It is difficult to quantify the risk of breath-hold diving after scuba diving because of the wide range of potential exposures of both types of diving. This makes it a thinking person's conundrum. Breath-hold diving to significant depth are best avoided after compressed gas dives with significant inert gas uptake. The challenge is to define "significant" both for the compressed gas dives and the breath-hold dive depth. It is probably safe to say that there is little concern over breath-hold dives to depths no greater than 30 fsw that are conducted one hour after a 30 minute dive to a maximum depth of no more than 40 fsw. The decision on where to draw the line for dives involving greater degrees of inert gas uptake could keep a room full of decompression experts entertained for some time. We also have to be cautious with our advice since the data are limited and suggestion or guidelines - intentional or not - are often given a greater weight than appropriate for the dynamics of diving exposure.
Having said all of the above, your "normal" breath-hold dive to 10 or 15 feet would cause me little concern even immediately after a compressed gas dive that was a couple of steps or more below the U.S. Navy no-decompression table limits or one step below the DCIEM no-decompression table limits. This is simply a rule of thumb. We have no hard data assessing this pattern of exposure.
My concern would increase as your compressed gas dives got closer to the no-decompression limits or your breath-hold dive depths went beyond the 10-15 fsw range. My degree of discomfort would reflect the increased presumed relative risk. Your plan to breath-hold dive in the morning and scuba dive in the afternoon would give me little concern. While some of us as humans have made an art out of finding ways to increase our risk unnecessarily, it would take substantial exposures to produce significant risk with the 16 hour surface interval that you mention. Please note, the risk could be generated; it would simply be unlikely for relaxed, recreational dive profiles that we like to see people doing. We have increasing evidence, for example, that many technical dives produce significant decompression stress as indicated by intravascular bubble formation. I would actively discourage individuals doing such dives from any post-dive breath-hold diving.
Increasing the breath-hold dive depths can also become problematic. While the 10-15 fsw dives likely produce only small to modest degrees of risk post-dive, many breath-hold divers are dramatically extending their range. With a world record depth (sled and lift bag assisted) exceeding 700 fsw, we know that the potential for significant risk can be generated. Assuming, though, that you stick to the 10-15 fsw normal depth and occasional 40-45 fsw depth, a separation into morning and afternoon activities would likely be of little concern. The working practice can be fairly straightforward: the more aggressive the scuba diving, the longer the surface interval and the more conservative the breath-hold dives. You should be able to develop a comfortable rhythm over time. Do remember, though, that you can "get away with" a given pattern of exposures nine times out of 10 or even 99 times out of 100 only to be surprised one day. It is important to remember that decompression sickness is a probabilistic event. The best way to be safe is to appreciate all the elements that may increase your individual risk and focus on keeping the risk of each element as low as is practical. And, finally, if you do have a bad day and end up with a problem, accept that the probabilities worked against you. It is important to control our tendencies to deny a problem and/or to blame a problem on a single factor. Decompression sickness is much too capricious to make these useful patterns of response.
I end with the discussion of the capriciousness of decompression sickness to remind you of the power of the old adage of an ounce of prevention being better than a pound of cure. You are ahead of the game by asking the question. You should now think about all the little things that can all work together to keep your risk at a comfortably low level.
Neal W. Pollock, Ph.D.
Divers Alert Network