Moisturizers fall into two main categories: occlusives and humectants. Occlusives trap water in the skin and prevent it from evaporating, while humectants draw moisture toward the top of the skin to plump up those cells. Since petroleum jelly is lipid-based, it's insoluble in water and therefore considered an occlusive ointment. The thick salve also qualifies as an emollient, which means it can soothe and moisturize the skin.
As a matter of fact, petroleum jelly ranks as one of the most effective occlusives for extremely dry skin. To evaluate how well a lotion moisturizes, scientists test for transepidermal water loss (TEWL) on a patch of skin after application. On average, petroleum jelly reduces TEWL by as much as 98 percent, outperforming both lanolin and mineral oil [source: Kraft and Lynde]. Petroleum jelly works so well because it not only blocks water loss, but the lipids also fill in spaces between the skin cells on the upper layer of the epidermis [source: Mayo Clinic].
Since petroleum jelly does such an excellent job softening skin, people slather it on areas susceptible to drying out, such as the feet and hands. In cold, windy weather, petroleum jelly also protects against wind burn and chapping. But despite its impressive moisturizing properties, many people refrain from applying the ointment to their faces for fear of clogged pores. Many dermatology resources also advise keeping pure Vaseline away from pimple-prone facial skin.
Yet, according to a study in the Journal of the Society of Cosmetic Chemists, petroleum jelly's pore-clogging reputation may be as mythical as wrinkle-free aging. Two groups of 10 acne patients applied different concentrations of petroleum jelly to their faces twice a day for eight weeks. At the end of the experiment, both groups showed significant zit reduction [source: Kligman]. Cosmetics shoppers who pay careful attention to labels might not be surprised by those study results since petroleum jelly is classified as non-comedogenic, meaning it won't clog pores. That's the same label given to light, nongreasy moisturizers as well.
Clear, healthy skin depends on two things: gentle cleansing and adequate moisturizing. What is doesn't rely on, however, is luxury lotions and creams. Petroleum jelly applied to slightly damp skin can lock in the moisture for that youthful glow at a pennywise price.