There's scientific testing for tea tree oil that's not hard to find. I'll add my experience to what others are saying which supports it's both useful and probably toxic for same reason.

I got athletes foot a while back. Severe stuff with swelling and bleeding of the feet. They even glowed under a UV light. Trippy. I tried natural solutions like vinegar or baking soda first since that stuff solves lots of my problems. Didn't work. Then, the OTC medicines didn't work. If anything, it's like it went down slightly, then flamed up with more damage in response. It got a holy crap response from lots of people, even some with a version of it before. Like it was a living thing that wanted to pay back torture with torture. Got doctors to prescribe the standard stuff with it not working either. They also prescribed antibiotics in case of infections from the bleeding. Had to bleach and throw away lots of socks.

A friend told me some "folk" story about tea tree oil. Skepticism up. He handed me a study comparing it to a common treatment for my condition with it working in the study. Studies get BS'd a lot I thought. Told me several people he knew had skin conditions treated that way. Placebos or something else not that product? Said I wouldn't figure out what he was giving me. Anyone knowing me or my posts here probably guessed I figured it out before that day was over: a furniture cleaner with tea tree oil as a main ingredient. Lol. So, big container of water for my feet, a few capfuls in it, soak 30 min, and repeat for certain amount of time (can't recall). What to loose...?

Unlike the other treatments, my feet soaking in this stuff felt fairly good. After the time period, it was... gone. Recall I tried acids, alkaline solutions, OTC medicines, and prescription medicines. Then, this furniture cleaner based on tea tree oil killed it in a shorter time than the meds were supposed to with no negative effects. Hell, my feet were even shiny. :) I mean, it did worry me where I wanted to wash it off but they said leave it on. I also kept it up a week or so after the symptoms disappeared just in case. No long term effects that I can see.

"No real purpose." "Folk remedies." "Toxic to humans." These are good replies to all the shit the drug companies and doctors told me to use. None of it worked. It was all damaging in some small or big way. The big being enough that it took more drugs to prevent major problems. The tea tree oil handled the job well fast and with no effects. I'm not saying I trust it in general, think it's a miracle drug past killing the fungi kingdom on my feet, etc. I'm just saying it provably did it's job on living organisms that resisted 8+ months of doctor-prescribed treatments with no sign of weakness until the tea tree oil. I recommend more research be done on applying blends to stuff like that.