This is a super broad question, with a frustratingly wide array of answers.
Some ingredients we can pop fairly safely in the “no” category—things like asbestos or agent orange.
For everything else, the answer is “it depends”. Yes, everything—even water can be dangerous if it’s in your lungs or you drink far too much of it! Remember that natural ingredients can be toxic, too (hemlock, poison ivy, nightshade, and even mistletoe!). Natural is not an indicator of safety!
The first thing you should consider is dose; how much of said ingredient are you using? Ibuprofen is considered a very safe medication, but if you take too much you can damage your stomach and intestines. The dose makes the poison.
The safety data sheet for an ingredient can be a good place to start your research, but remember that they are dealing with the ingredient at 100% concentration. If the SDS says an ingredient in irritating to the eyes at 100% concentration, but you’re planning on using it on your hands at 5% concentration, that isn’t hugely relevant. Chilli peppers, for instance, would really suck if rubbed directly into the eyes, but many people enjoy them at lower concentrations in spicy foods!
The second thing you should consider is the usage. Titanium dioxide (and many other fine powders) are perfectly safe as long as you don’t inhale them. Tea tree oil is toxic if consumed. Some citrus essential oils are phototoxic in leave-on products. Water is safe to drink, but not to inhale. Anyhow, you get the idea, but do your research and ensure you are using the ingredient safely—something that is perfectly safe for topical application may become dangerous if inhaled or ingested, but that does not mean that the ingredient is inherently unsafe.
The third factor to consider is your personal sensitivities. Are you allergic to nuts? If so, it should go without saying that you should avoid all nut products, even though they are perfectly harmless to anybody without nut allergies. The same philosophy applies to all ingredient sensitivities; some people find sodium lauryl sulphate (SLS) to be very irritating, while many people do not. Some readers have told me they’re allergic to vitamin C, that doesn’t mean vitamin C is bad for everybody!
All that said, some ingredients are certainly more dangerous than others if used improperly. They might have a much narrower margin of dosing error, making it easy to use too much (think morphine vs. water; one has a far smaller margin of error than the other!). Some have much scarier side effects than others (endocrine disruption, carcinogen, etc.). Some are well-documented irritants/health hazards and would be best avoided in a home making situation.
In the end, I’d recommend doing proper, science-backed research and deciding what you’re ok with. Cosmetics Info is a great database; the EWG’s Skin Deep Database is not. I’ve written a post on how to evaluate sources to help prevent you from falling into the fear-mongering wormhole that can be the internet.
I suppose the general gist of this is “think critically, do research, and make your own decisions.” I hear from some readers who won’t make anything with water in it because they refuse to deal with any preservatives, and that’s fine. I hear from some who are really enjoying playing with synthetic ingredients, and that’s fine, too. This is you, your body, and your hobby.
Posted in: Safety