“Every con­tact leaves a trace” is how the Locard Exchange Prin­ci­ple which is really a hypoth­e­sis is summarized.

It has been writ­ten as follows:

Wher­ever he steps, what­ever he touches, what­ever he leaves, even uncon­sciously, will serve as a silent wit­ness against him. Not only his fin­ger­prints or his foot­prints, but his hair, the fibers from his clothes, the glass he breaks, the tool mark he leaves, the paint he scratches, the blood or semen he deposits or col­lects. All of these and more, bear mute wit­ness against him. This is evi­dence that does not for­get. It is not con­fused by the excite­ment of the moment. It is not absent because human wit­nesses are. It is fac­tual evi­dence. Phys­i­cal evi­dence can­not be wrong, it can­not per­jure itself, it can­not be wholly absent. Only human fail­ure to find it, study and under­stand it, can dimin­ish its value.

The hypoth­e­sis goes that there are basi­cally two types of phys­i­cal evi­dence that come as a result of the Locard Exchange Principle:

1. Gross Evi­dence
2. Trace Evi­dence

Hairs are a type of trace evidence

When a hand touches wet cement is a good exam­ple of gross evidence

By plac­ing ones hands in wet cement, one leaves the imprint in the cement and exchanges for wet cement on one’s hands

If one is will­ing to accept this hypoth­e­sis, then it becomes quite clear and evi­dent that espe­cially with trace evi­dence the pos­si­bil­ity of con­t­a­m­i­na­tion is high and great pre­ven­ta­tive mea­sures must be taken by first respon­ders and sub­se­quent crime scene proces­sors so as to not neg­a­tively impact the phys­i­cal evi­dence at the scene.  This is why ded­i­cated clean suits and the like are nec­es­sary when pro­cess­ing a scene.

Dumb foren­sic sci­ence collection

Proper suit­ing up in a clean suit at a crime scene

The whole point is to try to have lit­tle to no impact on the scene or the col­lected evi­dence so as to not con­t­a­m­i­nate the scene or the evidence.