Buildings that were affected by the 1974 Brisbane floods tended to have wooden walls or asbestos cement walls, though brick veneer with plasterboard was becoming popular. Since then, a lot of houses have been renovated and plasterboard is very popular. From talking to people affected by the January 2011 floods, it seems they will be content to clean down internal walls with a pressure washer and then perhaps repaint, when the plasterboard appears dry.
I suspect that fungal spores in the flood water will have reached the cavity behind the plasterboard and there will be a significant number of people who find they start to get unexplained headaches and respiratory problems that originate from these fungi. Even if the walls are cleaned with bleach or if the cavity is disinfected with bleach, then the problem may not go away.
Much of the official advice on killing moulds after floods indicates dilute bleach is satisfactory. This advice appears to be dated and inappropriate. Please read the following notes carefully, as wrongful mixing of chemicals can lead to toxic exposures of chlorine.
An Australian mycologist Dr Heike Neumeister-Kemp from Mycologica notes in Myths about Mould:
The widespread use of chemicals fails to correct the original reason why the mould grew there in the first place. It also introduces additional air pollution into the indoor air. Specifically, bleach has a high pH which makes it ineffective to kill mould. The mould detects the bleach as a chemical attack and defends itself with exo-enzymes and a good defending membrane. The exo-enzymes makes the chlorine compounds in the bleach inert which then the fungi uses it as a food source. So when we put bleach on mould we are actually feeding it. Visually it looks like the mould is disappearing because bleach “bleaches” which means it strips the melanin compounds out of the hyphe membrane (just like the melanin in our skin when we get a sun tan). Three weeks later the fungi hyphe recovers the melanin content and the mould becomes visible again so it was actually never gone.
Dr Kemp also notes the use of vinegar to be effective.
The most effective cleaning solution that we have against mould so far is our favourite salad dressing – vinegar. This is claimed to be the most effective because it actually kills mould, but doesn’t introduce a new chemical pollutant into the indoor air. Vinegar is even used by some European hospitals as one of their main disinfectants… A point of note is that only white fermented vinegar seems to work, as synthetic acetic acid does not appear to be effective. Diluted alcohol comes a close second…”
It is possible that a combination of bleach and vinegar is the best approach. The Vinegar Institute points to research by Dr Norman Miner at MicroChem, a laboratory that ” specialized in efficacy testing of disinfectants and other antimicrobial products“.
“Adding white vinegar to diluted household bleach greatly increases the disinfecting power of the solution, making it strong enough to kill even bacterial spores. Researchers from MicroChem Lab, Inc. in Euless, Texas, report their findings today at the 2006 ASM Biodefense Research Meeting.
Sodium hypochlorite (NaOCl) in the form of laundry bleach is available in most households. The concentrate is about 5.25 to 6 percent NaOCl, and the pH value is about 12. Sodium hypochlorite is stable for many months at this high alkaline pH value.
“Laundry bleach is commonly diluted about 10 to 25-fold with tap water to about 2000 to 5000 parts per million of free available chlorine for use as an environmental surface disinfectant, without regard to the pH value of the diluted bleach. However, the pH value is very important for the antimicrobial effectiveness of bleach,” says Norman Miner, a researcher on the study.
At alkaline pH values of about 8.5 or higher, more than 90 percent of the bleach is in the form of the chlorite ion (OCl-), which is relatively ineffective antimicrobially. At acidic pH values of about 6.8 or lower, more than 80 percent of the bleach is in the form of hypochlorite (HOCl). HOCl is about 80 to 200 times more antimicrobial than OCl-.
“Bleach is a much more effective antimicrobial chemical at an acidic pH value than at the alkaline Ph value at which bleach is manufactured and stored. A small amount of household vinegar is sufficient to lower the pH of bleach to an acidic range,” says Miner.
Miner and his colleagues compared the ability of alkaline (pH 11) and acidified (pH 6) bleach dilutions to disinfect surfaces contaminated with dried bacterial spores, considered the most resistant to disinfectants of all microbes. The alkaline dilution was practically ineffective, killing all of the spores on only 2.5 percent of the surfaces after 20 minutes. During the same time period the acidified solution killed all of the spores on all of the surfaces.
“Diluted bleach at an alkaline pH is a relatively poor disinfectant, but acidified diluted bleach will virtually kill anything in 10 to 20 minutes,” says Miner. “In the event of an emergency involving Bacillus anthracis spores contaminating such environmental surfaces as counter tops, desk and table tops, and floors, for example, virtually every household has a sporicidal sterilant available in the form of diluted, acidified bleach.”
Miner recommends first diluting one cup of household bleach in one gallon of water and then adding one cup of white vinegar.
In an email to me today (February 1 2011) Dr Miner notes:
Our initial work with acidified bleach was in reference to the anthrax attacks in the USA. One cup (8 oz) of bleach in one gallon (128 oz) of water is excessively strong for killing fungi/molds, and releases too much chlorine for that application. For acidified bleach for fungi/molds/vegetative bacteria/viruses (everything except bacterial spores) use 2.0 oz bleach in 128 oz water, and then acidify with 2.0 oz of 5% vinegar. This will decrease the pH of the solution to about 6.5 to 7.5, where most of the bleach then exists as hypochlorous acid (HOCl) which is about 200X more antimicrobial than bleach at 9.0 or 10 pH, where most of the bleach is in the form of the chlorite ion (OCl-). Make fresh solution daily. Follow all of the safety precautions as found on a bottle of bleach. My sympathies for the terrible floods you have experienced recently in Queensland. This acidifed bleach will help people recover from the molds that naturally follow too much water.
It appears the addition of a relatively small amount of vinegar to dilute bleach avoids most of the generation of chlorine. Many sources warn of mixing chemicals like vinegar and ammonia or bleach. I have not actually tried making up acidified bleach yet, but I suspect cold water may avoid chlorine production better than warm water.
It would be interesting to trial bleach, vinegar, acidified bleach, with and without a surfactant, with cold and warm water, and with and without scrubbing on different surfaces to see which factors were most important. The information would be of international importance.
I am trying to solve a problem with the “comment” field of this blog. Danny Gazzi (UK) emailed me about the issue and said:
The suggestion of adding vinegar to bleach surprised me. Adding an acid (e.g. toilet cleaner) to bleach has often resulted in the production of chlorine gas, and several fatalities among cleaners as a result of inhalation. Presumably vinegar has a sufficiently high pH to avoid this? What pH of the resulting mixture is suitable? I consider that it would be appropriate to add a warning to this section. The correct dilution could be critical.
I was also surprised to read that “only white fermented vinegar seems to work, as synthetic acetic acid does not appear to be effective”. Acetic acid is acetic acid, although the synthetic stuff might not taste as nice. If it’s the acidity that is the key factor, then the correct dilution of either, whether fermented or synthetic, should work equally well. If they don’t, then there is something else involved.
Update: Febrary 21 2011
I found the 1999 US EPA report “Alternative Disinfectants and Oxidants Guidance Manual“. Section 2.7 notes:
Chlorine gas hydrolyzes rapidly in water to form hypochlorous acid (HOCl). The following equation presents the hydrolysis reaction:
Cl2(g) + H2O HOCl + H+ Cl- Equation 1
Note that the addition of chlorine gas to water reduces the pH of the water due to the production of hydrogen ion.
Hypochlorous acid is a weak acid (pKa of about 7.5), meaning it dissociates slightly into hydrogen and hypochlorite ions as noted in Equation 2:
HOCl Û H+ + O Cl- Equation 2
Between a pH of 6.5 and 8.5 this dissociation is incomplete and both HOCl and OCl- species are present to some extent (White, 1992). Below a pH of 6.5, no dissociation of HOCl occurs, while above a pH of 8.5, complete dissociation to OCl- occurs. As the germicidal effects of HOCl is much higher than that of OCl-, chlorination at a lower pH is preferred.
Digging back historically, it appears that the original work on hyocholour acid was by Faraday:
It is already a known fact that hypochlorous acid (HOCl) is a strong oxider, disinfectant, antimicrobial, etc. This technology was originally discovered by Michael Farraday when he developed his Laws of Electrolysis in 1834.
Looks like I should get the original work. I find the the original researcher often has insights that have been overlooked by others that quote their work.
