Some contaminants don't wash. Certain kinds of bacteria, blood, etc. would be relatively more hazardous than, say, everyday filth. Ever hear of people complaining they can't get the faint smokey odor off of furniture they received from people who smoked in their living rooms? My home has (for guests) a specialized smoke room for this reason (think a bathroom but just for smokers, not for people who have to use the bathroom).