Tea Tree Oil and Lavender Oil are the only two essential oils I have ever used or recommended for direct skin contact application without dilution. The problem you can run into with this is that different makers produce different strengths of concentration. "100% essential oil" isn't necessarily a guarantee that you are getting the strongest possible concentration of natural oils from plant. The actual effective chemical compound ingredients can vary.
I am the forum contributor who used tea tree oil on my daughter's horrific spider bite a couple of years ago. She had wounds that were at one point as large as the bottom of your average coffee cup, red and purple to the center and oozing in a way I have never seen before and hope never to see again. We used ichthammol to draw out the poison and necrotic material, puss, etc. and then cleaned the wounds with tea tree oil each day. Treatment also included daily immune system boosters, ice packs to hold down swelling, a highly beneficial diet and lots of rest. It was a holistic treatment that included tea tree essential oil as an antibacterial. The final result is that my daughter has only one nickle-size scar that is barely discernable, a healthy immune system and a very good eye for spiders.
The tea tree oil was tremendously effective and we use it today anytime we come up with an apparent bite or sting. As we are an active outdoor family, we are out where the insects are and in contact pretty much daily. I carry a small bottle with me everywhere I go, as do my children. I have used tea tree on small wounds on both my cats, my small dog and plenty of relatives. None has ever experienced any negative effects and the wounds healed without infection. One sniff kept the animals from licking.
The diffuser program using tea tree as an ingredient is intriguing and I wouldn't doubt that it can be very useful. I agree with Marlyn 100% that essential oils are very potent and pack as great or more of a punch than some over the counter typical medications. Before you use any essential oil, research it and get information. Educate yourself.
As far as how long they last, each brand is blended with dilution basic oils like jojoba, apricot kernel, grapeseed, olive oil or some other type. Some have their own antibacterial properties and so don't break down as quickly. Others need to be kept in a cool dark place and will last for years. One important thing is to apply them as you would any other medication; don't put your finger on the applcator lid - that contaminates it. Don't rub the top of the bottle along your wound...allow the drops to fall and then apply with a sterile instrument. Simple common sense things and people tend to forget.
I was glad to see this string, and hope that I've contributed positively. I know absolutely that my medicine cabinet is a lot smaller and more powerful than the boxes of junk many of my friends keep in their homes.
See ya! Kathe