisture or food source.
Oh, and borax discourages those bloody ants who want to shack up with me too. Geez, so when my own kids move out, these critters and their grand kids will still be about. Too bad they don’t do stupid pet tricks or something.

Yeah, so search those terms: wood hardener, brown mood, kill fungus, tea tree, grApe seed extract, …

One more thing…. Our skin is an ecosystem.
Instead of topical steroids or expensive shampoos, I now use some vinegar on my scalp to kill the fungus thAt causes dandruff , zinc oxide(diaper ointment) on my skin to correct psoriasis.

Monica November 17, 2015 - 8:27 am

Betty Haston,

I have the same problems and concerns as you. I see you have yet to get a response, so I hope this response finds you. I’m providing a link that has a little bit of info about it. It at least gave me an idea of where to start. Just be sure to scroll down a little ways so you can get to the ceiling section. Good luck!



Lori Barker March 7, 2016 - 6:42 am

Great usage of borax! I didn’t know that can be used to kill mold, which is exactly what I need right now. Thanks a lot for sharing!!

Anne March 18, 2016 - 5:18 am

What is used with 20 mule just mix it and scrub or what ?

seriously April 27, 2016 - 4:01 pm

HOW are people confused by this… HOW!? mold in the brains? re read the damn article, its simple, TOO SIMPLE, mix and use, mix and use, THATS IT.

Adam C Baker July 5, 2016 - 7:47 am

I had luck wiping down a very moldy area with salt water, completely saturating the water with as much salt as it would hold, and it to dry. May be wrong, but I’m sensing mold isn’t a fan of salt.

Eric T July 16, 2016 - 3:25 pm

I hope it has mold capabilities other than just its PH a PH of 9.3 is kinda lame. The PH scale goes up in factors of 10 times. IE- a ph of 10 is 10 times alkaline as a PH of 9 even soapy water is way more alkaline than borax

10 000 000 battery acid
1 1 000 000 gastric acid
2 100 000 lemon juice, vinegar
3 10 000 orange juice, soda
4 1 000 tomato juice, acid rain
5 100 black coffee, bananas
6 10 urine, milk
7 1 pure water
8 0.1 sea water, eggs
9 0.01 baking soda
10 0.001 Great Salt Lake, milk of magnesia
11 0.000 1 ammonia solution
12 0.000 01 soapy water
13 0.000 001 bleach, oven cleaner
14 0.000 000 1 liquid drain cleanerTable 1. The pH Scale: Some Examples

David Silver September 4, 2016 - 9:19 am

Thanks for the info on using Borax. As I understand things, killing mold is only part of the battle. Removing dead, fragmented mold spores is the other. I understand that the dead, fragments are undetected by air sampling yet just as toxic as live spores.
How does one (if possible) remove them?
Much gratitude,

Petro Swanepoel September 19, 2016 - 3:38 am

Can anybody tell me why people used borx for curing biltong in the old days? is it save?

Karin September 24, 2016 - 5:23 am

I want to clean non-porous surfaces which only have mold “dust” on them, and no live spores. Is this borax solution effective in removing that
Microscopic dust? I am very skeptical.

Edward John JESKE October 7, 2016 - 11:12 pm

Interesting ideas especially clarification on items and there ph.
I have been sick for a long time because in part of some type of mold smell out in the woods and therefore in my air stream. I hope someone is going to tell me what to use to bolster my immune system against these molds without being exotic, cumbersome methods, and more expensive supplements with a lot of artificial things in the formula. There is no history of the Native White or Red Men having any problems to do with mold, only later Mexican, Italian, and European bacteria.
An interesting note about one of the types of black mold turned into by meat head the plumber in 2012 is that they allege it actually grows in the defunct Chernobyl reactor and feeds directly off the radiation being able to convert the energy to food. Imagine if I could breed a species variant that you could eat other fungus but only feeds on radio nucleitides, radiation, other fungus, and cellulose but doesn’t affect our healthy cells or rob us of are nutrients like a parasite does! Another strain I could breed to use for as a food source like some products made from different yeasts which is mold!

Edward John JESKE October 7, 2016 - 11:16 pm

what about different oils. I cant get any hits that directly attest to people having success with some oils to cure mold in the body and prevent future occurrence. The best I could previously feel about tee tree oil is that it causes my lungs to violently close down from an already not full open position.

Tyler December 12, 2016 - 9:42 pm

Hi, we had our washing machine break down and I forgot the clothes in there for a few days. The strongest mildew smells I have ever encountered – slimy even! Most reading I did on the web suggested borax and vinegar. It definitely helped a lot! But now I’m realizing they likely offset eachother and perhaps inhibit eachother from being as effective as possible. What is your take on this? What would you suggest? I had soaked them in a tub of borax and a lot of vinegar for 2 days. Then washed them. Some of them still have mildew smell. Some seem cured.

Demetra March 31, 2017 - 11:44 am

This is Dee, can someone PLEASE tell me how to get rid mold quick and for good. Do you have to through all your stuff away? and leave the only place you have to live. I really need to know grantee cheaper way to get rid of the mold of any color. i really can’t pay a expert .we all need money, for everything is so high. lets be real and be like the bible tell us to be, our brother’s keeper. thanks for your help’

rockstar chef April 8, 2017 - 1:16 pm

will borax take mold out of a canvas tent trailer I have tryed the mixture does it need the sun to help kill the black mold quicker. Just do not want to do it to many times on the canvas do not want to destroy it

Jen June 21, 2017 - 8:16 am

WARNING: Do NOT attempt to self-clean a mold infestation (any more than one section 2’x2′) ! Some mold strains are highly toxic. I became chronically, mysteriously unwell for two years after moving in. After the moldy wall was recently opened up, I suddenly developed urgent, new symptoms-facial swelling, “intoxication,” and numbness/tingling in limbs. I have stage 2 Mycotoxicosis. Professional house mold tests revealed to be high levels of Aspergillus Penicillium and Stachybotrys in our house. We had to evacuate our house, and hire a mold remediation company. Very little mold was visible- 4 square feet. We also live in a very dry, northern climate. But microscopic, airborne mold spores circulate through the house, as our air samples proved. Dust is largely comprised of mold spores, so all fabric, including carpet, is full of spores that cannot simply be “cleaned.” 25% of people, like me, have HASE in our genes, making us very susceptible to toxic mold exposure symptoms, especially if we also have asthma. The other 75%, like my husband, are not genetically sensitive to mold. Not all mold is highly toxic, but some is, like the two strains we found: one is a nuerotoxin, the other carcinogenic. Stage 3 Mycotoxicosis is irreversible. Get your mold professionally tested. The expense is worth it- your money or your life.

cbbrooks August 28, 2018 - 9:38 am

How to know how much water is too much? I put 2 tablespoons in a 2 cups of hot water..it sort of dissolved but not completely. it didn’t look like any borax was being applied. so I wiped off the “mold” and solution and started again with alot more borax and it kinda made a paste but not a “smooth ” paste. the solution was pretty gritty. I mixed it really well. should i just damp off the “wet ” areas on the wood or will that take off the borax? thanks

Stephanie September 28, 2018 - 6:48 pm

Some (as in many) would disagree with you on that last bit “don’t eat it”. Essentially, you’re right. One should not actually “eat” borax. However, ingesting it in minute quantities, dissolved in water can offer a multitude of health benefits. Just read “The Borax Conspiracy”.

rae October 23, 2018 - 5:33 pm

I have noted you said to NOT eat it. Well no…not normally. But ingesting a small amount
will not hurt you. In fact it is good for the muscles and the joints. The Boron in the Borax molecule is used in the muscle cell calcium movement, and thus facilitates the removal
of excess calcium….so will assist with relaxing the muscles in those persons who have a
condition known as hardening of the muscles.
The boron also is important in joint chemistry and may relieve oesteroarthritis pain.
(google it if you want more information)
Salt will make you quite sick if you were to ingest 50 gms of it in a sitting. Likewise Borax
would make you feel ill if you ingested that amount….but it would NOT kill you (although
ants and termites might disagree)

raelph houghton October 23, 2018 - 5:37 pm

if borax is on a painted surface….will the mold be able to get thru the paint?

Anne January 28, 2019 - 10:46 pm

Thanks for this article, Brian! Do you recommend wiping the mold-infested area again with a vinegar and water solution? I saw a few articles that recommended this and wondered whether it’s worth it and/or counterproductive.

Gloria camacho February 26, 2019 - 10:48 pm

is Borax more powerful than vinegar? How long will it take for mold to come back if cleaning is not successful? Humidity is a huge problem and most attempts have failed to reduce it. Cleaned with bleach but mold keeps coming and spreading. Will a humidifier and cleaning with Borax/vinegar help?

Sandra February 19, 2020 - 7:24 pm

I have often heard that borax and vinegar kills mold in laundry. Is it important to use them separately since they have the opposite PH directions? I have been using them together and I would like better results.

fry July 4, 2021 - 2:15 pm


The homemade BoraCare is the ticket for dry wood – kills and repels mold and any wood eating critters. Read the full article for a better understanding.
4 1/2 pounds borax
3 1/2 pounds boric acid
Mix the ingredients and heat till boiling gently. Boil off water until a candy thermometer shows 260°F. This removes most of the water of crystallization in the borax.
This solution is stable at 40°F and has a borate content of 26%. This is equivalent to Bora-Care® at about $100/gal. for the concentrate. The concentrate must be diluted with an equal volume of water before being applied.

Application: Add 1 gallon of water to every gallon of concentrate and stir thoroughly until solution is completely uniform. Always use diluted within 24 hours after mixing. If kept for longer periods of time, the active ingredient can drop out of the solution.
I put the WARM solution in a pump sprayer and cover all surfaces liberally, sometimes following up later as time/construction allows for a second application, particularly on old or contaminated wood. The glycol causes the borates to penetrate deep into the wood.

This solution is non toxic and could be used on a variety of surfaces as a preventive – I haven’t used it on unpainted drywall but plan on experimenting. I imagine you’d want to apply more lightly on drywall than wood. Perhaps a few light coats and allow to dry between applications.

I nixed a really nasty cockroach infestation in an old house while replacing rotted subfloor – ripped out all old cabinetry and rotten subfloor boards, sprayed everything as well as I could cover, underneath, ends, painted WALLS etc. The glycol will penetrate finished surfaces except for epoxy and polyethylene with no damage to the finish. Also sprayed all new lumber going down on all sides. In a week there was not a bug to be seen and they had literally taken over the house – one wall and ceiling in a bedroom was covered with roach frass, ecch.

I now use this as often as possible in my projects, a couple hundred and some time spent up front is well worth the peace of mind going forward. We do a crappy job in the US of preventive NON TOXIC treatments, just not in the culture. I’m in favor of changing that.

annekee November 5, 2023 - 5:18 am

Could i sprinkle dry borax on the back of an art canvas, leave it on for a r of hours and vacuum it off. There are some microscopic spots of mould and I am reluctant to use a liquid solution


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