Smoker's forearms. Thermogram of forearms seen before (left) and 5 minutes after smoking a cigarette (right). The thermograms show the skin's temperature by recording its emission of infrared radiation. This heat radiation is displayed with each temperature in a different colour. Here, the temperature scale runs from white (over 34 degrees Celsius) through red (33 degrees), yellow (32 degrees) and green (30-31 degrees) to blue (28-29 degrees). After smoking, the blood vessels have undergone vasoconstriction (narrowing), cooling the forearms as less blood is flowing into them. The narrowing is mainly due to nicotine, the addictive nerve stimulant that cigarettes contain.