Ever since I started blogging about skin care, I’ve grown accustomed to seeing the same ingredients pop up over and over again. One of them is glycerin. A few months ago, a reader contacted me with an intriguing question: What’s the typical concentration of glycerin that would be included in a rich facial moisturizer? In this post, we’ll ask my dad that very question and give this ingredient some overdue attention.

Preservatives are necessary ingredients in personal care products. Would you want to find mold thriving in your face cleanser (or in anything else you’re applying directly to your skin)? Even so, it’s probably still better to ensure that your skincare and beauty products don’t contain preservatives that are likely to irritate your skin. I didn’t even know that some ingredients were only permitted in “rinse-off” products (like shampoos), until I asked my dad…

Do you ever notice the same ingredients appearing in lots of skincare products? Stuff you never noticed before? This happened recently with phenoxyethanol, a preservative that is often used as an alternative to parabens. Now it’s happening again with allantoin—I’m suddenly noticing it in lots of products that I write about on this blog. Allantoin is a white powder that has no odor, and provides conditioning properties to the skin, according to the National Eczema Foundation.