David,

I entirely see your point/perspective and that can be true from a lay perspective. From a pure toxicological/human biochemistry perspective the difference is important: where a cumulative toxicant such as lead, cadmium, mercury is concerned the body eventually excretes it, though it may have a biological half life of 40 years. Also most of the cumulative toxicants have different target organs (e.g. lead and bone marrow; cadmium and kidneys) and health effects which are typically chronic in nature. On sensitization, you have an overall systemic, acute reaction which may or may not be life threatening. Sensitization might occur in as little as one or two high doses with lesser adverse reactions (or almost none) and then wham! a bigger reaction.

From our modeling point of view, we want to avoid multiple exposures (as in prevent the "cumulative effect", whether it's technically cumulative or sensitization) and take typically the same precautions. So it's 6 and 1/2 dozen as far as we're concerned.

Chuck Carlisle, CIH, CSP, CHMM