Chapter 27

MINING 5,000 m below surface and the risk of barotrauma

R.M. Franz
CSIR Division of Mining Technology Environment Control Occupational Health & Safety Johannesburg, South Africa

ABSTRACT

The paper reports on that part of the South African DeepMine Research Programme investigating potential barometric pressure effects on mineworkers at depths of 3,500 to 5,000 m. It considers changes in pressure between a surface altitude of approximately 1,500 m and workplaces as much as 3,500 m below sea level (indicating a 75 percent increase in pressure), in terms of potential risk of barotrauma. Mechanisms for barotrauma are reviewed, risks examined and possible control measures considered.

The work drew from previous investigations of hypobaric and hyperbaric conditions, to assess the potential impact of pressure variations anticipated for ultra-deep mines, including effects on workers' ears, paranasal sinuses, gastrointestinal tracts and teeth, as well as possible complications during the evacuation of injured mineworkers.

Findings indicate the likelihood that individuals with chronic or temporary predisposing medical conditions would be subjected to an increased risk of barotrauma. Accordingly, recommendations are offered for incorporating risk-based medical examinations into present medical screening procedures, and improving workers' awareness and understanding of barotrauma to enable behaviour-based control strategies. The paper also discusses the need for precautionary measures during the evacuation of injured workers, and considers technology-based means to limit and monitor the rate of pressure changes that workers would be exposed to in ultra-deep mining.

KEYWORDS

Barometric pressure, barotalgia, barotrauma, barotitis, mediastinal emphysema, pneumothorax, pulmonary barotrauma, ultra-deep mining

INTRODUCTION

Barotrauma is broadly defined as any injury
to organs or tissue resulting from large changes in atmospheric pressure that cause the destructive contraction or, more commonly, expansion of gas
or air contained in body or tissue spaces. Concerns that the risk of barotrauma among workers could increase as mining depths are extended to 5,000 m led to the present investigation. It was originally envisaged that ultra-deep mining would adopt
a “greenfields” approach, i.e. a single shaft sunk from surface to a depth of 5,000 m. However,
a subsequent analysis of the engineering and financial implications indicated that a “brownfields” option, i.e. the extension of sub-shafts from existing main shafts, would be more viable. The assessment was adapted accordingly, to better enable planning for the control of barotrauma risks in ultra-deep mining.

METHODOLOGY

Sources of information included journal articles and texts, as well as consultations with mining industry medical personnel and specialists in the fields of hyperbaric medicine and otorhinolaryngology (disorders of the ears, nose and throat). Internet websites for high altitude medicine also proved useful. Information was evaluated in terms of its relevance to the intended design criteria for ultra-deep mining, particularly the planned hoisting speed of 1,000 m/min or 16,7 m/s, with two-stage hoisting over a total altitude range of 5,000 m and pressure range of 62 kPa.

Efforts to determine the current incidence of barotrauma relative to depth proved unsuccessful and were ultimately limited to a review of experiences among the industry's medical personnel. This was due to a lack of pertinent information in the medical records, which prompted recommendations for appropriate revisions to recording and reporting procedures.

Findings and discussion

Overall Considerations

The rate of increase in atmospheric pressure for a hoisting speed of 60 km/h would be 0,11 ATA/min. This corresponds with an underwater descent rate of 1 m in seawater (msw) per min., which is one-tenth the normal and safe rate for underwater divers (10 msw or 1,1 ATA/min.). In air, descent rates among skydivers commonly exceed 180 km/h or 50 m/s (equating to a pressure increase of 0,329 ATA/min.), normally with no incidence of barotrauma.

It must be acknowledged that populations of underwater divers and parachutists cannot be regarded as representative of underground mineworkers. Nevertheless, the consensus among medical specialists consulted is that excursions between surface and ultra-deep workings would only present an appreciable risk of barotrauma for individuals with a predisposing medical condition (chronic or temporary), or certain anatomical abnormalities.

The ears are normally the organs most susceptible to barotrauma, and they are most likely to be affected by increasing pressure, i.e. during descent. Potential pressure-induced effects and their implications are considered for the ears and for other organs in the paragraphs that follow.

Ears

Barotrauma to the ear (barotitis) results from pressure differences between the atmosphere and air in the middle ear cavity. During descent, increasingly dense atmospheric air exerts greater pressure against the tympanic membrane. If this pressure is not equalised by the movement of air from the back of the throat through the Eustachian tube and into the middle ear cavity, discomfort, barotalgia (pressure-induced middle ear pain) and barotitis can occur, depending on the pressure gradient. A difference of 90 mm Hg or 11,96 kPa (as would occur during a descent of 1,087 m with no equalisation) causes pain and closes off the entrance to the Eustachian tube, thereby preventing equalisation by any means other than re-ascending to reduce the pressure gradient and re-equalising.

Depending on the eardrum's condition, subjecting it to a pressure difference of 100 to 500 mm Hg or 13,28 to 66,42 kPa (corresponding with a 1,200 to 6,000 m increase in depth) would cause the membrane to rupture, immediately relieving the pain, but often resulting in nausea, vomiting, dizziness, vertigo and partial loss of hearing (Anon., 1994).

Scarring on the tympanic membrane as a result of previous barotrauma, middle ear infection or damage caused by a foreign object would render it more susceptible to rupture, as the interface between scar tissue and normal tissue would constitute a weak point in the membrane. Risk of middle ear barotrauma would be greatest among the most susceptible individuals and, accordingly, it would be appropriate to adopt a conservative criterion level. This indicates that the eardrum's lower limit of tolerance to pressure differences (100 mm Hg or 13,28 kPa, corresponding with a 1,200 m increase in depth) is potentially critical.

Pressure equalisation of the middle ears during descent often requires deliberate effort, but in the absence of congestion in the Eustachian tubes or swelling of surrounding tissue (which would constrict these air passages), most individuals are able to equalise with little difficulty. Equalisation can be achieved through deliberate swallowing actions and/or movement of the lower jaw to open the Eustachian tubes and admit outside air to the middle ear cavities. Where such actions do not have the desired effect, it is possible to perform Valsalva-type manoeuvres (deliberate pressurisation of air against blocked nostrils) to force air into the middle ear clefts.

However, where even active equalisation is prevented by congestion of the Eustachian tubes (caused by the accumulation of mucus), their constriction (by the inflammation and swelling of surrounding tissue), or their closure (by an excessive pressure difference resulting from failure to equalise incrementally), a risk of barotitis would exist. This can either take the form of barotitis externa (pressure-induced traumatic inflammation of the external ear canal and tympanic membrane), or barotitis media (pressure-induced traumatic inflammation or bleeding in the middle ear).

Both conditions are caused by a pressure difference across the tympanic membrane), but barotitis externa is most commonly associated with a coinciding imperforate blockage of the external ear. Such blockage could be the result of impacted cerumen (wax) in the ear canal, a congenital absence or deformity of the external ear opening, or by the inappropriate use of insertable hearing protection devices (e.g. during descent or while suffering from an acute congestive condition). Barotitis media, commonly known as aviator's ear due to its original association with aeroplane descent, can be accompanied by intense pain in the ear, tinnitus (a perceived ringing or hissing sound originating in the inner ear) and, occasionally, by a loss of hearing sensitivity and vertigo.

Medical conditions that can contribute to the occurrence of both forms of barotitis include middle ear infection, head cold, flu or an allergic response such as hay fever. The tissue swelling that accompanies these conditions can cause constriction of the Eustachian tubes, thus impeding equalisation. Where rupture or perforation of the tympanic membrane does occur, this would normally repair itself without medical treatment, provided the healing process is not impeded by infectious bacteria common to middle ear infections.

Barotrauma effects on the inner ear have also been demonstrated. Repeated pressure-induced trauma can eventually lead to the development of micro-fistulae (abnormal interconnections between normally separate structures) inside the cochlea of the inner ear. These cause localised loss of function within the organ of Corti (the hearing organ) and corresponding discrete (relative to audio frequency) hearing loss. Such effects, are relatively painless in comparison with those involving the middle and outer ear, but are of a more serious nature in terms of the permanent structural damage and resultant hearing loss they cause.

During ascent, decreasing ambient pressure normally allows passive equalisation of the middle ears, by the venting of higher-pressure air via the Eustachian tubes. In the absence of occlusion, this occurs once the pressure difference reaches 15 mm Hg or 1,993 kPa, approximately corresponding with a 180 m ascent. Only in cases of severe congestion and occlusion of the Eustachian tubes developing during the underground shift (e.g. extreme onset of flu or head cold symptoms, or a histamine response) would there be any risk of high-pressure air being trapped in the middle ears during ascent. In such an unlikely event, the risk of barotitis would be still be minimised by the tendency for higher-pressure air to vent itself, indicating a minimal risk of barotitis during ascent.

Sinuses

The paranasal sinuses comprise four pairs of cavities or air spaces within the skull, the members of each pair being symmetrically positioned lateral to and behind the nasal passages, communicating or interconnected with the respiratory tract. The paranasal sinuses are:

• Frontal sinuses, encased in the frontal bone (forehead), medial to and slightly above the eyes, extending approximately half-way up the forehead

• Ethmoid air cells, situated in the ethmoid bone (which forms the lower half of the ocular orbit or eye socket), medial to and slightly behind the eyes

• Sphenoid sinuses, located in the sphenoid bone (frontal portion of the skull's base) medial to the eyes, approximately mid-way between the eyes and ears

• Maxillary sinuses, located in the maxilla or upper jaw, below and behind the eyes at a similar level as the lower half of the nose

The sinuses' inner structure is somewhat convoluted by folds in the mucous membranes that line them, creating the potential for entrapment of air behind swollen tissue or abnormal accumulations of mucus. The frontal and maxillary sinuses are those most commonly affected by barotrauma, which causes sinus linings to swell and bleed. The frontal sinuses are the most susceptible, due to their long and tortuous ducts.

The development of sinus barotrauma under conditions of increasing ambient pressure involves a sequence of changes described by Garges (1985). Firstly, some absorption of intra-sinus air by the sinus lining occurs (exacerbating the pressure gradient) and the mucous membranes become engorged and swollen, causing inflammation and fluid accumulation in the sinus cavity. If ambient pressure increases as a result of further descent, pain and bleeding into the sinus cavity ensue. When this sequence is followed by ascent, decreasing ambient pressure allows accumulated fluid to block the sinus duct, thus preventing equalisation and causing further barotrauma. Such injuries commonly lead to infection, exacerbating sinus blockage through inflammation, swelling and purulent discharge, all of which impede the healing process.

As with the ears, active equalisation of the sinuses can sometimes be accomplished by pinching the nostrils and closing the mouth or throat, then forcing higher-pressure air into the sinuses. However, the success of such efforts would depend on the extent of congestion, and it should be recognised that inadvertent over-pressurisation of the middle ears could occur, possibly resulting in barotitis media.

Lungs

By virtue of their structure and normal manner of function, i.e. by constant and unrestricted communication with the atmosphere, the lungs would not be routinely exposed to any risk of barotrauma. However, under certain circumstances, mainly limited to cases of traumatic injury involving the chest or throat, such a risk could arise.

An increase in atmospheric pressure during descent, even with the obstruction of normal communication between lungs and outside air through traumatic injury or bronchospasm, would pose no risk of pulmonary barotrauma. This may be a moot point, since an injured or ill worker would normally be transported to surface rather than to a deeper level. Furthermore, an obstructed airway would, in itself, constitute an immediate threat to survival. Nevertheless, the compression of entrapped air by increased ambient pressure would be of no consequence for the magnitude of pressure changes anticipated for ultra-deep mining. However, this is far from true for the expansion of trapped air, as would occur during ascent.

Pulmonary barotrauma, i.e. the rupture of lung alveoli by their containment of high-pressure air, requires a pressure difference of only 0,1 ATA between the lungs and atmosphere. Such a condition would occur where the airway is obstructed during an ascent of approximately 1,000 m, implying that a worker holding his breath for one minute (at the intended hoisting speed of 1,000 m/min) could suffer pulmonary barotrauma.

This is a serious injury, not only in terms of the immediate damage it causes to lung tissue, but more for the risk of arterial embolism that it presents. Occlusion of a blood vessel by air bubbles introduced into the bloodstream at the ruptured alveoli could restrict blood flow to the brain, with immediately fatal or permanently debilitating results. Accordingly, there would be a need to inform workers of the potential risk and consequences of such behaviour. To provide for a situation where workers are exposed (or believe they are exposed) to an irrespirable atmosphere, they should be forewarned that exhalation will not admit harmful gases or combustion products into their lungs, but failure to exhale during ascent could be fatal.

In the event of evacuating an injured mine worker from ultra-deep areas, there would be a critical need to ensure that the individual is not suffering from any obstruction in his airway prior to ascent. In addition to the obvious need to restore breathing, an unobstructed airway would reduce the risk of life-threatening complications or exacerbation of existing injuries.

Besides traumatic injury involving the throat or chest, the airway can be obstructed by the presence of blood, vomit or a foreign body, or by bronchospasm caused by the inhalation of noxious gases such as chlorine, or oxides of nitrogen or sulphur. The serious nature of pulmonary barotrauma and its likelihood in the event of air entrapment indicate a life-threatening risk. However, it is one that would be largely confined to the unusual circumstances presently considered. Certain trauma-induced injuries could be severely complicated during evacuation to surface, including:

• Arterial air embolism: a potential complication of pulmonary barotrauma, which could result from premature evacuation to surface. An existing embolism would likely be exacerbated by the expansion of air bubbles in the bloodstream as ambient pressure decreases, further obstructing blood flow and possibly posing an immediate threat to life, depending on what part of the body is deprived of blood.

• Pneumothorax: the presence of air between layers of the pleurae (membranes enveloping the lungs and lining the thoracic cavity). Air introduced between layers of the pleura as a result of traumatic chest injury would expand during ascent, possibly leading to barotrauma of the pleura and its further damage, likely affecting the lungs as well.

• Mediastinal emphysema: an accumulation of air in the tissues of the mediastinum (space in the middle of the chest between the left and right pleurae). Air introduced to the thoracic cavity as a result of traumatic chest injury would expand during ascent, possibly exacerbating the original injury and causing further damage through barotrauma, likely involving the pleurae and lungs.

The potential for barotrauma-induced complications during casualty evacuations already exists in mines deeper than 1,000 m. However, the greater pressure changes occurring during ascent from an ultra-deep mine indicate the need for a risk-based assessment of potential risks and, subject to its outcome, the revision of current stabilisation and evacuation procedures to incorporate appropriate preventative measures.

In the first instance, correction of an airway obstruction or relief of bronchospasm prior to ascent would be essential in any pressurised situation. This indicates the need for expert medical assistance at depth prior to evacuation, to ensure appropriate measures for preventing complications or further injury. Pre-evacuation correction of airway obstructions is already standard practice among medical personnel. However, preventing the barotrauma-induced exacerbation of pneumothorax or mediastinal emphysema, or reducing the risk of an arterial air embolism during ascent may require additional precautions. These could include insertion of a chest drain to vent trapped air (a sensitive and potentially dangerous procedure that should only be performed by a doctor), and/or use of a portable hyperbaric chamber, as illustrated in Figures 1 and 2 (Bartlett/HMS, 1999 and CMG, 1999, respectively) to enclose the injured miner during evacuation and transport to hospital.

Figure 1. Portable Altitude Chamber® with foot pump

Figure 2. Gamow Bag® prepared for use

Devices such as those illustrated would require some modification for the application being considered, including an increase in pressurisation capacity from the current maximum of 22 kPa (above ambient) to something approaching 55 kPa, the approximate difference between surface and a mining depth of 5,000 m. There should also be adequate provision for medical personal to monitor the patient and administer any required treatment during evacuation. Clearly, input from mining industry medical personnel would be essential for successfully adapting current pressure bag designs for ultra-deep casualty evacuations.

Gastrointestinal Tract

The influence of pressure changes on a normal gastrointestinal tract would be negligible for the magnitude of pressure variations in ultra-deep mines. Risk of barotrauma would be virtually non-existent during descent, since gases resulting from digestion could only be subjected to compression, which would have no harmful or discomforting effects.

During ascent there would be a tendency for intestinal gases to expand as ambient pressure decreases. This could lead to gastrointestinal symptoms largely limited to abdominal discomfort and perhaps some degree of flatulence, possibly accompanied by pain. The severity of symptoms would depend on the amount of intestinal gas present (determined by content of recent meals, including food taken during the shift), as well as on the individual's state of health with regard to the gastrointestinal tract.

The potential for some discomfort notwithstanding, risk of gastrointestinal barotrauma would not be a serious issue in ultra-deep mining. Those tending to suffer from indigestion and other forms of gastrointestinal upset should be advised to avoid foods they know to induce such problems. All individuals should refrain from taking foods associated with intestinal gas, including legumes, cabbage and carbonated beverages, during the hours before and while working in ultra-deep areas.

Teeth

Pain caused by pressure-induced variations in the volume of air trapped within teeth is known as barodontalgia or, among underwater divers, as "tooth squeeze". Conditions that allow air to enter the interior of a tooth include caries (decay), defective margins of tooth restorations, periodontal abscesses, lesions in the pulpa (fleshy interior) of the tooth, as well as incomplete or improperly performed endodontic (root-canal) therapy. Pressure-induced movement of a loose filling can admit air, along with oral bacteria and the carbohydrates they metabolise in producing lactic and pyruvic acids (the chemical agents of tooth decay). All of this promotes underlying decay and increases the risk of barodontalgia.

Among underwater divers, teeth temporarily sealed after uncompleted endodontic treatment have been known to explode from the expansion of trapped air on returning to surface. Full-porcelain crowns have also been known to shatter after relatively shallow dives of 20 m or less. While such outcomes would be impossible for the pressure variations anticipated for ultra-deep mining, they highlight the importance of good dental health for individuals regularly exposed to significant changes in ambient pressure, including mineworkers.

In cases of existing decay, its extent and proximity to a nerve would determine whether filling movement and resultant compression/expansion of trapped air or gaseous products of bacterial metabolism could cause discomfort or pain. In some individuals dental pain may induce sympathetic symptoms in the form of headache or gastrointestinal upset. Such effects, depending on their severity, could range from mildly discomforting to temporarily debilitating. Where these effects are limited to discomfort or pain and the individual continues with his appointed tasks, there could be some risk of negative impact on his ability to work safely and productively.

The latter situation may indicate some potential for secondary risk as a result of trying to cope with pain through the use of analgesics while continuing with normal duties. This could render an individual less capable of productive work or contending with any unexpected threats to safety that might arise. Accordingly, workers should be advised to refrain from the unsupervised use of pain-relieving preparations for symptomatic treatment of dental problems, and be encouraged to seek professional assistance in resolving the underlying cause.

Prevention of pressure-induced effects involving the teeth requires good dental care and hygiene, including professional intervention where fillings become loose or show signs of leakage. To reduce the risk of barodontalgia, carious or decaying lesions in tooth enamel should be restored, ill-fitting crowns replaced, active periodontal lesions surrounding the teeth treated, and all endodontic therapy completed before exposure to significant changes in barometric pressure (Rottman, 1981). This implies the importance of regular examinations to identify and rectify problems before they become discomforting or temporarily debilitating. Where these requirements are met, there would be no risk of dental barotrauma, and even where they are not, risk would be largely limited to the possible occurrence of pressure-induced discomfort or pain.

Controlling the Risk of Barotrauma

Three fundamental strategies can be adopted to control the risk of barotrauma in ultra-deep mining, as considered in the sub-sections that follow.

Identification of high-risk individuals: Risk-based medical examinations (RBME) for workers being considered for ultra-deep areas would serve to identify high-risk individuals and enable the concentration of risk management efforts on those who are most susceptible. Where an unacceptable risk of barotrauma is demonstrated, the RBME would provide information necessary for the valid exclusion of such individuals from ultra-deep areas.

The RBME as contemplated would focus on the ears, given their greater vulnerability to pressure variations, but should also consider the paranasal sinuses, as well as any history of chronic conditions, e.g. allergies, colds and flu, or head injury. A review of medical history would rely to a large extent on individuals' medical records, which should include information on any previous headache or sinus complaints, upper respiratory tract infections and allergies. Symptomatic details, the circumstances of onset and the efficacy of any treatment administered should also be considered.

RBME procedures for the ears should include an otoscopic visual examination of the tympanic membrane, to assess its condition and determine the presence of any pathology indicating previous barotrauma, e.g. scar tissue, or signs of middle ear problems that could impede equalisation. In this regard, the tympanic membrane should be observed during swallowing actions to assess Eustachian tube function.

The use of tympanometry would quantify aspects that are considered subjectively during an otoscopic examination. A tympanometer comprises a sealed probe inserted into the opening of the external ear, and a microprocessor-controlled pump to alternatively pressurise and depressurise the ear canal, thus displacing the tympanic membrane and inducing changes in ear canal volume. These are measured to evaluate eardrum compliance and provide an indication of any middle ear pathology, e.g. a damaged or scarred eardrum, as well as abnormalities in the middle ear ossicles or the Eustachian tube. The tympanometer immediately generates a test report and can be linked to a computer for record-keeping and data management purposes.

Meaningful assessments by otoscopy rely on the knowledge, experience and interpretational skills of the examiner, implying the likely need for an ENT specialist. In contrast, tympanometry does not require the same level of expertise and the results are quantitative, thus enabling the use of pre-determined action and referral levels.

Occurrence of sinus barotrauma is mainly influenced by the function of mucous membranes in the nose and sinuses, as well as the magnitude and rate of pressure changes. Predisposing factors include allergy, upper respiratory tract infection, chronic irritation from smoking, diesel fumes, chemicals or the prolonged use of nose drops and nasal sprays, as well as mechanical blockage and vasomotor problems caused by chronic tension, stress or anxiety. Accordingly, the RBME should identify any presence of these factors.

Some individuals secrete large quantities of obstructive mucus, either in response to an allergy or upper respiratory tract infection, while others display obstructions that are a result of a nasal septum deviation, polyps or tumours blocking the openings of the sinuses. Identification of such conditions during the RBME should be followed by an evaluation of their potential to increase the individual's risk of sinus barotrauma, with consideration of the potential for medical intervention to eliminate or reduce such risk.

With regard to barotrauma involving the teeth, regular check-ups to identify potential problems, restoration of any loose or leaking fillings and treatment of any periodontal abscesses would largely eliminate such risks. Accordingly, the RBME should seek to identify any relevant dental conditions, and provide for the referral of affected individuals.

Management of high-risk individuals: The normal symptomatic treatment for conditions involving congestion of the Eustachian tubes or paranasal sinuses (excluding flu, which requires rest) is to administer decongestants or anti-histamines and, where infection is deemed to be the cause, an appropriate antibiotic. However, some of these preparations can cause drowsiness or lead to diuretic effects, both of which would be disadvantageous for persons performing potentially dangerous work under hot, humid conditions. This would indicate that workers suffering from conditions that prevent the equalisation of pressure differences (e.g. occluded Eustachian tubes or congestive sinusitis) should be excluded from work in deep-level areas until their condition has been resolved.

Individuals with chronic predisposing conditions should, at the very least, be advised to be vigilant for signs of barotrauma and symptoms of their underlying medical condition. They should also be counselled regarding the risk of aggravating their condition, with extreme cases referred for specialist evaluation to enable a participative decision regarding fitness for work in ultra-deep areas.

In some instances, surgically implanted grommets may offer a viable means of controlling the risk of middle ear barotrauma. Commonly used to treat children with chronic middle ear infections, these devices are inserted in a tiny incision made in the tympanic membrane, normally an outpatient procedure performed under local anaesthetic. In addition to draining purulent discharge from the middle ear, thus helping to clear an infection, grommets can provide for the equalisation of middle ear pressure differences, irrespective of Eustachian tube function.

Acute illness involving the upper respiratory tract can render normally healthy workers temporarily susceptible to the risk of barotrauma. To counter seasonal increases in the incidence of influenza and resultant risks, the prophylactic vaccination of workers should be considered. Provision should also be made for the administration of antibiotics to treat secondary infection where flu does occur, and the availability of medicinal preparations to control symptoms that could increase the risk of barotrauma.

Behaviour-based control strategies: A behaviour-based control strategy requires the promotion of rational, risk-based modifications in behaviour among workers and supervisors. Of primary importance would be the need for affected individuals to report any pressure-related problems to supervisors and medical personnel. Accordingly, employee awareness of the barotrauma hazard and knowledge of relevant signs and symptoms would be prerequisites to a behaviour-based control strategy, as would workers' awareness of their state of health.

Supervisors should encourage workers to report any problems, as failure to do so could result in the aggravation of acute conditions or development of debilitating chronic ones, in either instance extending the recovery period and absence from duty. Accordingly, a culture of early reporting should be developed, to enable early treatment and prevention of complications that could have lasting impact on workers' health and productivity.

Health and safety training for workers in ultra-deep mines should include explanations of the mechanisms by which barotrauma occurs, with particular emphasis on the ears and paranasal sinuses. Associations between symptoms of the upper respiratory tract (i.e. colds, flu, allergies and nasal irritation) and the risk of middle ear and sinus barotrauma should be made clear to workers, using appropriate visual devices and vernacular-based explanations. In addition, the advantages of prophylactic flu vaccinations should be discussed.

Training should explain and demonstrate practical means for coping with pressure changes, particularly middle ear equalisation techniques, with provision for workers to practice these tactics under the supervision of a suitably competent person.

In addition, workers should be made aware of the risks created by inappropriate behaviour, including the use of earplugs or holding one's breath during vertical conveyance, and failure to report medical problems. The increased risk of upper respiratory tract infections posed by smoking, and the dangers of using certain medications in hot, humid areas (especially without medical supervision) are other issues that should be addressed during workers' education and training.

While education and training would be required for all employees exposed to significant variations in barometric pressure, special attention should be given to those identified as susceptible during risk-based medical examinations. Additional requirements would be to regularly reinforce workers' awareness of the barotrauma hazard and their training in coping strategies, particularly during seasonal increases in the incidence of colds and flu, and to ensure that employees are cognisant of reporting procedures.

Technology-based control measures: Controlling the magnitude of pressure variations between surface and ultra-deep workings is beyond the reach of foreseeably practicable and affordable technology. Such an approach would require the artificial reduction or limitation of barometric pressure in mine workings, greatly complicating ventilation and environmental control systems. It would also necessitate the regulation of pressure changes during vertical conveyance, which would require sealed and pressure-controlled cages.

Micro-climatic pressure control by means of pressurised body suits would limit workers' mobility and cause an increased sense of isolation, with negative impact on safety and productivity. Such suits would require some means of thermal regulation, increasing their complexity, and be vulnerable to mechanical failure or physical damage. They would also be prohibitively costly to implement and maintain.

However, the second physical factor in the occurrence of barotrauma, rate of pressure change, is amenable to practicable control measures that are compatible with planned hoisting speeds and conveyance schedules. Risk would already be limited by the 1 000 m/min hoisting speed, which could be controlled by tamper-proof speed limiting and recording devices similar to those already in use. Furthermore, stops at intermediate levels, combined with the need to descend and ascend in at least two stages, would provide additional time for employees to adjust to pressure changes.

An additional control measure to be considered is equipping man-cages with battery-powered electronic barographs to log changes in barometric pressure. Such devices should provide sufficient resolution to confirm compliance with prescribed hoisting speeds on a trip-by-trip basis. They would need to be protected from damage and tampering, and be installed so as not to interfere with the loading of men or equipment. Access to information could be gained by downloading at regular intervals, as dictated by battery life and scheduled to coincide with shaft inspections when there would be no need to monitor rate of pressure change.

CONCLUSIONs and RECOMMENDATIONS

Despite the considerable depths being contemplated for ultra-deep mining, the relatively low density of air, together with incremental hoisting procedures, would tend to minimise the risk of barotrauma while travelling between surface and underground workings, except under certain circumstances considered above. In accordance with the findings of this investigation, it is concluded that the potential risk of barotrauma can be limited to acceptably low levels, through a multi-faceted risk management approach based on:

• Workers' understanding of the mechanisms by which barotrauma can occur, and their knowledge of behaviour-based preventative measures

• Prospective identification of susceptible individuals and management of their predisposing conditions

• Prophylactic seasonal use of vaccinations for influenza, use of antibiotics to counter secondary infection where required, and medically supervised treatment of symptoms conducive to barotrauma

• For the most susceptible individuals, a specialist evaluation of fitness for work in ultra-deep mines, considering surgical intervention where warranted, followed by a participative decision on allocation

• Implementation of vertical conveyance procedures and suitable devices to control and monitor the rate of barometric pressure changes

• A thorough assessment by medical personnel of the potential for barotrauma during evacuation of injured workers where traumatic chest injuries are involved, and the appropriate revision of current procedures to control such risks where indicated

REFERENCES

Anon, 1994, “Warnings from the FDA,” Medical Sciences Bulletin, Pharmaceutical Information Associates, Ltd., Nov., pp. 7-11.

Bartlett, C.E. and HMS (Himalayan Medical Supplies), 2000, Victoria and Repton, Australia, http://www.bartlett.net.au/pac.html.

CMG (Chinook Medical Gear), 1999, Edwards, CO, USA, http://www.chinookmed.com.

Garges, L.M., 1985, “Maxillary sinus barotrauma: a case report and review,” Aviation, Space and Environmental Medicine, 56: pp. 796-797.

Rottman, K., 1981, “Barodontalgia: a Dental Consideration for the Scuba Diving Patient,” Quintessence International, No. 9, pp. 979-982.

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