Other research suggests that smoking may actually cause skin to become thinner. A study undertaken by scientists at The Twin Research Unit at St. Thomas' Hospital in London, headed by Dr. Tim Spector, studied 50 sets of identical twins, which were divided into long-term smokers and non-smokers. Ultrasound was used to gauge the inner-arm skin thickness of the twins and the results were compared. The Twin Research Unit discovered that those subjects who smoked had skin around 25%, and even up to 40%, thinner than their non-smoking siblings. The skin of the twin who smoked also exhibited the usual smoker's characteristics of being less supple and more wrinkled.