"Alcohol" refers to a group of organic compounds with a wide variety of forms and uses in cosmetics and in other types of products and solutions.
For skin, there are good alcohols and bad alcohols, corresponding respectively to high molecular alcohols and low molecular alcohols, which can have emollient properties (cetyl alcohol) or act as cleansing cleansers such as isopropanol.
There are also benign forms, including glycols, which are used as humectants to help hydrate and deliver ingredients to the skin's top layer. Low molecular weight alcohols - the bad for the skin - can be drying and sensitizing. The alcohols to be concerned about in skin care products are ethanol or ethyl alcohol, denatured alcohol, methanol, isopropyl alcohol, SD alcohol, and benzyl alcohol. The concern is when one or more of these are listed among the main ingredients; small amounts in an otherwise good formula are not a problem.
In addition to being drying and sensitizing, these alcohols can disrupt the surface layer of the skin. Alcohol helps ingredients like retinol and vitamin C to penetrate the skin more effectively, but it does so by breaking down the skin's surface layer - destroying the very substances that keep your skin looking healthier and younger in the long term.
Alcohols like SD and "denatured" immediately damage the skin and start a chain reaction of damage that continues long after it has evaporated. A published study from 2003 found that with regular exposure to alcohol-based products, cleansing becomes a damaging ordeal - the skin is no longer able to prevent water and cleaning agents from penetrating it, further eroding its surface layer.
It also destroyed the substances in the skin that help soothe and defend it naturally against the visible effects of environmental damage.
If that wasn't bad enough, exposure to alcohol causes healthy substances in the skin to literally self-destruct. The research also showed that these destructive, aging effects on the skin's substances increased the longer the exposure to alcohol lasted; that is, two days of exposure was dramatically more harmful than one day, and that's only from exposure to a 3% concentration (most denatured alcohol skin care products contain higher amounts than that).
Simply put, for healthy-looking skin at any age, avoiding products that contain large amounts of the drying, sensitizing types of alcohol is a skin care must.