[ninetenfour]

Another solution is the spray them with isopropyl alcohol. I do this with sandals if they ever start to smell. Works great. Smells for a bit while it evaporates but then it is fine.

Boric acid still makes nervous but it is completely safe?

[Renaud]

Boric acid is very inert. It tastes like chalk. The Wikipedia page says there are some some/weak evidence that it could affect reproductive systems, but you would need to ingest it for any effect (and a lot of it apparently).

It's not toxic otherwise. You need very high concentrations: lethal dose is 2.6g/kg. To put that in perspective, table salt is lethal at 3g/kg.

https://en.wikipedia.org/wiki/Boric_acid