If there is any risk of lactic acid absorption, it should be in the pamphlet that comes with the product. Salicylate poisoning is a real concern with salicylic acid products, but lactic acid is something normally found in the blood (leg cramps after running are due to lactic acid buildup). It takes a while to clear, and there is a potential for lactic acidosis. Anything that knocks the pH out of whack can cause respiratory and heart problems, and I did find a case in a science journal where that did happen:http://cat.inist.fr/?aModele=afficheN&cpsidt=17901938
Here's the abstract:
Percutaneous poisoning is quite rare, and in particular, topical lactic acid poisoning is even more so. We report a young girl with lamellar ichthyosis and symptoms of poisoning, with clinical signs of irritability, agitation, myoclonia, and difficulty in walking, accompanied by lactic acidosis as a result of the more frequent than recommended application of emollients containing lactic acid. The clinical symptoms resolved upon discontinuation of the topical treatment. Among the possible causes of metabolic acidosis, health care providers should be aware of the possibility of percutaneous poisoning.