How do you derive a practical DNEL?

The first step is to ensure that the ADME substance is similar in the animal tested and in humans. In the absence of such information, we will consider that the ADME is similar in humans.

If we only have a NOAEL from an oral study in animals, which is the most common experimental route, and we want to switch to another route, inhalation or dermal, we'll have to take into account the level of absorption (ABS) of the substance by all routes and equalize to derive a new NOAEL for the inhalation or dermal routes.