I'm looking at whether mineral oil, an incredibly common moisturiser - possibly the most common moisturiser, check your moisturiser's ingredients - is dangerous again. Read part 1 here!
3. Mineral oil is comedogenic and will make you break out - FALSE
Mineral oil appears on a large range of "comedogenic ingredients" lists. Once upon a time (well, in the 1970s), cosmetic companies noticed that a lot of women started getting acne from their makeup products. One scientific study on comedogenicity used the inside of a rabbit's ear to test whether products caused pimples, and this quickly became the test of choice. However, later on, they found that sometimes the results on a rabbit and the results on a human were different. (Lab Muffin loves rabbits, and this made her sad, because an awful lot of rabbits got ear pimples for no good reason!)
A later study tested products containing between 0 and 30% mineral oil, and found that it wasn't comedogenic on human skin. The best thing about mineral oil is that (unlike a lot of plant oils) it's incredibly stable - it doesn't oxidise, and stays liquid. In other words, it's not likely to clump up later on, after reacting with oxygen and light, and clog your pores! However, this doesn't mean that it won't cause you to break out, since different people respond differently to certain ingredients.
4. It just sits on top of skin - it doesn't moisturise! It suffocates your skin - PARTLY FALSE
Remember this post on moisturisers? There are three ways in which moisturisers moisturise - occlusive (covering your skin up so water can't evaporate), humectant (grabbing water and keeping it next to your skin) and emollient (makes your skin feel soft) actions. Mineral oil is an excellent occlusive, so yes, it does just sit on top of your skin - but it definitely moisturises! In fact, scientists often use it as a standard for comparing other moisturisers. Of course, if you have dry skin to begin with, just putting mineral oil isn't going to work so well (if there's not enough water to begin with, there's not much water to keep in!).
As to whether skin can be suffocated - skin is porous, but it doesn't really need to "breathe". What people usually mean by "letting your skin breathe" means washing off the dirty gunk from your pores... dirt can stick to mineral oil, just like it can stick to anything else on your face.
Because mineral oil is really good at being an occlusive, it's possible that it can block certain nutrients in your cream from reaching your skin - the solution is to put on the active ingredient first, then cover it up with the mineral oil, and the mineral oil will keep that stuff on your skin.
A. L. Agero and V. M. Verallo-Rowell. A randomized double-blind controlled trial comparing extra virgin coconut oil with mineral oil as a moisturizer for mild to moderate xerosis. Dermatitis 2004, 15, 109.
J. C. DiNardo. Is mineral oil comedogenic? J Cosmet Dermatol 2005, 4, 2.