Dr. Nicholas Ashford, my co-author of the book Chemical Exposures: Low Levels and High Stakes, presented a thought-provoking analogy about how the same chemical exposure can affect people differently. His comments came at a National Academy of Sciences workshop in April 2012. I presented at the same workshop on TILT, or Toxicant-induced Loss of Tolerance; the QEESI, or Quick Environmental Exposure and Sensitivity Inventory; and the need for EMUs, or environmental medical units.

Here are Dr. Ashford’s remarks:

“Let me ask you to do a thought experiment. Suppose there were 10 Apple computers lined up along that front table and I were to open the motherboards of all those computers. Now suppose I were to take 10 pairs of those little Scottie magnets that your kids play with and I threw a pair of magnets into the motherboards of all 10 computers and then closed the computers and asked the first computer to add 2 and 2 and it gives me ‘minus 5.’ Then I asked the second computer how much is 2 and 2 and it says, ‘Well over 3.’ The third one just whirs. The fourth one doesn’t do anything, and so on, for each computer, down the line.

“Now if we had computer epidemiologists they would look at these 10 computers and they’d say they are all sick and each appears to have a different disease, i.e., each gives a different wrong answer to the question how much is 2 plus 2. If the epidemiologists didn’t see me throw the Scottie magnets into the computer, they would say these are unrelated diseases. Not only that, but the incidence of the malfunction — each rendition of a wrong, but different answer to the question how much is 2 plus 2 — would not be significant enough to say there was any disease in the population at all because not enough of the computers gave the same wrong answer. That’s what we’ve got with brain-mediated, brain-damage disease and I would argue that one of the bases for Toxicant-induced Loss of Tolerance is brain-mediated damage.

“When you throw a neurotoxic chemical into the brain, and you know a lot of them get into the brain, including the limbic system, which is where the immune system, the nervous system, and endocrine system converge, they may make the brain misbehave in a number of different ways. One of the ways in which it may misbehave may be endocrine disruption. Another way that it may misbehave is attention deficit hyperactivity disorder (ADHD). A third way in which it may misbehave is that autism can develop.

“Now I said it was a thought experiment, but probably the organ we know the least about and which is computer central to our entire existence is our brain. Toxicant-induced Loss of Tolerance can manifest in a number of seemingly unrelated ways, unexplained by classical toxicology that assumes a one-hit or single insult resulting in a single kind of pathology.”