Attention recreational divers: You may now begin making deco stops. In fact, please do. The National Association of Underwater Instructors (NAUI) and others are now recommending that recreational divers should stop their ascents for one minute at about half their maximum depth before continuing upward to the 15-foot mark for a second safety stop. Computer makers Suunto, Mares, Dacor and Zeagle also recommend similar stops for recreational divers and the Professional Association of Dive Instructors (PADI), says it "probably won't hurt."

Why? New research into the mechanics of decompression sickness (DCS) suggests that by adding this deep stop to a slow ascent and the traditional three- to five-minute safety stop at about 15 feet, recreational divers will enjoy a greater margin of safety against DCS and leave the water with less nitrogen.

"We think this is a significant change in the way people will dive," says NAUI's Tim O'Leary. "This is a quantum leap forward."

From Haldane to Wienke

To understand why NAUI and others recommend this new deep safety stop, you have to understand the basics of decompression theory, how it has evolved and how it has led to current dive practices like the 30-foot-per-minute ascent rate, traditional safety stops and now the deep safety stop.

Most modern dive tables and algorithms (the basis of dive computer programs) are based on the work of physiologist J.S. Haldane in the early 1900s. He started from the premise that DCS was caused by the formation of nitrogen bubbles in the blood and tissues when a diver ascended too rapidly to a lower pressure. It's still not proven, but modern researchers accept the theory that the "bends" is caused when bubbles of inert gas press against nerves, clog your blood circulation and panic your immune system, causing pain, numbness and paralysis.

Haldane's critical assumption was that by controlling the release of dissolved gas from a diver's tissues, we could prevent bubbles from forming. And, in that, Haldane was probably "only half right," says O'Leary.

The reason? We have bubbles Haldane didn't know about. J.B. Bateman and A.R. Behnke theorized as early as 1951 that many small, harmless bubbles were forming on many dives, even "no-decompression dives." They called them "silent bubbles" because they did not seem to cause DCS. They were proven right in the late 1960s when Doppler ultrasound detectors made it possible for the first time to "see" (actually, hear) and count these bubbles in the bloodstream. Since that time, the study of bubbles has taken center stage among decompression researchers.

Basic science tells us that bubbles are much more likely if they have a tiny "seed" to form around. That's why sprinkling salt in a flat beer gives it a new "head"--the salt crystals are seeds around which new carbon dioxide bubbles form. This is the critical insight behind the latest "two-phase" or bubble theories of decompression, including the most recent--the Reduced Gradient Bubble Model (RGBM) of Dr. Bruce Wienke, a NAUI instructor trainer and Los Alamos National Laboratory physicist.

These theories assume that bubble micronuclei or "bubble seeds" are created all the time by such things as turbulence in the bloodstream, whether we are diving or not. As divers ascend, dissolved nitrogen forms larger bubbles around these nuclei.

"We all know that bubbles are formed on every dive, whether the person's got DCS or not," says Wienke. "Doppler counts show it, even for divers that haven't pushed their no-decompression limits. Bubbles are always there."

Bubbles Block Decompression

Even before they get big enough to cause DCS, these bubbles are slowing down your offgassing. Here's what happens: In your tissues, bubbles act like lock boxes. Once inside a bubble, a gas molecule tends to stay there instead of leaving your body. "It gets isolated from the circulation," says Dr. Richard Vann, vice president for research at Divers Alert Network (DAN). "It has to diffuse back into the tissue before the circulation can cart it off to the lungs."

In your bloodstream, bubbles clump together, get trapped where capillaries branch or narrow, and prevent gas-carrying blood from reaching your lungs, where the nitrogen can be unloaded. It's all still theory, but "I'm convinced this is the way things go," Vann says. "There is experimental evidence that if you form bubbles, you retard inert gas elimination," he says.

When gas elimination slows down, bubbles grow even bigger and circulation is blocked more. At the end of the downward spiral is DCS--including those infamous "undeserved hits" that occur in defiance of all predictive DCS models.

For divers on a Haldane-based profile--one that tries to deal only with the mechanics of eliminating dissolved gas--bubbles present a problem. Unlike dissolved gas, bubbles do not leave your system faster as you ascend. Instead, they leave more slowly, Wienke explains in his book, Reduced Gradient Bubble Model in Depth. If you stop briefly to decompress deeper, dissolved gas is eliminated more slowly, just as Haldane assumed. But bubbles will actually leave your system faster. That's because small bubbles, kept small by the pressure at the deep stop, will have a chance to collapse as the gas in them redissolves.

Haldane's original tables have been modified numerous times to reflect experience and growing awareness of the role bubbles play. When divers got bent, exposures were shortened, stops were added, ascents were slowed. Now-common practices like the 30-feet-per-minute ascent rate and the 15-foot safety stop were both concessions to experience. The addition of a deep safety stop, says O'Leary, is the latest evolution, designed to deal with the physics of both bubbles and dissolved gas.

Deep Stops in Practice

Experience with deep stops comes from the technical diving community where divers have been incorporating "dual-phase" models like Wienke's RGBM for almost a decade. Their experience has revealed the surprising fact that spending extra time making a deep stop actually shortens the overall time a diver needs for safe decompression.

"We have divers using RGBM down at 300 feet for five hours," says Wienke. "Haldane deco, U.S. Navy deco, would ask them for 20 hours of decompression time. But using RGBM, they decompress and come up safely in 12 hours."

Dr. Peter Bennett, former CEO of DAN and an early proponent of deep stops, says divers in the Adriatic working with DAN Europe have come to the same conclusion, that a deep stop accelerates decompression. "Gas saturation in the fast tissues was reduced quite low indeed and bubble counts were reduced virtually to zero."

You read that right--if you spend more time stopped at depth you can end up with less nitrogen. It sounds counterintuitive--if not nuts--and, of course, it's not that simple. The depth and time of the deep stop do matter. Here's how Wienke explains it:

"Let's say a diver has been down to 90 feet until his no-decompression limit and now he's coming up. Bubbles are excited into growth by the reduced pressure. They are of varying size. The smaller ones don't grow so fast, but the bigger ones grow faster, and the dissolved gases feed into them.

"Then he stops halfway to the surface. Because he's holding himself at 45 feet, he's minimizing the gradient for in-gassing so the bubble isn't growing now. Pressure is holding the bubble, preventing it from expanding," he says. During the deep stop, gas is squeezed back out of the bubble and it begins to shrink. With less obstruction, blood flows faster, carrying redissolved gas to the lungs where it's eliminated.

Now consider that same diver following the traditional direct ascent. "If he continues up to a safety stop at 20 feet instead of stopping for a minute, he's dumping gas into the bubble, the bubble is growing, and then anything he does in that shallow zone is treatment of bubbles, not prevention of bubble growth," Wienke says.

Vann says the ongoing Doppler studies in Europe, so far totaling 25 divers and 1,600 dives since 2000, confirm the basic theory. "There's no question that deeper stops will reduce the number of bubbles."

What Freeway Traffic Tells Us

If this still sounds like voodoo economics, consider the experience of millions of motorists every day in rush-hour traffic. Think of parking lots full of cars as being like tissues saturated with nitrogen. If all the cars try to get on the freeway at once, traffic slows to a crawl. Though there are more cars on the road, the "throughput," the number of cars passing a given point in a minute, falls--just as the total amount of nitrogen leaving your body falls if too many molecules try to crowd into your circulatory system at once. Sometimes cars collide and sometimes bubbles clump together, stopping the flow entirely. But that is how rush-hour traffic "moved" for many years, and that essentially describes the Haldane method of decompression as Wienke and others see it.

Beginning with a 1969 experiment in Minnesota, traffic engineers have learned that using stoplights to meter on-ramps, to delay cars trying to enter, keeps congestion down, increases the throughput and gets everyone home sooner. Like decompression "engineers," they use algorithms to keep traffic density below predefined saturation levels, and they do it for the same purpose--faster transport results in less pain. It's sometimes difficult for the commuter to accept that by waiting a minute on the ramp--the motoring equivalent of a deep stop--he will get home quicker, but those who have experienced it know it to be true.

How Much Safer Is It?

Deep stops were developed to handle the high gas loads of tech divers. Given the conservative nature of recreational profiles, there is some question as to whether deep safety stops will appreciably improve safety for no-decompression divers. To date, there is no definitive proof that bubble theory and deep safety stops will help recreational divers, and there probably never will be. The number of DCS cases is already so low--estimated at somewhere between one and three cases per 10,000 dives--that "the data is lost in the noise," Wienke says.

"Is it safer? Time will tell," says O'Leary. "We believe in it so much that we say 'absolutely.' "

And here's another statistic to think about: According to the most recent DAN injury report, 79 percent of decompression sickness "hits" were the more serious type II variety involving the central nervous system and often leaving permanent injury. If paralysis is on the table, every safety margin helps. "I'm quite convinced," says Vann, "that bubble algorithms will overall give us safer dives and make a fairly safe situation even safer."

That's the Theory, At Least

CAN DEEP STOPS REALLY MAKE RECREATIONAL DIVES SAFER? All the known science indicates that it should, but researchers are quick to admit there's much we don't know about DCS and pretty much everything we think we know is unproven. "We're very, very lucky that we get away with such a low incidence of decompression illness," says Dr. Peter Bennett. "A lot of people will speculate, but nobody understands decompression. It's all theory."
In weighing the value of deep stops, it's important to remember a few key principles.

> ALL DIVES ARE DECOMPRESSION DIVES. A technical diver engaged in staged decompression stops and a diver following his recreational tables are both attempting to do the same thing--eliminate dissolved gases at the fastest possible safe rate. That's what we mean by decompression, and when we call some dives "no-decompression," we really mean "no deco stops required."

> A SAFETY STOP AND A MANDATORY DECO STOP ARE NOT THE SAME. Or not quite the same. The difference is the degree of risk in missing the stop. Because the tech diver's nitrogen load is so high that a DCS "hit" is thought likely if he misses a stop, we call the stop mandatory. Recreational divers within no-stop limits have a nitrogen load small enough that a hit is thought unlikely, even if he misses safety stops, which we call optional. A safety stop is meant to add additional safety to an already safe dive. But remember ...

> IT'S ALL A MODEL, NOT A MONITOR. All decompression theories, tables and dive computer calculations are--at best--a mathematical model of what's believed to be happening inside your body. Not even the best dive computer on the market today can look inside your tissues and see what's really going on. Instead, they calculate what decompression theories and experience tell us usually happens in similar situations. Both technical divers and recreational divers have undeserved hits--and misses.

How to Make a Deep Stop

NAUI, PADI and the dive computer manufacturers who have adopted RGBM algorithms all have slightly different ideas on the best way for recreational divers to make a deep safety stop.

> NAUI: Recommends a one-minute deep stop at half your maximum depth on all dives deeper than 40 feet. Once the stop is complete, you should ascend slowly (no faster than 30 feet per minute) to the traditional safety stop at 15 feet. NAUI's protocol allows you to subtract the deep stop time from the 15-foot stop, but there's nothing wrong with staying longer at the shallow stop, says O'Leary.

If you follow NAUI's recommendation on a dive to 90 feet, you'll stop your ascent at 45 feet and hold there for one minute before continuing to a traditional safety stop at 15 feet for two minutes. That means you would surface at the same time as another diver who started with you and made only the traditional three-minute stop at 15 feet.

O'Leary stresses that the deep stop is a recommendation and not mandatory if conditions aren't right. "Make an intelligent decision," says O'Leary. "If there's current at 40 feet and you're drifting away from your vessel, this may not be the time."

> PADI: Sees less value in deep stops and dual-phase models like RGBM for recreational divers. Karl Shreeves, vice president for technical development at PADI's Diving Science and Technology (DSAT), says, "Deep stop models offer a lot of promise for decompression stop diving, but when we talk about no-stop diving, it really doesn't seem to have much to contribute.

"If somebody within the no-stop envelope wants to do a deep stop around 12 meters [approximately 40 feet], it probably won't hurt anything, but we haven't seen anything empirical that suggests a huge or significant safety benefit," Shreeves says. If you decide to make a deep safety stop, he suggests a simple, math-free rule of thumb: 40 feet for three minutes.

> COMPUTER MANUFACTURERS: Mares and Dacor dive computers incorporating a new "Mares-Wienke RGBM" algorithm sound an alarm for a deep stop at half your maximum pressure change, but only when less than three minutes of no-decompression time remains. They suggest a one-minute stop, followed by the full three minutes at 15 feet for an increase in total dive time of one minute.

All the features of new RGBM algorithms for Suunto and Zeagle computers were not completed by press time, but reportedly both will signal for the new deep safety stop. Zeagle computers will allow you to custom-set the depth of the alarm and turn it on or off.

And what if your present dive computer has a Haldane-type algorithm? Will it penalize you for stopping deep? Probably not. Assuming you ascend normally before and after the stop, your computer will probably average the stop with the ascents and credit you with a slower ascent rate.Bruce R. Wienke's book Reduced Gradient Bubble Model in Depth (ISBN 1930536119, 96 pages), is available in hardback from Best Publishing and can be purchased on Amazon.com.