Most skin benefits from using moisturizer, but not all moisturizers are alike. Although people with oily skin may not think they need a facial moisturizer, environmental factors—heat, cold, wind, sun—as well as indoor heating and cooling deplete moisture, leaving skin dehydrated (lacking water), even if it's oily. The principal ingredients in most moisturizers are water and emollients (animal, vegetable or petroleum-based oils), which act as a barrier to moisture loss. Oil-free moisturizers contain humectants (such as glycerin, propylene glycol and lactic acid) that work by drawing water into the outermost layer of the skin. Some moisturizers contain both emollients and humectants. Facial moisturizers designed for day use are usually of a lighter consistency than facial night creams or eye creams. Moisturizers are also formulated by skin type: more emollient formulas for dry skin; noncomedogenic, oil-free formulas for oily skins.