Formalin in leave on product as supportive preservative.
In the book cosmetic Microbiology it said: formalin is used 0.1-0.2% in shampoo.
My question is
can we use this amount of formalin in leave on product like face and body lotion at pH 5 and 4.1?
if no then how much formalin is safe in leave on product with phenoxyethanol and caprylhydroxamic acid or Piroctone olamine as supportive preservative?
In future i will use DMDM hydantoin but currently doesn’t have it available. Only formalin.