It's very comparible to the LD50 for drugs. When they try to figure out the average dose that will kill 50 percent of lab rats, some of those lab rats will be well above the LD50 and some will be well below the LD50.

It very similiar with mercury sensativity. This gov't set bar, saying if you are below a certain dose you aren't toxic, it's false.

As little as 0.1 ug on a hair test is enough to get some people (like in my case).