Hair samples on violent offenders?

A few weeks ago while at the hospital having lunch with some physician colleagues discussion turned to the recent Colorado school shooting. I argued that violent criminals should have hair samples tested for toxic metals. Today, Andrew Blankstein of NBC News published the article Are the Xbox and unleaded gas helping keep you safe from violent crime? Of the "seven other seldom-mentioned factors" that may be responsible for the decrease in U.S. homicide rates, Blankstein writes:

In a landmark 2007 study, Amherst College Public Health Professor Jessica Wolpaw Reyes found a remarkable correlation between lead exposure and violent crime. According to her calculations, exposure to the heavy metal could have accounted for between 28 percent and 91 percent of the 83 percent increase in violent crime in the U.S. between 1972 and 1992. And as lead exposure dropped, so too did violent crime, falling 56 percent during the 1990s, she found.