Non-Carcinogenic Hazards
The first step in characterization of non-cancer risks is to evaluate, for each receptor, the dose for each chemical via each route. Say we have a site with toxic metals and TCE. First we compute a hazard quotient ( HQ ) by dividing each intake by the RfD.
An HQ of 1.0 indicates that the expected dose is the same as the safe dose. Therefore an HQ of less than 1.0 is considered safe and greater than 1.0 is unsafe. (Safe indicating it is acceptable to whoever devised the RfD.) Unlike the slope factor, an HQ is not a risk or probability. A HQ of 0.1 does not indicate a 10% probability of anything. Rather it says that the dose is 10% of the acceptable dose.
So the HQ for lead by drinking water route would be the intake of lead via the drinking water divided by the RfD for lead. We might subscript the RfD as RfD lead, drinking water and so on. Often, for each class of receptor, the HQ's from several chemicals are summed and their total is called the hazard index ( HI ) :
Like the HQ, the HI is not a probability. If the HIwater is less than one, it is "safe" to drink the water.
Finally, you might sum the HI's from the different routes and:
If you were doing a risk analysis for a contaminated site, your baseline risk assessment might have several overall HI's: one for children near the site, one for adult workers at the site, one for trespassers, and so on. You do NOT sum these overall HI's for the different receptor types, you would express them separately. Clearly if all these overall HI's were much less than 1.0, the site is probably not a significant risk and does not need to be cleaned-up, at least not for any human health risk-related reason.
Let's consider some fine points next: