WHAT IS THE LATEST ON THE USE OF BLEACH TO REMOVE MOLD?
The term “bleach” refers to chemicals that are strong oxidizing agents and, in the process of this oxidation, kill many microorganisms and decolorize many chemicals. There are two common types of bleach: Oxygen bleach, which contains hydrogen peroxide, and chlorine bleach, which contains sodium hypochlorite. Cleaning products usually contain chlorine bleach because it is strongest.
Chlorine bleach remains a good disinfectant and will kill many but not all molds. It also apparently damages the allergens associated with mold. However, chlorine bleach is a hazardous substance. The solution we know as “bleach” is a solution of hypochlorite, chlorine, and water. As long as the solution remains basic, the equilibrium remains in the hypochlorite direction. If you mix chlorine bleach with any acid (vinegar, drain cleaners, lemon juice, etc.) chlorine gas is produced. Chlorine gas is extremely dangerous and exposure can be fatal.
The bleach bottle also says not to mix bleach and ammonia. This is because the bleach reacts with the ammonia to produce nitrogen trichloride, which is explosive, and extremely irritating to the mucous membranes (it has been used as tear gas!)
These are the reasons I discourage the use of bleach in household cleaning. Another reason is the fact that it will NOT kill all mold. In fact there are several small studies that indicate as little as 50% of mold growth is killed on, for example, wallboard. On wallboard, nothing you do is going to remove all mold growth. You either must replace the board, or (and this is often the method of choice) fix the water problem, remove the visible mold with soapy water, dry thoroughly, and repaint. The mold will not come back as long as the water problem remains fixed.
You can use bleach to help remove staining caused by mold. However, my experience has been that whatever you do, at least on wallboard, you will have to paint over the remaining yellowish stains that bleach does not remove. Note also that whenever you do use bleach, it must be mixed with a detergent. Mold spores are generally hydrophobic and will be aerosolized if wetted with water or bleach alone.
Weekly use of bleach or bleach-containing products will keep mold out of shower tile and from any other place that has a hard surface and is more or less continuously wet. Note that bleach by itself does not stay on surfaces. It degrades and turns into plain water. Some of the bathroom and kitchen disinfectants have quaternary ammonium compounds that are silanized so that they stick to the surface. These last longer, but the quats kill fewer molds than bleach, and you will still probably have to use stuff weekly and mechanically remove any mold that has appeared. Peroxide-based bleach is less hazardous, but also less effective as a mold-killer than chlorine, and is probably a waste of time for mold remediation.
Dr. Harriet Burge is director of aerobiology at Environmental Microbiology Laboratory Inc. and associate professor and director of the microbiology laboratory at the Harvard School of Public Health. Widely considered the leading expert in IAQ, Burge pioneered the field more than 30 years ago. She has served as a member of three National Academy of Sciences committees for IAQ, including as vice chair of the Committee on the Health Effects of Indoor Allergens.
To submit a question to Dr. Burge, write to her by e-mail at askdrburge@emlab.com. All questions posed to Burge will receive a reply, although space limitations prevent us from publishing them all. By submitting a question, you agree to have your question and its answer published in a future edition of IE Connections.
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