The book says that "heavy exercise" can predispose a diver to CNS/O2 toxicity, but it doesn't really say how or why.
DCS and CNS/pulmonary O2 toxicity are
completely different beasts. I think that some responses in this thread have focused more on the association between exercise and DCS.
To be honest, the mechanisms of CNS and pulmonary oxygen toxicity are still unknown. IMO, one of the most interesting hypothesized mechanisms is one which involves
nitric oxide (NO) signaling. If you're up for the academic legwork, I'd suggest learning all you can about how NO works, what enzymes (
nitric oxide synthase) produce it, where those enzymes are located, how expression of those enzymes changes with exercise, the effect of NO on neuronal functioning, and how NO affects other very important cellular processes (stem cell differentiation, cell death, etc.). The
Wikipedia page on NO is a little thin, but it's probably worth a quick read anyway. A lot of interesting research on NO is going on
right now. (Do a
PubMed search if you don't believe me.) It's cutting edge stuff that goes way beyond what any scuba instructional agency would ever include in course materials. This shouldn't be too surprising since we still don't know enough to make informed recommendations on diving practices based on NO research. Give it a few years, though, and things will change. You'll see.
You asked about how exercise can be linked to CNS and pulmonary oxygen toxicity. Based on what I've read, here's how I would make the connection...
Many studies have shown that exercise can increase NO production in blood vessels. NO has the effect of relaxing endothelium (muscular lining of blood vessels). This is most likely how mediators of NO (nitroglycerin pills) function in the context of coronary artery disease. So it would seem that when it comes to cardiovascular health NO is a "good" thing. However, that's not a complete picture of how NO functions in the entire human body.
NO has very important functions in the CNS as well -- some good and some bad. Just as in peripheral blood vessels, NO causes vasodilation in CNS blood vessels. This has important physiological implications that I won't get into here. In addition, researchers have found that, in the brain, breakdown of NO by the enzyme superoxide dismutase can produce a
radical oxygen species (
ROS) known as peroxynitrite, which is actually
toxic to neurons. There is a positive correlation between levels of brain peroxynitrite and seizures associated with hyperbaric oxygen treatment.
Prolonged NO production in the lungs results in diffuse tissue damage (pervasive inflammation) due to breakdown of the "alveolar-capillary barrier," possibly related to the generation of ROS. This eventually causes fluid in the lungs (pulmonary edema) and consequently impairs gas exchange. It should be noted that under
normobaric conditions, NO effects predominate in the lungs, whereas under
hyperbaric conditions, CNS NO effects are seen.
Given all this mechanistic info regarding how CNS and pulmonary oxygen toxicity proceeds, it begs the question
how exercise can impact any of these processes. I speculate that the key here is the generation of ROS. With exercise, more oxygen flux occurs on a metabolic level and consequently more ROS are produced. Over time and with increased training, the body can upregulate enzymes which help keep the ROS safely in check. There are likely differential limits on the extent of upregulation of such enzymes (blood vessels vs. lung tissue vs. brain tissue). If those "good" enzymes are overwhelmed, it's not difficult to imagine the ROS running amok. This could explain the complicated relationship between the timing/extent of exercise training and CNS/pulmonary oxygen toxicity. Put simply...Too much exercise in the short-term can be bad, but exercise training over the long-haul can be good.
Here's a link to
a nice article on the relationship between NO, exercise, and cardiovascular disease.
Allen et al. wrote a very nice
mini-review of how NO is implicated in oxygen toxicity. I've linked directly to a PDF version of this article, which unfortunately lacks the figures. The text alone, however, contains a lot of good info.
As with most things in science, this post probably raises more questions than it answers. At the very least, I hope you find it entertaining.
) but I'll run just before and make sure my profiles are tame. The real message shouldn't be that running is bad, teh real message should be that pushing the NDL's if you run is bad.... Everything in moderation. 
