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The truth about bleach & mold

You’ve probably heard a lot about using bleach as a means to effectively kill and remove mold growth from your home. Whether as a result of major flooding, or minor water damage – bleach shouldn’t be used as a means to kill mold and stop its spread.

Can bleach stop your mold problem?

The short answer is no. Bleach simply is not designed to be an effective commercial mold killer. In reality, using bleach on your walls to clean up mold growth may actually be doing more harm than good.

A quick refresher

Before we explain the science behind using bleach on mold – it’s important to remember that mold needs two things to grow and spread: moisture and warm temperatures.

Just like an normal plant, mold fungi has a root system (mycelia) that provides the structure for mold growth. Mold fungi grow and colonize the area surrounding it by spreading microscopic spores along the surface and through the air.

With those facts in mind, let’s turn back to bleach.

Designed for “non-porous” use.

When you look at a bottle of bleach, the label states that bleach works best on a hard or non-porous surface. So that’s something like bathroom tile, glass, a bathtub or a kitchen counter-top. On a porous surface like wood or drywall – bleach is not designed to penetrate deep down beyond the surface to clean and disinfect.

The problem is that when bleach is used on an area of mold contamination, it doesn’t actually kill the mold growth or the sub-surface mycelia (root system of the mold colony). The typically recommended bleach mixture only bleaches the existing mold white or clear – just like bleach is designed to do with clothes – but leaves the mold to continue growing.

Check out Clorox’s website when you get a chance, you’ll notice that they don’t list bleach as a treatment for indoor mold growth anywhere on the site.

The Garden Weed Analogy

A very good example to way to explain how bleach is ineffective at mold removal is to compare it to weeds in your garden. If you took a pair of scissors and cut the above ground part of a weed off, it will grow back in a few weeks time. The root system of the weed is still there, it’s going to grow back.

Bleach does the same thing – removes the green or black stain that mold causes, but doesn’t kill the embedded root system of the mold fungi.


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Types of Mold

Types of Mold

While not all are allergenic or toxic, here’s a few common harmful indoor molds:


Mycotoxins are the poisons produced by molds. The EPA states that mycotoxins can be ingested, inhaled or absorbed through the skin – all with harmful effects. Here are a few examples of the most harmful mycotoxins produced by indoor molds:

Mold needs water to grow

But the issue of porous surfaces isn’t even the biggest problem with using bleach to kill mold on wood or drywall. The active ingredient of bleach is chlorine – the same stuff you put in a pool to clean and disinfect. But do you know what makes up 90% of a bottle of bleach? Water! And I’m sure you remember that mold requires moisture to grow and spread. So once you scrub a patch of mold with bleach, the visible mold will be bleached and wiped away, but the root system will remain untouched. Untouched and given a fresh supply of water to expand its roots. Give it a few days, that patch of mold will be back and larger than it was before.
Mold Spores

Oregon State University Study

In 2004, OSU released a study regarding the effectiveness of bleach to remove mold growth from wood surfaces. The entire study can be found here.

Feel free to read the entire study (it’s only a few pages long), but we’ll summarize a few important points from this research.

• They measured the effectiveness of water and water/bleach mixtures (from 2.5% to 20% bleach) to remove the visible staining that mold causes on wood.

• They discovered that while washing the boards reduced staining/discoloration by 5 to 15% – there was no significant difference in the effectiveness between all the treatments.

• So from 2.5% bleach, all the way up to 20% bleach – water was just as effective in reducing discoloration as a bleach mixture was.

• Additionally, they concluded that bleach does not eliminate surface microflora on wood. Microflora being the microscopic fungi that grows into the large mold patches you see.

• Finally, the study notes that reducing the moisture level of wood (below 20%) is necessary for proper mold remediation.

Doesn’t it seem a little silly to douse mold with a cleaner that’s 90% water when the goal is to remove the moisture from the surface?

We think it is.

But what about the EPA, CDC, etc?

People often say, “if bleach doesn’t kill mold, why do all these government agencies tell us to use it to kill mold?” Which is an excellent question, because for a long time, bleach was the standard recommendation for mold cleanup.

In the last few years, the perception of bleach as a mold killer has changed with some of these agencies. For example, the EPA does not advise using bleach to remove mold – these days they suggest a detergent mixture. The Occupational Safety & Health Admin. (OSHA) clearly states they do not recommend the use of bleach.

The CDC hasn’t completely gotten on board with not recommending bleach for mold cleanup, but their website does clarify that bleach should only be used on hard, non-porous surfaces. So that’s tile, glass, counter-tops, etc. – not wood or drywall.

The Garden Weed Analogy

As you can tell, it probably doesn’t make much sense to use a cleaning agent that is almost entirely water to kill a fungus that needs moisture to grow.

For example, when a commercial company remediates a building with mold contamination, we use a commercial fungicide that was specifically designed to kill mold. And because of that, the cleaner we use is not water-based.

Simply put, a water-based cleaning agent should never be used to tackle mold growth.

Have Questions Contact Us Today!

Feel free to contact us with any questions or comments you may have.

We contacted Roland about possible moisture/mold issues in our home. He promptly arranged to inspect and found some serious issues that needed to be addressed and corrected. Novos is the best company we have ever worked with. Prompt, professional, trustworthy and great at communication. All issues were documented by photograph and sent to us, together with progress photos and detailed plans to fix any further issues. Not only did they correct the moisture and mold issues, but they also corrected structural and foundation issues to ensure our home is safe from mold and moisture in the future. We got our home back thanks to this company. I highly recommend InspectRite and Novos.



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