Non-Carcinogenic Hazards, Continued.
RfD is based on the NOAEL from toxicity testing. But often there are many tests, and these were conducted for different lengths of time. Sometimes there are important differences between the NOAEL from chronic or lifetime tests and NOAEL from tests of a shorter duration. If those differences are known and assumed important by the toxicologist, both a chronic, sub-chronic, and sometimes short-term RfDs are presented in the toxicity evaluation. The Intake used to compute the HQ would correspond to the RfD. The RfD is assumed to be chronic, unless stated otherwise.
For the inhalation route, you compute the HQ by dividing the ambient concentration by the RfC. The process is otherwise the same and you can add the HI from the inhalation route to the HI from the oral route when arriving at the overall HI. Dermal is more problematic. You seldom find relevant RfDs for the dermal route. Get your resident toxicologist to make relevant assumptions and computations to help you through this one. Note what you did earlier with dust and dirt referred to oral exposure from ingesting the soil, not absorption through the skin.
If the HQ's for different chemicals are added, this is assumed conservative. That is, if the chemicals have different health effects or target organs, the HI's for the chemicals might be presented separately. I will mention here that much less is known about the combined effects of chemicals. When something is known, as it is for many drugs, the combined effects might be more or less than the sum of the individual effects.
The level of uncertainty in the expression of the RfD, the HQ's, and of the HI is high. Therefore they are not usually expressed to greater than whole number precision.