In 1985, Dr. Douglas Model found he could identify smokers by their facial features alone. People with "smoker's face" had lines at the corners of their eyes and mouths, grayish skin, and a gauntness in their cheeks. Cigarette smoking causes blood vessels at the surface of the skin to constrict, reducing the oxygen level in the blood there. This thickens the blood and reduces the collagen levels in skin. All of these contribute to making skin look older more quickly.