Although some authors quote the large US asthma death statistics, based originally on University of Rhode Island studies, the compilation of these data in fact allowed hardly any conclusions regarding such contributing factors as asthma.
As most of the information was obtained from newspapers, the likelihood of a documented medical history was close to nil. However, this has not prevented some authors from quoting these figures, ignoring the asthma death statistical adage that absence of evidence is not evidence of absence.
A more recent collection of US fatalities from the DAN database are more reliable, as at least some attempt is made to obtain a past medical history. This was admitted to be not very successful, however. In this survey four of the 60 cases were recorded as having asthma. During the last year of the fatality statistics from DAN (1998), of the deaths on which information was available, 7 per cent were taking medication for asthma.
One series of 100 consecutive recreational diving deaths was recorded in Australia, and an active attempt was made to obtain the full medical history of all the divers, together with an assessment of the various factors contributing to the diving death. It revealed that eight (of the 49 cases in which the medical history was available) had asthma as a likely contribution to the death. One other case clearly had asthma, but because it had never been diagnosed as such, was recorded merely as a 'respiratory disease contribution. This would suggest that a minimum of 9 per cent of the deaths had asthma as a contributing factor in this series. In 51 of the cases the past medical history was unavailable, and was not forthcoming for a variety of reasons, including medicolegal. Thus, the true incidence of the contribution of asthma death statistics could be well in excess of the 9 per cent.
In comparing the Australian data on diving deaths, with the presumed incidence of active asthma in the diving population (see above), it was concluded that the asthma cases were more highly represented, than they should have been by chance alone. This is also consistent with the individual clinical cases demonstrating the existence and effect of the asthma. The death statistics highlight the difficulty in obtaining precise information, and also the time-consuming nature of this approach.
A microcosm of the problem is evident in the New Zealand reports. Those who denigrate the contribution of asthma to diving deaths, quote a figure of 1 out of ll, but those who are more impressed with its relevance quote 5 out of 20. The difference in the statistics may reflect the thoroughness with which the medical history is sought.
Insisting on autopsy data to implicate asthma, as has been suggested by some, is illogical. The first pathological sign of an acute episode of asthma, is eosinophyllic infiltration of the mucosal lining of the lungs (diving medical standards). This usually takes some 24 hours to develop, and so an acute attack initiated during diving would therefore not likely be detected at autopsy. Evidence of chronic asthma effects, albeit indicative of the disease, do not necessarily imply that this has contributed to the death.
Prospective surveys of asthmatics who do dive, have been under way now for more than 10 years. Apart from the obvious selection, i.e. these are 'survival' statistics, this approach could produce valuable information, both negative and positive, if the researchers are fully informed by the divers, impartial, and adhere to conventional statistical guidelines.
Asthma Population Statistics
It has been estimated that 4-7 per cent of the population has clinical asthma. However, the incidence in Australia and New Zealand is higher, and is increasing in children; there is less evidence of such an increase in adults, however. In one symposium on asthma and diving, Professor Des Gorman referred to an 'infamous UK epidemiological survey which is believed as a role model of how not to do such studies'. Unfortunately most of the other population 'statistics' available are no better, do not warrant any extrapolations, and probably would not pass adequate peer review.
The incidence of asthma in divers is not known. Self selecting surveys - such as those asking a large population about a disease X - will automatically select disproportionately more responses from people with disease X. Such surveys often also suffer from a very low percentage response rate, and are virtually valueless or misleading in determining prevalence figures. A number of such surveys have been performed.
Questionnaires sent out in a skin diving magazine with a response rate of less than 5 per cent, indicated that 3.3 per cent of the respondents were current asthmatics.14 A UK prevalence of asthma in about 4 per cent(31/813)of divers, was deduced from a survey of selected diving physicians, though details were not available to identify whether these were historical (childhood) or current asthmatics.
Surveys with a higher percentage respondence among divers still have the disadvantage of screening a survival population (e.g. those who have not died or retired from diving because of the illness under question or from prudence).
Some scientists assume, against all logic and even their own statistics, that the diving population (trainees or experienced) has the same prevalence of asthma as the normal population. One would expect that many asthmatics would not wish to even contemplate the possibility of scuba diving, because of increased respiratory difficulties. One review10 which is frequently quoted, claims a prevalence of asthma in divers as 5-8 per cent, but this is not evident from the references cited.
Most of the current statistics are contaminated by selection of the data, or not discriminating between a past history of asthma and current asthma. Frequently these reports of population statistics are in the form of abstracts (no detailed data available), or unaccompanied by details of the questionnaire supplied.
A more comprehensive UK survey was also less biased as it did not focus on asthma, and analysed 2240 responses (21 per cent); this indicated an asthma prevalence of 1.7 per cent amongst divers.
A DAN survey indicated that 13/696 respondents were currently asthmatic (defined as having an attack within one year or using bronchodilators). This equates with a 1.9 per cent incidence, but there was also a 5.3 per cent incidence of respondents with a past history of asthma.
An analysis of the DAN membership survey, in a 1989 report by Wachholz indicated that only 1.2 per cent of the 2633 respondents were active asthmatics, and 3.9 per cent had a history of asthma. The DAN survey had all per cent response and was multifactorial, avoiding specific self-selection of respondents for asthma.
In Australia, where pre-diving medical examinations are mandatory for scuba training, 68 of 2051 (3.3 per cent) admitted to a history of asthma - some of which were childhood. Only 1.4 per cent gave a history of asthma since childhood, but among those who gave a history of childhood asthma only, 50 per cent were ultimately considered fit to dive. Parker's survey verified not only the sometimes innocuous nature of the asthma history, but also that the inci-dence of asthma in diver trainees did not reflect the normal adult population.