By Alex Brylske
Q: Deidre Levin had a question that I wish more divers considered. “I was on a live-aboard dive trip recently where most of us made between 20 and 25 dives during the five days we were aboard. One evening toward the end of the trip, our discussion centered on an issue that I’ve never seen addressed in any publication or discussed by any diving authority. But it seems like a simple and important matter. Can a diver do too much diving? I’m not talking about decompression sickness. Assuming that’s not an issue — the person feels perfectly fine — is there still a point when, like most things, there’s too much of a good thing?”
A: You have entered a very interesting realm of discussion, and you’re right; it’s rarely if ever addressed. Perhaps the reason almost no one ever talks about it is because we know so little about the long-term effects of diving in recreational divers.
It is, however, well documented that some commercial divers do suffer long-term effects. Hearing loss is one common ailment. However, another is a lesser-known condition called dysbaric osteonecrosis, literally, “bone death due to uneven pressure.” While to my knowledge it hasn’t been proven conclusively, many medical authorities blame this disorder on the extreme and continued conditions encountered by commercial divers, especially those who have a lot of deep-saturation experience. (By the way, it’s even been reported in sperm whales.)
The good news is that it’s believed that recreational divers spend far too little time underwater in their careers for this to be of any concern. However, there has been some evidence that dive professionals who spend extreme amounts of time at depth, such as divemasters and instructors, may be susceptible.
Still, my concern gets back to the issue that you dismissed: decompression. Even if someone does not manifest any symptoms of bends, we really don’t know what the cumulative effect may be. We do know that silent bubbles develop commonly in recreational divers, and that they most often resolve without any problem. But could they have a cumulative effect over many years or decades? Could age have any effect on this mechanism? Or, could more diving actually help build some form of immunity to bends? We just don’t know.
Personally, I’m a cautious person. And, probably like a lot of folks, I’ve gotten more so as I’ve gotten older. Now, in my diving, I almost never make more than two dives a day, and three is the absolute limit. I know that may sound unduly conservative — and I do have the advantage of diving just about any day of the week I care to — but that’s my decision. So, does the concept of too much of a good thing apply to diving? Probably, but we have no idea where that limit lies, and I personally don’t plan to provide any evidence.
Q: Dave Lewis sent in a question about altitude and diving. “I wonder about the oxygen content of compressed air in a scuba tank when it is filled at altitude where the lack of oxygen tires out hikers. Do dive centers at altitude enrich compressed air to normal levels?”
A: Your confusion is understandable given that hypoxia (low oxygen) is a common concern for anyone at high altitude. After all, who hasn’t watched a film about pilots or mountain climbers in which they didn’t see oxygen masks at the ready? As we’re reminded in the required announcement prior to every flight we’ve taken, supplemental oxygen is readily available in the event of loss of cabin pressure.
But let’s think more closely about this for a second. The concentration of oxygen in the atmosphere everywhere on the planet is 21 percent. (If you’re a stickler for accuracy it’s really 20.946 percent.) What’s more, that value holds true no matter where you are in the atmosphere, be it at the surface or about to step on the peak of Mount Everest. That means no matter where you fill a scuba tank, as long as it is somewhere on Earth, it will always contain 21 percent oxygen (assuming it’s not a nitrox fill, of course). So, if the oxygen fraction of air is always the same, where does all the concern over hypoxia come from?
The answer involves not pressure, but percentage. The physiological effect a gas has on our body depends not on the fraction of a gas within the mixture, but on the actual number of molecules we breathe. This is a function of the partial pressure of the gas in question within a gas mixture. An example will illustrate the point.
If you’re a nitrox diver you know that the oxygen partial pressure (ppO) at the surface, 1 atmosphere, is 0.21 ATA (reflecting its gas fraction, 0.21 x 1.0 = 0.21). At 2 atmospheres (33 feet [10 m]), where the pressure doubles, this increases to 0.42 ATA (0.21x 2.0 = 0.42). However, if you ascend in the atmosphere to a height of about five kilometers (16,400 feet), the pressure will halve. This means the oxygen partial pressure drops to 0.105 ATA (0.21 x 0.05 = 0.105). While we’d experience no problem whatsoever at the surface or at 33 feet [10 m], at altitude this low value is insufficient to maintain consciousness and we’d black out. Remember, however, that the percentage of oxygen in every case has not changed; it’s always 21 percent.
Q: Patti W. writes, “What should I use to rinse the inside of my buoyancy compensator (BC) after diving in salt water?”
A: First of all, it’s always good to give your gear a quick rinse in fresh water after an ocean dive, to rinse away the seawater. If you aren’t in the habit of rinsing your gear, you should be, because salt crystals that form on your gear when it dries can abrade surfaces, wear out threads and cause corrosion. A longer soaking at home after a dive trip — ideally in warm water — is also recommended.
Now, to the question of rinsing the inside of the BC, I’ve never actually used anything but fresh water.
Some divers use the eco-friendly product Simple Green® or similar mild detergents. There are also a few commercial products available just for rinsing scuba gear, so ask at your local dive center. Some have speculated that because a BC might be inflated orally, its interior should be disinfected periodically. So, a very mild bleach solution could be used for this purpose. In addition, some rebreather manufacturers offer nonbleaching disinfectant solutions, which are a better choice but, again, I think that good-old water should do the trick.