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 second thing you should consider is 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. 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 sulfate 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 regardless of does or safe usage rates (parabens come to mind).
In the end, I’d recommend doing proper, science-backed research and deciding what you’re ok with. The EWG’s Skin Deep database is a good place to start; they bring together health ratings and warnings from organizations like Environment Canada, the FDA, and PubMed to help you get an idea of any potential hazards and usage considerations. Don’t just look at their number ratings, go in and read up on the ingredient. Sometimes it gets a good rating because there’s almost no data on it. Sometimes it gets a low rating but is still listed as “Classified as expected to be toxic or harmful”. Also, read the MSDS sheets for your ingredients; any reputable supplier should provide these for everything they sell. They will include all potential hazards, safety precautions, and how you should deal with any accidents.
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