Moldy drywall - don't just bleach (C) Daniel FriedmanHow to Kill Mold - Do We Want to? Is Dead Mold Dangerous?

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Mold killing guide:

This document explains a very common mistake in mold cleanup jobs: relying on bleach, biocides, or fungicides to "kill" toxic mold.

This is a chapter of the Mold Action Guide, a document which provides an easy to understand step-by-step guide for dealing with toxic or allergenic indoor mold and other indoor contaminants: what to do about mold.

The steps in this article series will be sufficient for many building owners who want to do their own mold investigation, mold testing, mold cleanup, and mold prevention in their home or office.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

HOW TO KILL MOLD: Should We be Trying to Kill Mold?

Severe mold contamination in a home (C) Daniel FriedmanMany readers have contacted us to ask about using bleach, fungicides, or biocides as a "mold remediation" step and many mold remediation projects we've seen have placed too much reliance on attempts to spray or gas mold to try to "kill the mold and render it harmless" that we've added this brief article on what's "right" and "wrong" about relying on any spray, gas, chemical, or "treatment" to treat problem mold in buildings.

Here is what the U.S. EPA says about using biocides, bleach, mold killers, mildewcides:

Cleanup and Biocides

Biocides are substances that can destroy living organisms. The use of a chemical or biocide that kills organisms such as mold (chlorine bleach, for example) is not recommended as a routine practice during mold cleanup.

There may be instances, however, when professional judgment may indicate its use (for example, when immune-compromised individuals are present). In most cases, it is not possible or desirable to sterilize an area; a background level of mold spores will remain - these spores will not grow if the moisture problem has been resolved.

If you choose to use disinfectants or biocides, always ventilate the area and exhaust the air to the outdoors. Never mix chlorine bleach solution with other cleaning solutions or detergents that contain ammonia because toxic fumes could be produced.

Spraying, Bleaching, or Killing Mold Does Not Kill All of the Spores

First of all, after spraying or gassing "toxic mold" or "black mold" in a building, the bad truth is that the mold is not dead - at least not all of it. Even if you spray bleach all over your "black toxic mold" you won't kill every single spore.

I've examined bleached mold samples in the lab. I can usually tease out viable spores from the supposedly "dead" mold sample. To kill every single mold spore using bleach, for example, you'd have to use such a high concentration of disinfectant and you'd have to keep it on the surface for so long that more likely you'd damage the structure - and still miss some toxic spores.

This is particularly true when people rely on spraying rather than physically cleaning or removing moldy surfaces or materials.

To be scrupulously fair, if we sprayed a surface with sufficient force as to actually physically clean it (and if we removed all of the sprayed-on liquid and debris, we'd do a credible job of cleaning the surface of problem mold. Of course in fact, you could use water or deck cleaner or a dry product like baking soda for such purposes just as well as a disinfectant. See our Report on Media Blasting for an example of using a "spray" approach to physically clean a surface.

- Thanks to Arlene Puentes for technical edits.

Killing Mold Spores Leaves Toxic or Allergenic Particles in Place

Second - even if in theory we could "kill" every spore, the assumption that they are unimportant is highly questionable. "Dead" spores often contain allergens or toxins that are just as harmful to someone breathing them or getting such mold in one's eye or in a cut, as before.

Relying on Mold Sprays, Bleach, Biocides, Fungicides Risks Taking an Ineffective "Short Cut" to Mold Removal

Third - when I hear remediators or testers focused on whether mold is "alive" or "dead" we are immediately concerned that they do not understand the best practices regarding mold problem diagnosis and cure.

The object is not to "kill" mold, it is - to remove the mold reservoir in the building by physical cleaning or in cases of items that can't be cleaned, such as drywall, soft goods, carpets, furniture, or insulation, remove the moldy material - to identify the cause and make sure that's been corrected (which in your case you think has been done but I'm doubtful where a crawl space is involved - wet conditions that made part of a building wet have often affected other building areas that are less obvious).

It is Important to Find, Identify, and Remove All of the Problem Mold Reservoirs in a Building

Fourth and very important - we need to be very confident that ALL of the substantial mold reservoirs at a building have been identified and cleaned-up or removed.

My experience is that very often people focus too quickly on the mold that they see, say on wood in a crawl space, and fail to detect an as large or larger and as problematic or more harmful mold reservoir that they didn't "see". Examples of this mold killer approach error include:

Hire a Competent Professional for a Thorough Visual Inspection for Problem Mold and Its Causes

SO what you need (and might already have) is great confidence that the expertise of the mold or building inspector and the scope of the investigation have been very accurate and thorough before any remediation project is begun. Otherwise the risk is that you have to repeat the process again later.

Even if you clean or remove problem mold thoroughly, if you do not correct the original cause of mold growth, you are likely to face this trouble and cost all over again.

How Should Small Areas of Mold be Handled?

Small Areas of mold: clean and disinfect surfaces: small areas of mold (less than 10 sq.ft. or less than 30 sq.ft. in some guidelines)can be cleaned by most property owners using common household cleaners or simple soap and water; if using bleach, extra caution is in order.

While demolition/removal of building components in of small areas of mold may not be required, in some cases removal/replacement of moldy materials permits valuable additional investigation for hidden mold, and may be easier than cleanup, particularly where surfaces or materials are in poor condition or where mold is on drywall or other components such as fiberglass insulation or duct work which can't be cleaned. If further investigation, renovation or repair discloses mold or conditions which cause growth of large areas of mold, professional cleanup/removal by an expert remediator would be needed.

How Should Large Areas of Toxic or Allergenic Mold be Handled?

Large areas of mold (more than 30 sq.ft. of contiguous problem mold): professional cleanup is needed by an expert remediator and is likely to involve significant expense.

"Large" is more than 10 sq.ft. according to the US EPA, or 30 sq.ft. per the NY City Guidelines. For instructions for remediation see the NY City guidelines at If your remediator is not familiar with these guidelines s/he may not be properly informed to do the job.

What Good is Bleach in a Mold Remediation Project?

Though normally weI do not enthuse about "bleaching" mold (since leaving a dead mold spore in place can still be harmful and can leave toxins), careful cleaning of otherwise already HEPA-vacuumed debris-free surfaces with a proper bleach solution (5%) is an effective end measure for hard surfaces - not much use on upholstered furnishings.

Watch out: do not accept a mold remediation job that relies principally on bleaching surfaces.

The mold reservoir needs to be found, removed, its cause corrected, and all related surfaces cleaned (or non-cleanable materials thrown out). Then using a biocide, sanitizer, or bleach solution for final cleaning is ok provided the solution is used according to the manufacturer's directions and applied safely.


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