evidence: Firstly thinking that putting onions on your feet would purify your blood is just simply impossible as the liver and kidneys handle that. More so Phosphoric acid is more of a irritant than anything else its commonly found in soft drinks, in the link found here you can find a compound summary for phosphoric acid from pubchem, and if your interested you can find many more possible uses of phosphoric acid at [wikipedia].2 [...] To conclude sleeping with onions touching your feet or anywhere else for that matter isn't going to do much, although eating them will give you some health benefits.