Lately there has been some discussion on safe levels of essential oils. Depending on the intended application of the end product is going to determine how much essential oil is used. When we get into talking percentages, we're talking about the combined amount of the essential oil, not a percentage of each essential oil. For instance. It is commonly accepted practice that small children only receive 1.5% essential oil in a product, so your combined total cannot be above that level if you are making something for a child. And because children are so much more sensitive there is a list of essential oils that are the only ones that should be used for children (ref: Valerie Ann Worwood - Essential Oils for Healthy Children). There are a lot of people (unqualified) that like to create pages about things they may not be qualified to write about and want their opinion to be heard. Not everything we read on the internet is the truth and there is a lot out there written by quacks with their own agenda. In fact who knows maybe I'm a quack (I expect people who don't like me or agree with my line of study call me that). Your best resources are from trusted authors, and even better than that by taking a course in aromatherapy, even if it is just an introductory course so that when you use essential oils in product you understand safe handling and how to find the information you need to understand what you are using and why. Now let's touch on Photosentizers. All citrus oils are photosensitizing. What this means in layman's terms is that you are more likely to get a some burn when using a product with citrus in it if that are of the skin is exposed to direct, or even indirect, sunlight. Remember we can burn on a cloudy day (been there done that and it took me by surprise). So if you are using citrus in a product and you are using more than 2% concentration you should have a warning on the packaging. Is that the law - no. Is it responsible - yes. May Cause Irritation This one means that it can cause contact dermatitis which can run from a little bit of burning, or irritation all the way up to full blown sensitivy which can be an allergic reaction. Mind you anything that says "may cause irritation" is unlikely to give you blisters as would something that is a sensitizer. Sensitizer To become allergic to something we must be exposed to it more than once. Each time we are exposed to it we react stronger. An oil that is a sensitizer usually means that if you already prone to reacting to this ingredient or something within its botanical family you will be either have a reaction or make you more sensitive to the other allergy. For instance. I have an orange and lemon allergy - when I've had a reaction either mild or strong then I become more sensitive to my other allergies for about 3 months, for other people it can be 6 months to a year. The allergic reaction has my histamine levels up and reactive which is what an allergy is - your body rejecting a substance. This is so much a shortened version, but I hope you find it helpful and at least a little informative. My certification is Clinical Aromatherapist so I am by no means a master and my certification is only a year old so I too still have a lot to learn. The good news is that my training revealed how much more I need to learn If you get Saponifier then you will have read my articles on essential oil monographs which is a great way to learn about an oil. Take one oil that you want to use on a regular basis and learn everything you can about that oil, then move onto the next.