Will a Hot Shower Always Destroy DNA Evidence?

Analysis of a DNA Profile by a Forensic Scientist

Whenever a rape occurs there is a potential exchange of hairs, fibers, DNA, even fingerprints and shoe or tire impressions from the suspect to the victim (and vice verse). Additionally, if a date rape drug was used then there will be traces of that in the victim’s bloodstream. Because of the fragility of these types of evidence, rape victims are encouraged to seek medical and/or police attention immediately. This is to ensure that the evidence necessary to identify the suspect can be preserved before it is lost.  Unfortunately, notification may not be immediate if it occurs at all. It’s understandable that some women do not want to report a rape and there are a number of contributing factors to that decision. It’s also understandable that some women want to take a shower after such an attack and you certainly can’t expect a victim to be thinking about the preservation of evidence after being violated.

Rape victims are generally examined by trained physicians or nurses who collect what we dispassionately call a “rape kit”. They take swabbings of the vaginal vault, mouth, anus, or areas where the suspect may have licked or bitten the victim, hair samples, blood sample, fingernail scrapings, etc. They take photographs of bruises or patterned impressions (impressions made by certain objects that may be linked to the scene or suspect). They will collect the victim’s clothing if available. This is all done by a same sex professional.

Now I have read scenes in several books (and one movie comes to mind) where a victim comes in the day after a rape to report it to the police. When she tells them that she had taken a hot shower they essentially throw up their hands and say there is nothing they can do now because the evidence surely is lost. Sounds like common sense after all right? Admittedly, a hot shower can destroy trace evidence like hairs, fibers, and DNA from the outside of the body. However, if DNA (semen) is present in the vaginal vault or anus then it may be present for up to 72 hours following the assault.   There is no guarantee obviously, but it is possible and detectives should make an effort to collect it, despite the shower or bath.

If you wanted to use this in a story you could have an inexperienced detective, patrol deputy, or even a family doctor tell the victim that there is nothing that can be done. This is usually devastating information to a victim seeking help (they may feel victimized all over again).  You now have a clock running down from say72 hours from the rape. You could use that countdown as the CSI or detective tries to track down the victim before it is too late and the evidence is lost. If the evidence is not collected you could also use that as a legitimate the cause preventing a series of rapes from being connected to the same suspect and possibly his release without arrest from interrogations. Just some things to think about.