Zinc Oxide
ZnO is used in everything from paint to dental fillings to sunscreen. A dirty diaper contains stuff tantamount to toxic waste, and zinc oxide provides a powerful barrier between scum and bum.
White Petrolatum
This waxy yellow gunk was discovered coating the surface of oil-well drill bits and filtered into a colorless, odorless, jellylike mass that's impervious to water. You probably know it better by its brand name: Vaseline. Liquids increase the permeability of skin (allowing irritants like urine and fecal enzymes into the epidermis — ouch!), so keeping baby's skin dry yet not dried-out is one key to combating diaper rash.
Anhydrous Lanolin
Sheep exude this stinky yellow oil into their wool as a waterproofing agent. Purified into lanolin, a wax with a melting point around human body temperature, the moisturizing goo can also thicken the inner skin layer by accelerating the growth of new cells.
Stearyl Alcohol
Now easily extracted from plants, this fatty alcohol used to be distilled from sperm whale oil. Water utilities sometimes float a thin layer of stearyl alcohol on the surface of their reservoirs to retard evaporation. Note the water-barrier theme.
Bisabolol
An update on granny's folk remedy for diaper rash, this compound from the herb chamomile penetrates the skin and has anti-inflammatory, antibacterial, and muscle-relaxant properties.
Oat Kernel Extract
Of all the indignities caused by sitting in your own poop, the worst may be a Candida yeast infection. Not merely another skin soother, extract of Avena sativa seeds also contains a very mild antifungal protein.
Cholesterol
This vital component of our cell membranes is the ur-steroid, the material from which progesterone, testosterone, and other hormones are derived. But while some skin creams are steroid-based, here raw cholesterol is most likely just a moisturizer and emulsifier for keeping all the other ingredients properly mixed.
Polysorbate 80
A food additive from the atomic age, when artificial meant better, this chemical has been touted as a baldness cure (unproven), denigrated as a carcinogen (not), and held up as evidence that flu vaccine is part of a vast government conspiracy (isn't). It's actually just an emulsifier.
