Say you have 15 women and 5 men in a room. One of the men is a murderer. The rule is there must be less than 10 percent murderers. With all 20 people in the room, you are good.
The second you remove all the women, your murderer/civilian ratio jumps 4 times and you fail the check.
That’s what extracting does. It removes irrelevant material, leaving you with active chemicals/contaminants.