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Is handwashing with ash safe?

When I suggested that changing Sustainable Development Goals (SDG) Indicator 6.2.2 “Population with a hand washing facility with soap and water in the household” to “Population with a hand washing facility with soap or ash and water in the household”, it was brought to my attention to that this would be unwise because ash may be contaminated and therefore pose an unacceptable health risk.

My questions are:

  • Is the ash commonly used for handwashing likely to be contaminated? My assumption is that the ash comes from cooking stoves using firewood (twigs). This wood is likely to be safe unless it is sourced from a contaminated industrial site.

  • If the ash does contain trace contaminants like heavy metals, what is the risk that they may be absorbed by the skin taking into account that a only a small amount of ash is used for a very short period (the handwashing duration guideline is 20-30 seconds) and that the ash is then washed away with water?

  • If there are serious health risks, why does an organisation like UNICEF still promote handwashing with ash? In 2014 UNICEF promoted it in South Sudan -

  • Is there direct or indirect pressure from the soap-making private sector to discourage the use of (free) wood ash for handwashing?

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Unfortunately I am not able to answer your questions, but I really think you have brought really interesting points. I have the same concerns. Let's see if somebody can kindly provide some info.

alfirc gravatar imagealfirc ( 2015-09-01 01:40:22 -0600 )edit

7 Answers

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Interesting discussion. For the practitioner, I would suggest that the question is what is safer than what people are using now and sustainable. So if one can jump to soap and water in the house, great. But if soap is expensive and people poor, then perhaps ash is the best one can do for now if what people are using now is nothing! Or put another way are the risks associated with using ash greater than using nothing at all. Informed judgement?

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Ash is a strong alkali with Ph ranging from 10 - 12. Most bacteria cannot survive in such ph. Although alkaliphile bacteria such as vibrio cholerae survives at a Ph 9. Therefore ash acts as a disinfectant when used. However its possible corrosion of the skin can occur which depends on contact time and the Ph of the ash. Therefore in villages where availability of soap is a challenge ash is the best substitute.

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The only study I have come across on the literature was in a paper by Sandy Cairncross, which suggested that abrasion & friction were the mechanisms responsible for killing bacteria during hand washing. This would partially explain why there is little evidence for anti-bacterial soap or gels being superior to ordinary soaps in preventing diseases

Credible quantitative evidence for the benefits of using ash or sand, is likely to be difficult to obtain, as there is limited scope for providing a placebo for soap or Ash in a control group situation. I guess from a WASH perspective, promotion of the act of handwashing has the potential to build positive hygiene behaviours, the practice of which reinforces hygiene education messages and awareness of "F" diagram transmission paths within communities. Consequently rubbing hands with ash or sand as an alternative to soap at key times can be viewed as a positive hygiene behaviour, when the later is unavailable. According to the Cairncross paper it will have some beneficial impact. In most humanitarian contexts soap should be provided as a standard item in distributions (explicitly mentioned in the relevant sphere indicators and guidelines I seem to recall). With this in mind promotion of hand washing with materials other than soap should be seen as a stop gap measure while supply chains are established. I believe Ash has been used as toothpaste and for other hygiene uses, so it's promotion is better than nothing at least.

The risk from heavy metal contamination is likely to be low relative to consumption via foods, and the alkalinity of ash would tend to mitigate risks from contamination. In the context of setting post 2015 development Goals, surely access to soap and secure running water at the household level should be a minimum from the perspective of equity. I recognise that this is a question on belief's in response to your question on science, but the " soap or ash" element of your suggested wording strikes me as a serious lack of ambition in the post 2015 vision, surely we should be aiming for a world where everyone can maintain health and dignity?



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Hello Dietvorst,

Ashes are used as an alternative to soap in cases the latter should not be readily available, or in case of emergency, its advantages are ash being common and easy to find item in rural settings, and providing almost as good pathogen removal in handwashing as soap.

Regarding your questions, it is difficult to provide a general answer, as the risk of being exposed to heavy metals almost always depends on the location, type of firewood used, and type of contaminant in the firewood.

As a rule of thumb, when burning contaminated wood (e.g. railway sleepers, or any other treated wood), the main pathways for exposure are the products of combustion by inhalation and dermal absorption, and by ingestion of contaminated drinking water or via bioaccumulation trhough the foodchain.

That said, skin absorption can still play a role in chronic toxicity, mostly through the exposure to contaminated soils or clays, albeit once again this changes dramatically depending on the type of hazardous substance dealt with.

This paper provides an overview of the typical exposure pathways for the most common (and dangerous) heavy metals: while dermal absorption is mentioned as a possible pathway in the general exposure dyagram, it is then not mentioned once in each heavy metal description for lead, mercury, arsenic, and cadmium. It is worth noting, however, that some forms of lead (tetraethyl lead) and mercury are more prone to absorption than other

A good review of H&S and exposure pathways is the NIOSH pocket guide to chemical hazards, covering all possible substances being employed in the US manufacturing and service industry. I'm afraid there is no firewood ash in it, but it will definitely include heavy metals

So, to provide an answer to your four questions:

Ash may possibly be contaminated; the risk of absorbing heavy metals through handwashing is minimal compared to other exposure pathways; and I doubt there is a soap lobby pressuring for the use of soap, otherwise the ash reference would have never made it to the SDGs.

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The alternative to handwashing with wood ash is to produce lye from the ash and use it to saponify available fats to make soap.

There are several resources for local soap production. One of the most useful is from Practical Action:

I would suggest that the health benefits of handwashing far outweigh the possible harmful effects of using wood ash that may have become contaminated.

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Indeed, this article mentions that 'hyperaccumulator' species are used for phytoremediation of contaminated land, but assessment of the risk in a particular locality would require a detailed survey of soil composition and species harvested for wood, which probably ain't gonna happen.

While I agree with John that use of ash from any harvested wood is probably better than nothing, it does pose a direct threat to the user's skin. Anyone who has ever handled an alkaline solution will recall that it feels slippery. This is because it rapidly saponifies the oils in the user's own skin. In other words, the user is producing a small amount of soap from oils in the user's skin instead of from vegetable oil or another source of animal fat. (Remember, the Nazis were reported to have manufactured soap from human fat in their extermination camps.)

While "self-saponification" is kind to the environment, it clearly poses a risk to the integrity of users' skin, which could lead to dermatitis or infection by pathogens. I therefore think that users of wood ash would benefit from being advised to use minimal amounts and be vigilant that their skin does not deteriorate. Of course, external application of oil or fat to the skin would help to replace oils stripped from the skin.

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Ash is effective for removal, and often killing of pathogens like cholera bacteria, from hands. It is widely and frequently used, by the poor in e.g. South Asia. Drying kin can also be a problem for some with many types of soap, particularly the cheap and local. Heavy metals is not the problem from kitchen ash. Ash hand washing is of both recommended and neglected by the same organisations.

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