Participation in recreational scuba diving

Sports & Fitness Industry Association Report 2015

Every year the Sports & Fitness Industry Association (SFIA) releases a report that reviews participation data on various sports and recreational activities. The 2015 report pertains to 2014 participation data and is based on 10,778 online interviews (5,067 individuals and 5,711 households) among one million US online panel members. The survey asked about demographics and participation in various physical activities and sports.

Demographics of the survey participants included the following:

  • 49 percent male, 51 percent female
  • < age 18: 15 percent
  • > age 65: 17 percent
  • 77 percent Caucasian, 8 percent African American, 5 percent Asian/Pacific Islander, 8 percent Hispanic, 1 percent “other”
  • The results were calculated based on a U.S. population of 292,064,000 ages 6 and older. Various weighting techniques were applied. The sample provides a high level of confidence. A sport with a participation rate of 5 percent has a confidence interval of plus or minus 0.42 percentage points at the 95 percent confidence level. However, scuba diving participation was significantly smaller (1-1.1 percent of population)

According to this report, 3.145 million Americans (1.1 percent of population) participated in scuba diving once or more in 2014, which is a 0.9 percent decrease over 2013. However, the average participation for the last two years shows a 1.3 percent increase over the average for the previous five years.

There are 2.252 million casual participants in scuba diving (defined as making between one and seven dives per year) and 893,000 core participants (defined as making eight or more dives per year). Males make up 66 percent of casual and 74 percent of core participants.


Figure 1 shows the age distribution of casual divers versus core participants.

  • 1 percent of casual and 57 percent of core participants are between the ages of 25 and 54.
  • 7 percent of casual and 21.2 percent of core participants are younger than age 25.
  • 2 percent of casual and 21.8 percent of core participants are older than 54.


The rate of participation (the percentage of population that participate in scuba diving) by age group is shown in Figure 2.


In the 18 – 44 age range, participation for the casual divers is between 1.2 and 1.4 percent and for the core divers between 0.2 and 0.4 percent. Participation rates vary with age. Casual participation rates increase continuously until age group 35-44 and decreases sharply afterwards. The core participation practically does not vary between age 25 and 64 (3-4 percent) but drops at older age (0.1 percent). Interestingly, both casual and core participation rates for children (6-12) are several times greater than for  65+ age groups.

This may be partly because the later includes wider age range.


As shown in previous reports, scuba diving participants are on average wealthier and better educated than the general population.

Data about cross-participation of scuba divers in other activities shows 3 – 22 times higher participation indices in comparison to the general population. This pertains to aerobic activities, participation in individual and team sports as well as in extreme sports. Nearly 46 percent participate in running/jogging and 36 percent swim for fitness. It is not known what is the overlap nor how many divers do not participate in other sports. However, it is encouraging to see that divers participate in sports more than the rest of population because in the past the DAN medical emergency line and Fatality and Injury monitoring program has observed that some divers get injured due to inadequate physical fitness. More precise data are necessary to identify those who need some additional encouragement.

Gradient Factors can be used to control for depth-time exposure and alleviate the risk of decompression sickness in recreational scuba diving

An interesting study conducted by DAN Europe and presented at the European Underwater and Baromedical Society (EUBS) 2015 meeting in Amsterdam studied risk factors of decompression sickness in recreational divers and proposed how to reduce the risk of getting bent.

The study used data recoded in DAN Europe Dive Safety Laboratory (DSL) Database and compared dives that resulted with DCS to dives that left diver’s symptom free. There were 327 DCS cases (206 males and 121 female divers) and 65,304 symptom free dives. Data included electronic dive profiles, diver data and outcome data. Researchers explored possible contribution of dive profiles, age, gender, height, weight of divers, workload during dive, dry vs. wet suit, water temperature, acute health problems and other problems during dive.

The most significant difference between dives resulting with DCS and symptom free dives was in the dive profile. DCS dives were deeper (33.8 vs. 29.1 msw) and of longer run time (50 minutes vs. 39 minutes). Women were overrepresented among divers with DCS (37%) in comparison to their representation in DSL database (17%). It is important to note that most of DSL dives were collected prospectively while most DCS dives were include retrospectively and thus do not provide for calculation of rates. The mean age appeared higher in DCS cases but the difference was too small and of no practical significance. Other tested factors like weight, height, workload, type of dive suit, water temperature, acute health problems and problems during dive did not appear different between the two groups.

The study went a step further and tried to find a practical measure of dive exposure severity that divers could use to control their risk of DCS. They estimated the gradient factor (GF) in each dive and did group comparisons. The gradient factor is the mathematically estimated ratio of the maximum tolerable supersaturation in tissues, an estimated saturation during decompression from a given dive while assuming “standard” conditions. When saturation of the tissue equals the theoretical supersaturation the GF equals 1. When saturation of the tissue exceeds maximum tolerable supersaturation, theoretically gas cannot stay in solution any longer and free gas occurs in tissues and circulation. Over time, we have learned empirically that pushing the limits (diving until the GF gets close to theoretical limit) results in increased risk of DCS and that GF should be less than 1. The empirical wisdom was confirmed with this study too, as is shown in Figure 1. The GF was greater in the DCS group (0.47 to 1.15 and mean value of 0.79) compared to the symptom free group of dives (0.21 to 1 with a mean value of 0.67). The possible practical implication is that by setting dive computer GF limits to lower levels divers may reduce their risk of DCS. Depending on personal risk tolerance, the GF setting may vary but should not be greater than 0.80.

It is important to note that in deeper diving the GF game becomes more complex and the optimal GFs vary with depth. It is beyond the scope of this blog to discuss it. For more detail read the DAN Decompression Sickness Health and Diving References Series booklet. Divers must learn in-depth theory behind the GF practice before they start to make adjustments. It is also important to warn that a GF is not measured but rather mathematically estimated based on depth, time and inert gas without taking into consideration other factors that can greatly change gas solution dynamics.

This study also shows that the effect of depth-time on outcome of decompression is overshadowing possible effects of other observed factors.  However, this was an observational field study where such factors are not really measured and so the reader should not take it as far as to conclude that workload during dives does not matter. Instead, whenever exerting oneself during a dive more than usual, a diver should cut the time at depth short. This may compensate for the increased blood flow and on-gassing caused by exercise. The diver could also extend decompression time.

The presentation of this study at EUBS was a preliminary study. More thorough analysis will be presented soon in a scientific journal.

DCI quartiles



Pieri M, Cialoni D, Balestra C, Marroni A. Possible risk factor correlated with decompression sickness: Analysis of DAN Europe DSL Data Base. Presentation at 41st Annual Scientific Meeting of EUBS Amsterdam (August 19-22 2015)

Melatonin and Susceptibility to CNS Oxygen Toxicity

Seizure due to CNS oxygen toxicity in divers breathing a hyperoxic gas mix underwater is very likely to cause drowning. Some anecdotal reports indicate that CNS oxygen toxicity may be more common during night activities. A research team from Israel Naval Medical Institute in Haifa, studied the effects of night activities and melatonin on possible modification of susceptibility to oxygen toxicity in rats.

shutterstock_Melatonin_2Melatonin is widely present in animal and plant species, however its physiological function differs depending on the organism. In animals, it is produced by the pineal gland and is associated with regulation of diurnal (circadian) rhythm. With the onset of darkness, the pineal gland begins secreting melatonin reaching maximum production in the early morning hours and stops with the onset of daylight. Melatonin promotes sleepiness and contributes to a healthy and restorative sleep. It also contributes to antioxidant and anti-inflammatory processes.

In this study, rats were exposed to 12 hours of light/dark cycles to affect melatonin production over a three week period. The control and experimental groups were kept awake during the day and night, respectively. At the end of this period melatonin level in the blood was measured. Each group was divided into two subgroups, one receiving supplemental melatonin and the other a placebo before exposure to 5 ATA oxygen in a hyperbaric chamber. The time to onset of convulsions was measured. In addition, researchers measured the levels of enzymes affecting reactive oxygen and nitrogen species.

Oxygen convulsions occurred much faster in animals active at night. The rapid onset of convulsions was associated with a reduced level of melatonin. However, provision of external supplemental melatonin did not affect resistance to oxygen toxicity. The effects of external melatonin on the level of antioxidant enzymes varied. Oxygen reactive species were more affected than nitrogen reactive species.

The most important finding of this study was that night activity represented an additional risk factor for the development of CNS oxygen toxicity in rats. This was likely caused by changes in diurnal rhythm which could not be compensated by administration of external melatonin.

For divers this raises the question of whether diving at night increases the risk of oxygen toxicity as well. Although taking melatonin before a night dive may not provide any preventative benefits or protection against CNS oxygen toxicity, it can be beneficial in other ways. Melatonin may help with jetlag by supporting a healthy sleep cycle and by re-setting the body’s sleep-wake phases.


Eynan M, Biram A, Mullokandov M, Arieli Y. Susceptibility to CNS oxygen toxicity following a switch from day to night activity is associated with changes in melatonin and antioxidant enzyme activity. Oral presentation, EUBS 2015, Amsterdam.

Recompression Using Deep Heliox Tables and Treatment Outcomes

In most cases decompression illness (DCI) in recreational divers resolves with recompression to 18 msw (2.8 bar pressure) while the patient is breathing oxygen. However, severe cases of DCI can be more resistant to treatment and may leave the diver disabled. To increase the chances of complete resolution, some physicians advocate the use of deeper tables combined with heliox. Their rationale is based on physics and animal studies. Recompression to a greater pressure can lead to a larger decrease in bubbles and quicker elimination. To avoid oxygen toxicity, oxygen should be diluted at greater pressures. Theoretically, If a diver acquires DCI while diving on air, using helium — which is slow to enter tissues — as a diluent in treatment gas may more quickly eliminate from the tissues the nitrogen and bubbles that cause injury.

This theory, however, had not been tested with divers. At the annual scientific meeting of the European Underwater and Baromedical Society in Amsterdam this August, Emmanuel Gempp presented experience with use of heliox tables at Sainte Anne’s Military Hospital in Toulon, France. Toulon is on the Mediterranean coast, where a lot of diving activities occur. The emergency medical services in the region are well organized. In 85 percent of DCI cases the time to recompression treatment in a hyperbaric chamber is less than three hours, and almost all injured divers receive first aid surface oxygen.


Skin Mottling after Diving May Be Result of Brain Lesions Caused by Gas Bubbles

Cutaneous decompression sickness (DCS), or “skin bends,” most often manifests as skin mottling on the torso, upper arms and buttocks to various degrees. An associated marbled look to the skin is sometimes referred to as cutis marmorata. While cutaneous DCS is most likely related to gas occurring in body — after decompression or due to lung barotrauma or some medical procedures — there generally is no accepted explanation how the free gas is related to skin changes.

Possible explanations include the occurrence of gas bubbles in subcutaneous tissues, occlusion of subcutaneous arteries with circulating bubbles bypassing the lung filter (as with a patent foramen ovale), inflammatory reaction bubbles present locally or bubbles causing endothelial injury at remote locations.


Can a coronary calcium scan improve the prediction of heart attacks in older divers?

In the July 2015 issue of Undercurrent, an article titled “A better heart-check tool than a stress test?” discusses the possible benefits of a coronary calcium scan for older divers to reduce the risk of experiencing a heart attack while diving.1 This article is a follow-up to a May 2015 Undercurrent report about an overweight 65-year-old diver who died shortly into his dive while on a dive trip.2 That article, which considered preventive options such as a stress test, also presented views from Dr. Alfred Bove and DAN’s Dr. Petar Denoble and Dr. James Chimiak, who agreed with the American College of Physicians (ACP) guidelines that recommend a graded and individualized approach to preventive testing and diagnostics.

Another physician suggests in the July 2015 article, however, that older divers should have a coronary calcium scan, which he claims may provide information that will help them avoid a heart attack on their dive trips. Many walk-in clinics offer the test at a low price. “A coronary calcium scan can tell you years before a positive stress test that you are headed in that direction [of significant coronary disease] so that you can do some kind of intervention,” he said. While the statement has merit, it may be misleading in this context.


Can a Test Identify Divers Who May Be More Susceptible to DCS?

Are some divers prone — or resistant — to gas bubbles after diving?

Decompression sickness (DCS), which may occur in divers after decompression from a dive, is dependent on the combined dose of gas saturation during the dive and the rate and magnitude of decompression. However, there is a great variability of outcomes in subjects exposed to the same dive profiles. The variability decreases as the severity of exposure increases.

DCS is correlated with the degree of venous gas emboli (VGE), or “bubbles”, in circulation after a dive. Generally, the higher the VGE grade (more bubbles) the greater the probability of DCS, and vice versa. Similar to DCS, there is a great variance in the probability of VGE appearing postdive. Some researchers who practice VGE detection have hinted that some divers bubble after most dives and may exhibit a high bubble grade (HBG) and others tend not to bubble at all or rarely exhibit HBG. The former are often labeled as bubblers (or high bubblers), while the latter are labeled as nonbubblers (or low bubblers).