The human factors of diving accidents in pools: Diving accidents in swimming pools often cause serious injury This is somewhat surprising, in view of the widely accepted belief that diving is an inherently dangerous activity. Some states have even enacted laws restricting lawsuits related to diving, the rationale being that diving is an open and obvious hazard, so divers accept the risk - Aquatics

So why do accidents still occur with such regularity? Unfortunately, issues involved in swimming pool accidents are not always obvious. In this article, I provide a primer about diving accidents in swimming pools. Much of the discussion directly transfers to diving accidents in rivers, lakes and other natural settings.

The causes of accidents lie in two sets of factors. The first is physical, the pool design. Some are inherently safer than others, but, as explained below, none can be perfectly safe. The second set is psychology and human factors. Some factors are due to visual attributes of the pool environment, but most center on the way people see warnings, view risk, and decide on compliance. Below, I discuss the issues involved in each set of factors.

PHYSICAL FACTORS

The safety of diving varies with several aspects of pool design.

* Depth: Often, diving accidents are attributed to insufficient water depth at the point of the dive. Statistical analyses show that the vast majority of accidents occur in water depths of about 3 1/4 feet or less. One study reports only 13% of accidents occurred in depths of 3 1/4 -- 9 feet.

Since injury often occurs with head impact on the pool bottom, depth matters because water acts as a brake that slows diver velocity. Several diving biomechanics studies examined the velocity of divers as they enter the water and reach various depths. People diving from boards and decks hit the water at about 15 feet/second. Estimates of the point where downward motion ceases is between 12 -- 15 feet, although some place the depth for absolutely safe diving at 18 -- 20 feet. Of course, few can pay the expense of building and maintaining a pool of such depth. This leaves open the question of the water depth required to create an acceptable risk.

The answer to this question has proved surprisingly elusive. The minimums recommended by various sources cover a wide range, from the American Red Cross and FINA (Federation Internationale de Natation Amateur) estimates of 3 feet, compared to the Australian Diving Association estimate of over 9 feet. SportSmart Canada recommends diving only when the depth is twice diver height. Many standards do not discriminate among diving platforms, although the National Foundation for Spinal Health specifies a minimum 3 1/2 feet for any diving, 3 1/2 -- 4 feet for dives from platforms of 18 inches above the water, and 4 plus feet for platforms up to 30 inches high.

It is important to note that such estimates are merely "standards," and are not based on any physical or empirical science. Moreover, accident statistics are of little help. The fact that most accidents happen in three feet or less of water does not provide a true estimate of risk. There is no way to know how many safe dives were taken at each depth, and hence there is no way to know the probability of injury at each depth. There are no baseline controls on these surveys, so it is not scientific data. (Very few accidents occur in water depths below two feet. This is likely not because of greater safety, but because relatively few people dive into such depths.)

Lastly, safe depth depends at least partially on dive angle and technique. It is likely that short of making pools so deep that all vertical motion ceases, at approximately 15 feet, there will never be an absolutely safe depth for all divers. Moreover, issues of pool shape also can affect safe diving depth.

* Shape:

-- Bottom Contour: Although some accidents occur when the diver's head strikes bottom, others occur when the diver's trajectory runs him/her into an underwater contour. In most pools, there is a gradual slope to the bottom. For in-ground pools, this is intentional in order to gradually change depth from the deep end to the shallow end or because there is a diving well, an area made especially deep for diving. For above-ground pools, this is usually inadvertent, occurring when the middle of the bottom is dug deeper than area around the edge. The result is a deep spot in the middle of the pool, with grades going up to the sides. If the bottom grade is steep, it is sometimes called the "spinal wall." Collision with the spinal wall is a frequent injury cause and is probably the main cause of diving accidents in deeper water.

Another factor in such accidents is the absence of water depth markings. This not only makes judging depth at the dive location difficult, but also prevents a diver from estimating how quickly dive angle will bring him/her into shallow water. Even when markings are available, they are often located underwater where they are difficult to see.

-- Side Contour: Injuries sometime occur when the diver hits the side of the pool. In this case, it is often pool width rather than water depth that causes the injury. Artsy pools with irregular curves are becoming increasingly popular, but may leave insufficient margin-for-error if the diver's movement takes him/her laterally.

PSYCHOLOGY & HUMAN FACTORS

Regulatory bodies require swimming pool managers to post warnings about the safety of diving conditions. However, people frequently fail to notice or to comply, and accidents ensue. Reduction of diving accidents requires an understanding of how people respond to warnings and why these warnings so often fail.

To understand why warnings frequently fail, it is necessary to understand how the viewer mentally processes them. This requires a psychological perspective that looks into the viewer's head and examines perceptual and cognitive processes. From this perspective, the effectiveness of warnings depends on activity in four stages:

1. Notice the warning: The first stage is simply becoming aware that the warning exists. The key issue is conspicuity -- the ability of something to draw attention even when the user is not looking for it. This is a huge topic, but there are a few general points to make. First, attention is generally directed in the same direction as the eyes point. This means that warnings not located close to the line of sight will not be readily seen. Physical properties such as size and high contrast will likely further encourage conspicuity. However, much recent research suggests that many factors often thought to promote conspicuity, such as motion and color, do not always work.

2. Perceive the warning: Once the user notices a warning, it must be perceived. By this I mean, the words or graphics must be read -- the sensory input is given meaning by drawing on memory. In order to read a warning, it must be high contrast and, in the case of text, be large enough and rendered in a legible type font and family.

If people believe that there is little hazard, they are likely to refrain from bothering to "read the warning, even if they are aware that there is one present." Moreover, people often perceive the wrong part of the warning. There are many cases, for example, of a nurse looking at a vial and then injecting the patient with the wrong drug. In most cases, the nurse noticed the label but failed to perceive it. People have a tendency to generalize the cues that they use to make decisions. If it is the case that different labels have different colors, the nurse might unconsciously depend on color (or even more subtle cues like position on the shelf) to signal the warning and then cease reading the label text.

3. Understand the warning: Once the warning is perceived, the user must properly understand its message. The message should be clear and easily understood. It is difficult to ensure clarity for several reasons. Sometimes the message is simply too vague to convey. A warning that says "ensure adequate ventilation" is problematic because it fails to define "adequate." Moreover, sentences may be poorly constructed or contain words that are unfamiliar. "Flange?"

Users may also fail to understand the warning if they have little familiarity with the product and its operation. The words may be comprehended, but the relationship to product use unclear. Some users, such as children (with lack of language skills), older people (with declining cognitive abilities) and non-native speakers with insufficient English knowledge may also have difficulty in understanding warnings.

In order to address users with lower language skills, there has been great interest in rendering warnings with pictographs, small pictures, such as a person recoiling from an electric shock, or abstract graphics, such as the one used to note radioactive material.

It is very difficult, however, to ensure that that pictographs and symbols will be properly interpreted. One famous example is the traditional symbol for poison, a skull-and-cross-bones. Many children were poisoned because they interpreted graphic as meaning "pirate food," and therefore believed that the substance was safe and might be fun to eat. (The skull-and-cross-bones has been dropped for Mr. Yuck, a schematic, frowning face.)

To show the difficulty of creating understandable symbols, I have also run classes where students privately created symbols for a variety of concrete and abstract concepts. Each student then presented his symbols to class to test comprehension. Success rate was very low, usually 20% at best. Many research studies have recorded similarly low rates of graphic interpretation. The American National Standards Institute (ANSI) suggests that graphic warnings be tested and have a recognition rate of 85%, but there is no requirement that manufacturers test graphic warnings.

4. Comply with the warning: Even if the user understands the warning s/he may not comply. One common reason is the cost of compliance. If a warning says do not drive for two hours after taking a drug, then the likelihood of compliance lies in the time and expense of finding alternate transportation. If a warning says, "do not enter," then compliance likelihood lies in the availability of an alternative route. One study shows that the users are more likely to comply if their cost is directly lowered; users were more likely to comply with a warning to use safety gloves when the gloves were attached to the product.

While several studies suggest that warnings can reduce diving injuries, there are many factors which affect likelihood of seeing and complying with warnings. Warning research specific to diving has revealed the following:

Compliance with no diving signs: One study examined whether a group of high school students would notice "No Diving" signs located in three places around the shallow end of an in-ground pool. The majority of students failed to notice the signs, with females being significantly less likely. This runs contrary to studies for other products, which generally show females more likely to see warnings. Interestingly, males were still more likely to dive into the shallow end.

Risk Perception: Compliance depends the viewer's cost-benefit analysis of the situation. The lower the perceived risk, the less likely the compliance. Several studies have shown that people with a history of diving into both in-ground and above-ground pools were more likely do so in spite of the warning. In fact, one study found that diving team members were the people most likely to ignore a "no diving" warning. This is the "familiarity effect," where people having a benign history with a product are less likely to comply with warnings. Since their personal experience says that there is little risk, they are more likely to gage future risk as low. The finding that most accidents occur on a first dive of the day may also be a type of familiarization effect. However, it may also be a statistical anomaly -- there are probably more first dives taken than second, third ...

Visual Factors: Accidents often occur because visual factors make depth estimation difficult. Lack of lighting around a pool has been estimated to contribute to about 30% of diving accidents. Another more subtle effect occurs from the optical properties of water which effect depth perception. When a diver looks into a pool s/he is likely to overestimate the depth because water the pool bottom will appear to be further away than the actual distance. This effect is magnified by any cloudiness to the water, which results from poor maintenance or other sources.

Diver Sense of Control: People may be well aware of a hazard, but believe that they can minimize risk by using a product in a "safe" manner. Many accidents occur when a diver is aware that the water is shallow but decides that it is OK if the trajectory is shallow. In fact, most people have poor control over dive angle. Still, this may be another reason that diving team members were so likely to ignore warnings.

CONCLUSION

The notion that diving into a swimming pool is an obvious hazard overlooks much psychological reality. In most human factors design, the initial goal is to create a safe environment. Better pool design would decrease diving accidents, but economic realities and aesthetic goals limit the degree of safety achievable. Diving into a pool will always be hazardous behavior and safety will always require a psychological line of defense.

Ultimately, peoples' actions in hazardous situations are based on a cost-benefit analysis in which they place risks against gain. If psychological factors, personal experience, experience of friends and family, and feeling of expertise and control are strong, then the risk will be seen as low and compliance will not occur.

Visual Expert and University of West Virginia Medical School. Contact: vexpert@visualexpert.com

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