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Olmo Giovanni gravatar image

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 https://www.uic.edu/classes/pcol/pcol425/restricted/Tiruppathi/Metal%20Toxicity-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

https://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.