Collecting hair samples at a crime scene can be difficult if the crime occurred outdoors, where wind, rain and other natural elements can quickly contaminate the scene. While crime scenes can be carefully protected with special tarps, there is nevertheless a premium on securing the evidence from the elements and from possible human contamination as quickly and efficiently as possible. Under such circumstances, the hair should be collected as a clump and stored in a special sanitary container, which is then sealed to protect the integrity of the physical evidence within. Once in a lab, the hair strands can then be extracted individually and tested for DNA evidence. The clump of hair having been properly, according to established procedures, collected, secured, and transported to the lab, its integrity as having been found at the crime scene is secured. Any DNA successfully extracted from the hair strands -- and this is more complicated than it appears -- can then be certified as having come from the crime scene and from individuals identified as having been at that scene.
Clumps of hair found inside a building and not, or less, subjected to natural elements is also most likely to collected together, unless it is visibly distinct. In other words, if there are long blonde hairs found mixed in with shorter brown hairs, then an effort could be made to segregate the hairs for the purpose of identifying more than one individual. This is more likely to occur with pubic hair at a rape scene than with other types of hair.
DNA evidence collected from hair at a crime scene is used to prove that certain individuals were at that scene. This can be done whether the hair is collected individually or as a clump. If it is found in a clump, then on-site technicians are unlikely to try to separate out disparate strands at the scene out of concern for inadvertently contaminating the evidence. Hair samples are one of the less reliable sources of DNA evidence, saliva, semen and blood being more reliable. Only the part of the hair at the scalp, and still containing keratinocytes, provide reliable traces of DNA, in other words, the root of the hairs is reliable; the rest of the strand is not.