Scenario: You just got a new essential oil blend and can't wait to try it! You roll it all over your leg and massage the oil into your skin. A minute later, you notice red bumps raising up on your skin where you applied the oil. Wait, what??? Aaarrrggghhh! You realize you may be allergic to one of the ingredients.
While allergic reactions to essential oils are rare, they can occur. If you are allergic to certain plants or plant families, or have chemical sensitivities, you may have a higher possibility of reacting to essential oils. (This doesn't mean you will react - I tend to be very chemically sensitive, but have only once reacted to an essential oil blend, and that happened because I had let it oxidize.) Hopefully, you've already read in previous posts here to use whole milk to ameliorate a skin reaction, but it still stinks that you've spent money on this new blend and can't use it now! So, how can you be sure, before you buy an oil, that you won't react to it?
There are a few steps you can take. If you know what your allergies are, you can read the ingredients before you buy the product. For example, if you know you are allergic to plants in the Asteraceae family, you'll want to at least test oils that come from that plant family before using them. Reputable companies, like Dragoo's Oil Blends, will clearly list all of the ingredients in their products and gladly answer any questions you have. But what if you don't know what all of your allergies are?
This is where a skin test can be helpful. If you're buying from a local aromatherapist, (s)he may be willing to let you do a quick skin test before buying. I have samplers of my products at my flea market booth for that purpose, and before a customer buys a topical oil blend from me, I make sure (s)he does a skin test. However, buying oils online means you don't have that opportunity. In this case, I recommend you buy the smallest available quantity and perform a skin test when it arrives.
With a skin test, you put a drop of the blend on the inside of your wrist or elbow, rub it in, then wait 10 - 15 minutes to see if you're going to react. It's a quick, easy thing to do. If there's no reaction to the skin test, you can use the blend. If you do react, wash the area with whole milk, then soap and water and don't use the blend.
It's important to note here that you can develop a reaction to a substance, like an essential oil, even after you've been exposed to it many times. Regularly using an essential oil neat on your skin can cause you to become allergic to that oil. Whether it's the first or the 100th exposure, if you react to an essential oil blend, follow the steps above, then let the aromatherapist (or the company you purchased the oil from) know that you reacted and what the reaction was.