Does bleach actually kill mold?

The two federal agencies that publish mold guidance for homeowners differ in emphasis, not in substance. The EPA, on its page answering whether to use bleach to clean up mold, says using a chemical or biocide that kills mold, with chlorine bleach as its example, is not recommended as a routine practice during mold cleanup. The same page allows that professional judgment may call for one in some cases, such as when people with weakened immune systems are present.

The CDC is more permissive: its mold guidance says mold can be removed from hard surfaces with household products, soap and water, or a bleach solution of no more than 1 cup of household laundry bleach in 1 gallon of water. The 1 cup figure is an upper limit rather than a required strength, and the CDC adds that you should follow the manufacturer's instructions for any cleaning product.

Both agencies limit bleach to hard surfaces, both tell you to remove mold rather than just treat it, and both say the moisture problem has to be fixed. The disagreement is only about whether bleach belongs in the routine.

The EPA's Brief Guide to Mold, Moisture, and Your Home says dead mold may still cause allergic reactions in some people, so it is not enough to kill the mold; it must also be removed. A surface that has been bleached and left alone can look clean and still carry the same material that caused the reaction in the first place.

Sterilizing a room is not the goal either. The same EPA guide notes that in most cases it is not possible or desirable to sterilize an area: a background level of mold spores will remain, and those spores will not grow if the moisture problem has been resolved. The EPA's flood cleanup guidance adds that disinfectants only work on surfaces that have already been cleaned, and that many of them are not intended for surfaces that absorb water, such as fabrics or unfinished wood.

The concern behind all of this is health, which a contractor should describe carefully. The CDC says exposure to damp and moldy environments may cause a variety of health effects, or none at all. For some people, mold can cause a stuffy nose, sore throat, coughing or wheezing, burning eyes, or a skin rash, and people with asthma or mold allergies may have severe reactions. Questions about your own health belong to a physician.

You will read this on a great many websites: the chlorine stays on the surface, the water in the bleach soaks into the drywall, and the mold underneath is fed rather than killed. No government agency or standards body we could find supports that claim.

The EPA did not allow it on one product's pesticide label. In 2010, when a mold-product maker proposed label language saying "Bleach cannot penetrate porous surfaces like wood, drywall, grout, and concrete," the EPA's pesticide office required the statement deleted as a comparative claim. The lab evidence points the other way: University of Arizona researchers reported that household bleach reduced mold on drywall surfaces by more than 99 percent.

None of that makes bleach the right tool for moldy drywall. The real reason moldy drywall usually comes out is that mold grows into porous materials. The EPA's mold cleanup guidance says absorbent materials such as ceiling tiles and carpet may have to be thrown away, because mold can fill the empty spaces and crevices in them and be difficult or impossible to remove completely. The Minnesota Department of Health names drywall directly: porous items like carpet, drywall, insulation, and paper should be bagged and thrown away if they are growing mold.

The steps below follow EPA and CDC guidance for a small area on a hard surface.

Size is the first test. The EPA's mold cleanup guidance says that if the moldy area is less than about 10 square feet, roughly a 3 foot by 3 foot patch, in most cases you can handle the job yourself. For areas larger than that, the CDC suggests considering a professional.

The EPA is careful to say its size categories exist to help choose a cleanup technique, and are not based on health effects. Size is also only one consideration. The EPA lists other factors that matter regardless of square footage:

Testing is usually not the next step. The CDC does not recommend mold testing, and its page on Stachybotrys chartarum, a greenish-black mold, says it is not necessary to identify the type of mold, because all molds should be treated the same with respect to potential health risks and removal. When a job is bigger than a weekend cleanup, see how we handle mold remediation. If the patch is small enough to handle yourself, we will tell you so.

Standard policies usually do not cover mold unless it came from a covered event. According to the Insurance Information Institute, standard homeowners policies generally treat mold the way they treat rot and insect damage, as a maintenance problem that the policy does not cover. The exception is mold caused by a covered peril, such as a burst pipe, in which case the cost of removing it may be covered.

The line is usually between sudden and gradual. Liberty Mutual gives one example that is not likely to be covered: a leaky faucet that went unrepaired for months and led to mold damage on flooring, countertops, and cabinets. Liberty Mutual adds that mold claims are evaluated case by case and that coverage also depends on the limits in your policy, so check your declarations page and any endorsements before assuming either way.