Recently I was involved in an incident that made me think about safety stops and people's interpretation of when they are appropriate. In this particular instance, a diver became disorientated at 14m on descent, panicked and went up to the surface out of control. He arrived at the surface, where fortunately other people were able to make him positively buoyant and then tow him to shore. Once ashore, he was given first aid and while this was going on, his buddy arrived.
When queried what had happened, his buddy (a Rescue Diver) had noticed that he was missing and correctly enacted the "search for one minute before ascending procedure". However, during his ascent, he elected to carry out a safety stop, waiting for 3 minutes at 5 meters before ascending to the surface.
The potential ramifications of this should be obvious. If others had not been on hand to deal with him, there is a strong likelihood that in the 3 minutes he spent at 5 meters, his buddy would have drowned. Panicked divers typically fail to establish positive buoyancy and will rapidly exhaust themselves trying to remain at the surface. Once they become exhausted they will sink and typically drown.
So why do we do safety stops?
This is perhaps one of the worst "fudges" in entry level training and poorly understood even by experienced divers. The foundation of all recreational diving (for almost all agencies) is that if required, due to a contingency, a diver can always ascend directly to the surface. Carrying out dives with required "hard" decompression stops (or in overhead environments) requires additional training and equipment that is not incorporated into recreational diving programs.
So why then do we recommend a "safety stop" at 5 meters? The answer lies in the way decompression theory views bubbles. Most commonly accepted theories pretty much rely on the premise that if you stayed within certain time and depth limits, bubbles would not occur. Relatively recently, this has been shown to be incorrect.
Physics is quite clear. If you apply energy to a liquid containing a gas, some bubbling will occur as the increased energy level in the liquid makes it lose its ability to hold said gas in solution. Practically, as a heart beats, this is a form of energy transfer and it creates some bubbling in the blood. The heart is just an example, there are lots of situations where we have an energy transfer taking place in our circulation and hence where bubbling can occur.
These bubbles have nothing to do with the absorbing and releasing of inert gas at depth or during ascent and are present both when we are not diving and when we are.
The physiological response to these bubbles is to ensure they remain very small and to use the pulmonary capillaries in the lungs as a giant filtration network to remove them from the circulation. Although not exact, a complete filtration of the blood occurs approximately every 3 minutes.
When we take this phenomena underwater, the greatest pressure change occurs from 10m to the surface. These, up until now, harmless little bubbles can potentially expand to twice their size and cause decompression sickness (DCS). However, by hanging around for 3 minutes at 5m, we allow time for them to be filtered out.
In truth, modern research has shown that there is quite a lot of bubbling that occurs during ascent and post-dive. Curiously, no one can really explain why most of these bubbles do not cause DCS. In fact, no one can really explain why DCS occurs in general, beyond that it is a complex physiological event with links to excessive gas absorption and bubble formation.
So safety stops and slowing ascents in general are actually a method of limiting potential bubble size.
So should I do one on every ascent?
As a general rule, yes. It is best practice to slow your ascent from 10 meters to the surface (and arguably from the bottom to 10 meters too) in order to give time for any bubbles to be filtered out. However, this advice should always be viewed in the light of it being best practice, not mandatory.
In practice, we have to make a risk assessment in a missing buddy situation. A drowned victim is very hard to revive and their prognosis is poor. It is very unlikely, but possible that not carrying out a safety stop may cause DCS. However, DCS is very treatable and the chances of survival and subsequent full recovery are very good. So in balance, given a choice between carrying out a safety stop and ascending to assist a buddy who may be in serious distress at the surface, the priority should always be given to assisting the buddy.
Of course in a conventional missing buddy scenario, you have no way of knowing whether you have simply become separated due to a lack of awareness of each other's position, or if your buddy has had to go to the surface due to some kind of issue. Prudence would dictate that if you lose you buddy, you should assume that they have gone to the surface due to a problem and will need your assistance and therefore you should ascend as promptly as possible.
Failing to do a safety stop will typically not cause DCS. Failure to establish positive buoyancy for a diver in distress at the surface may well cause them to drown. Given these two choices, it would seem logical that in a missing buddy scenario, carrying out a safety stop is a bad idea.