Obagi Professional-C Serum 15% defends against harmful free radicals for a more youthful look. Free radicals come from environmental factors such as pollution, UV rays and cigarette smoke. They break down collagen, which leads to cell damage and loss of elasticity in mature skin. The body’s ability to defend itself against these molecules becomes weaker as we age. That’s where this vitamin C serum comes in handy. This formula breaks down environmental aggressors so the body is able to destroy them.