DNA is found in every cell in the human body including skin cells. Contact or touch DNA refers to DNA that is recovered from skin cells that are left behind when a person touches or comes into contact with an object. The presence of DNA on an object may be circumstantial evidence that a person came into contact with that object; the absence of DNA on an object may lead to the conclusion that a particular person did not come into physical contact with that object. Detecting contact DNA is variable and certain factors can influence the ability to detect contact DNA on an object. For instance the type of contact: the amount of pressure and friction, as well as duration of contact, can relate to how much DNA is transferred onto a touched object. An increase in the amount of pressure applied tends to lead to an increase in the amount of DNA transferred and the applicaiton of friction to the contact increases the amount of DNA transferred. The amount of DNA transferred onto an object increases with the amount of time an object is handled. Rough surfaces such as wood, concrete, grooved surfaces tend to collect and retain skin cell DNA better than smooth surfaces. Some individuals tend to touch their face, eyes, nose and hair more often than others, thus picking up DNA from those areas to be transferred onto the next item that is touched. Sweat is known to increase the amount of DNA available for transfer The more an individual sweats, the greater the chance that DNA will transfer onto an object he or she touches. Several studes have indicated that some individuals may be considered good shedders, people who naturally shed skin cells at a greater rate than others. DNA may persist on objects kept in a controlled environment.