This brief from Sian White on the Hygiene Hub/ Covid19 website gives some guidance. 
resources.hygienehub.info/en/articles/39...QM6wVqX8F8IfHCa75ZhE

Can ash be used for handwashing? Topic brief

Ash can be used effectively for  hand cleaning in general . However, soap and water in combination are particularly effective for  killing and removing SARS-CoV-2  (see ‘ Why does handwashing with soap work so well to prevent COVID-19 ?). Ash is thought to remove germs from hands in a different way to soap - primarily through friction. It is unclear if ash would have the same effect as soap on the coronavirus as no studies have been done on this. 
However in settings where soap is really scarce handwashing with ash is likely to be more effective than handwashing with water alone. If recommending ash to households make sure that they are using the white ash from the centre of a fire once cooled. This white ash is likely to be the most sterile as it was heated at the highest temperature. Be aware that handwashing with ash does not feel very nice and does not leave hands feeling and smelling nice in the way that soap does, as such promoting ash may actually discourage people from practicing handwashing. We recommend also reminding people that soap of any type can be used for handwashing. See our section on ‘ Are some types of soap more effective than others? ’ for more information.
Recommendations:

• There is no evidence on the effectiveness of ash for removing or killing SARS-CoV-2. 
• In settings where soap is really scarce, remind people that any type of soap is effective for handwashing. 
• Where there are no other options, handwashing with ash should be encouraged as it is more effective than handwashing with water alone. 

Want to know more about COVID-19 and handwashing? 

• Why does handwashing with soap work so well to prevent COVID-19?
• Are some types of soap more effective than others?
• Should we be promoting handwashing with chlorinated water?
• What can we do where soap is scarce?
• Can soapy water be used?
• Is alcohol-based hand rub better than soap?
• Should we be promoting handwashing at different times during the COVID-19 outbreak?
• What can we do in areas with real water scarcity?
• Can I use greywater or water that is not clean for handwashing?
• Is it safe for people to share handwashing water?
• Do public handwashing facilities pose a risk?
• Can bar soap spread COVID-19?
• What kinds of handwashing facilities should we construct?
• How should hands be dried?

Here is another response by Mike Clarke that came in via the HIFA Dgroup:

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There is a Cochrane rapid review, podcast and Evidence Aid summary on the use of ash
for hand hygiene:

www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD013597/full
www.cochrane.org/podcasts/10.1002/14651858.CD013597
www.evidenceaid.org/hand-cleaning-with-a...fects-are-uncertain/

Best wishes,
Mike

Mike Clarke
Director, Northern Ireland Methodology Hub and Northern Ireland Clinical Trials
Unit
Research Director, Evidence Aid
Podcast Editor, Cochrane Library

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I copy the authors' conclusions from the rapid Cochrane review:

Based on the available evidence, the benefits and harms of hand cleaning with ash compared with soap or other materials for reducing the spread of viral or bacterial infections are uncertain.

Regards,
Elisabeth

I am copying a further reply by Alberto ACQUISTAPACE:

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Hi Erin and Emma,

According to " COVID-19 Emergency Response UNICEF Hygiene Programing
Guidance Note -Revision 1/4/2020 - sanitationandwaterforall.org/sites/defau...ng-guidance-2020.pdf " currently no evidence is available on the effectiveness of handwashing
with ash in the context of Covid-19. Generally, handwashing with ash is bears
the risk of contracting soil-transmitted pathogens and exposure to heavy
metals. Hence, handwashing with soap is preferred over handwashing with ash,
but the latter can be promoted as a last resort.

Also LSHTM discourages use of ash and promote soap ( globalhandwashing.org/resources/qa-diffe...lternatives-to-soap/ )
Hope to have been useful!

Alberto ACQUISTAPACE
Référent Technique Eau, Hygiène et Assainissement / WASH Technical Advisor
SOLIDARITES INTERNATIONAL| 89 rue de Paris 92110 Clichy, FRANCE
WebSite : www.solidarites.org

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And a response by Juan Smulders from the same e-Discussion on the RWSN platform:

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I am not sure youwill find a clear answer.
If you do not have soap, washing with ash is better than only water. Ash is
a basic with high pH and can kill germs (less effectively than soap
ofcourse).

It also creates friction to wash of dirt.
I would also like to share the newly create hygiene hub with a lot of
answers to questions and they even have a expert helpline to assist with
COVID related queries.
resources.hygienehub.info/en/articles/39...used-for-handwashing
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I am cross-posting this contribution from an e-Discussion about Covid-19 and WASH that took place in April 2020 on the RWSN platform (more information about that e-discussion here ).

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Posted by Emma Kelly (from UNC) on 15 April:
 
This is a response to the question by Erin Huber Rosen (Executive Directorand Founder, Drink Local Drink Tap) on 15 April:

Does anyone have research showing the proof that ash is effective in
cleaning to the standards needed for this virus and others? 

Hello Erin,

Thank you for your very relevant question about using ash as a hand washing
material. There is evidence that hand washing with ash (and sometimes even
sterilized soil) is effective in removing fecal indicator bacteria (FIB).
Studies in India (1) and Bangladesh (2), for example, found that washing hands
using ash was as effective as using soap. Both studies assessed this by
testing the quantity of FIB on the hands of participants post-washing.
A more recent study in Bangladesh found that washing with ash and water
reduced concentrations of FIB on hands, and that diarrhea was equally likely
for children of caretakers who used ash versus soap (3).

A review published by the International Scientific Forum on Home Hygiene
compiles some of the data available on the effectiveness of using ash to wash
pathogens off of hands (4). However, the review also discusses risks
associated with using soil or ash as a hand washing material, which may
themselves be contaminated with pathogens, helminths and heavy metals.

References quoted above:
(1) Anuradha, P., Devi, P.Y. & Prakash, M.S. Effect of handwashing agents
on bacterial contamination. Indian J Pediatr 66, 7–10 (1999). doi.org/10.1007/BF02752341 .

(2) Hoque BA and Briend A, Comparision of local hand washing agents in
Bangladesh. Jr Trop Med and Hyg 1991; 94 : 61-64.

(3) Baker, K.K., Dil Farzana, F., Ferdous, F., Ahmed, S., Kumar Das, S.,
Faruque, A.S.G., Nasrin, D., Kotloff, K.L., Nataro, J.P., Kolappaswamy, K.,
Levine, M.M., 2014. Association between Moderate-to-Severe Diarrhea in Young
Children in the Global Enteric Multicenter Study (GEMS) and Types of
Handwashing Materials Used by Caretakers in Mirzapur, Bangladesh. Am. J.
Trop. Med. Hyg. 91, 181–189. doi.org/10.4269/ajtmh.13-0509

(4) Bloomfield, S.F., Nath, K.J., 2009. Use of ash and mud for handwashing in
low income communities International Scientific Forum on Home Hygiene (IFH).

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It seems fair to say that there does not appear to be much evidence about the safety of handling ash. The nearest I could find (with reference to the potential dangers of heavy metals from wood ash) is this paper, which considered the dangers to workers in biomass powered power plants: www.degruyter.com/view/j/bimo.2015.2.iss...1/bimo-2015-0001.pdf

It seems that dermal metal exposure is a real issue however it seems to me that the risk to a worker in a power station would be expected to be several magnitudes higher than someone who was using the ash they produced from their own cooking.

Therefore I'd think that the important question is the potential risk of a dermal heavy metal dose from using ash compared to the potential benefits (and perhaps the risk of not using it, if there was no soap available).

The evidence as to the effectiveness of ash for handwashing seems mixed. See www.ajtmh.org/content/91/1/181.full

Cor posted the same question on Knowledge Point and he received the following interesting answer there today (knowledgepoint.org/en/questions/3285/is-...shing-with-ash-safe/):

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by Olmo Giovanni


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 www.uic.edu/classes/pcol/pcol425/restric...Toxicity-M2-2011.pdf

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

www.cdc.gov/niosh/docs/2005-149/pdfs/2005-149.pdf

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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Many studies show that ash is an effective and accepted alternative for soap. See a recent overview by Torsten Mandal.

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 - www.unicef.org/southsudan/media_15566.html .


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