The Problem With Survival Charts
"Survival charts provide a very misleading picture of cold water immersion and they should not be used to educate the public about cold water safety."
“For over 50 years numerous attempts have been made to quantify the relationship between water temperature and survival time. Despite this, the goal remains elusive.”
- Golden and Tipton in Essentials of Sea Survival
How long can a person survive in 40F water?
It's an intriguing theoretical question, and as the quote above points out, a lot of researchers have taken a crack at trying to answer it. Along the way they've gained some very useful insight into the ways in which body heat is lost in cold water, and this research led to the development of computerized models like the Probability of Survival Decision Aid (PSDA) used by the US Coast Guard.
However, variables like water temperature, sea state, BMI, water movement, and movement of the person in the water make simple predictions very difficult, and it's unlikely that we'll ever have a simple chart that accurately predicts survival time in cold water.
The main problem with using survival charts to educate the public about cold water immersion is that they're based on an estimate of the amount of time it will take for a person to become hypothermic in calm water. Those estimates greatly understate the danger because:
They don't consider cold shock, swimming failure, and incapacitation.
They don't consider sea state and make no distinction between water that's flat calm vs rough water with breaking waves, and they also don't consider the relationship between wave splash, inhaling water, and drowning.
They fail to consider the fact that in cold water, wearing a standard PFD, most victims are likely to drown before they become hypothermic.
They underestimate the speed with which manual dexterity can be lost.
Their estimates of survival time are based on theoretical projections for “average” subjects in calm water.
Finally, as a practical matter, they also don't provide any safety information that's useful in increasing survival time or preventing cold water close calls and fatalities.
Most people, viewing many such tables or charts, will draw the optimistic but very mistaken conclusion that in an unprotected 53F immersion, they “have” 10–15 minutes before they lose the use of their hands, 1-2 hours before they become “exhausted or unconscious”, and 1-6 hours of “survival time”.
From that perspective, it's no wonder so many people conclude that they can just skip wearing thermal protection. "What the heck, I can be back in my boat in 5 minutes..."
However, unless you're wearing thermal protection like a wetsuit or drysuit, cold water immersion is immediately life-threatening. In fact, most people will experience maximum-intensity cold shock, including a complete loss of breathing control, at water temperatures between 50F – 60F (10C – 15.5C).
Survival Chart Examples
This is a typical survival chart, loosely based on Barnett's 1962 projection. It only considers hypothermia in predicting a time of death. No consideration is given to drowning due to cold shock and/or incapacitation, or rough water, or whether the person is wearing a lifejacket.
In this version, the made-up concept of the “Danger Zone” and the idea that it represents an area in which undefined “safety precautions and appropriate behavior can increase your chances of survival” has no scientific basis.
Even worse is the “Low Probability of Death” zone (Barnett called this a “safe zone”) which leads people to believe that there's no immediate danger of dying – even in ice-cold water.
This next chart adjusts the calculations to account for body size and percent of body fat, but it still gives a very misleading impression of the danger of an unprotected immersion because it's based exclusively on hypothermia:
Source: National Water Safety Congress
Another Typical Hypothermia Table.
This "Hypothermia" Table is very misleading for several reasons:
It only includes loss of manual dexterity, not incapacitation. Incapacitation means your arms and legs no longer work. Incapacitation is a progressive deterioration in your ability to control your arms, legs, hands and feet. At first your arms and legs work fine, but they progressively grow weaker. The colder the water, the faster incapacitation occurs. At the end, however long that takes, you're hanging helpless in your lifejacket, unable to move your arms and legs.
It doesn't distinguish between “exhaustion” and “unconsciousness”. This is a major flaw because they're not remotely the same. On a hypothermia-based survival chart, unconsciousness means that your body temperature has fallen so low that you “pass out”, so it makes no sense to place both in the same column.
The problem with the “expected time of survival” column is that once you become incapacitated your risk of drowning dramatically increases. Furthermore, as soon as you lose consciousness, you're very unlikely to survive.
To put this in perspective, imagine that you're wearing a Type III lifejacket – the kind that most recreational boaters wear. Your mouth is only 3” above the water surface, and as a lifejacket test conducted by BoatUS discovered:
"The type III inherently buoyant vest-style life jacket proved the real eye-opener for our test crew who had to work hard treading water to keep their faces clear of the waves when using this device. When another test was conducted simulating an unconscious victim, those wearing the Type III inherently buoyant devices repeatedly sank well beneath the surface as the waves rolled over them." (The test crew was made up of professional lifeguards.)
More Survival Chart Examples
Note that both these charts have the same flaws discussed above.
What The Experts Say
“Sources of error in the estimation of survival time: Several factors make the prediction of survival time in cold water difficult. These include the ambiguity of the definition of survival time, the lack of well-documented data from actual incidents, the consequent reliance on extrapolation of data from innocuous laboratory-based exposures, and individual variability. More important than all of these, however, are the variability in sea state and the exclusion of the possibility of death from drowning.”
“Even a cursory glance at the literature shows that a single lethal deep body temperature does not exist and that the temperature thought to be incompatible with life can vary greatly among individuals.”
“One common source of error occurs in interpreting the curve drawn on a graph depicting survival time. This curve can represent a limit of tolerance, a relative safe zone, a lethal zone, a marginal zone, or estimated 50% survival time. Other authors provide information relating to a “good prospect of survival”.
"The 50% survival curve has been mistakenly taken to represent the time at which an individual would have a 50-50 chance of survival rather than the survival time of 50% of individuals. Furthermore, people often forget that this time theoretically exceeds the survival time of 50% of individuals."
"The problems just outlined are exacerbated by replication of the various survival curves without the associated explanations and caveats and by the ill-conceived formulation of hybrid curves.”
- Golden, F., Tipton, M.J., Essentials of Sea Survival, (Human Kinetics, 2002).
“Graphs showing predicted survival times in cold water before hypothermia sets in do not account for cold shock, which can occur during the first minutes of
immersion. If not read with proper understanding, they may give the unwary boater a false sense of security.”
- Avery, W.M., Boaters Beware! Perils of Cold Water.
Encyclopaedia Britannica, Medical and Health Annual, 1991.
In Survival In Cold Waters, an excellent and very thorough report for Transport Canada, Dr. Chris Brooks concluded:
“These curves must be used with caution. All of these predictive curves are premised on the fact that the person using the curves is prepared to accept the assumption that death is due to hypothermia. Golden explained that a conscious survivor in a seaway will make the physical effort to keep his/her back against the waves, but when physically impaired through muscle cooling, semi-conscious and with a loss of determined will to survive, both of which occur after a body core temperature drop between 2-3°C (3.6 - 5.4F), then the survivor turns into the waves and drowns.”
- Brooks, C.J. Survival In Cold Waters. (Transport, Canada, 2003)
Brooks was referring to Golden's finding that waves exert a turning force on a body floating in water. Keeping your back to the waves to avoid water splashing your face requires physical effort, which is lost as incapacitation increases. Water inhalation from wave splash causes gradual drowning.