Large numbers of people work at high altitudes, particularly in the cities and villages of the South American Andes and the Tibetan plateau. The majority of these people are highlanders who have lived in the area for many years and perhaps several generations. Much of the work is agricultural in nature—for example, tending domesticated animals.
However, the focus of this article is different. Recently there has been a large increase in commercial activities at altitudes of 3,500 to 6,000 m. Examples include mines in Chile and Peru at altitudes of around 4,500 m. Some of these mines are very large, employing over 1,000 workers. Another example is the telescope facility at Mauna Kea, Hawaii, at an altitude of 4,200 m.
Traditionally, the high mines in the South American Andes, some of which date back to the Spanish colonial period, have been worked by indigenous people who have been at high altitude for generations. Recently however, increasing use is being made of workers from sea level. There are several reasons for this change. One is that there are not enough people in these remote areas to operate the mines. An equally important reason is that as the mines become increasingly automated, skilled people are required to operate large digging machines, loaders and trucks, and local people may not have the necessary skills. A third reason is the economics of developing these mines. Whereas previously whole towns were set up in the vicinity of the mine to accommodate the workers’ families, and necessary ancillary facilities such as schools and hospitals, it is now seen to be preferable to have the families live at sea level, and have the workers commute to the mines. This is not purely an economic issue. The quality of life at an altitude of 4,500 m is less than at lower altitudes (e.g., children grow more slowly). Therefore the decision to have the families remain at sea level while the workers commute to high altitude has a sound socio-economic basis.
The situation where a workforce moves from sea level to altitudes of approximately 4,500 m raises many medical issues, many of which are poorly understood at the present time. Certainly most people who travel from sea level to an altitude of 4,500 m develop some symptoms of acute mountain sickness initially. Tolerance to the altitude often improves after the first two or three days. However, the severe hypoxia of these altitudes has a number of deleterious effects on the body. Maximal work capacity is decreased, and people fatigue more rapidly. Mental efficiency is reduced and many people find it is much more difficult to concentrate. Sleep quality is often poor, with frequent arousals and periodic breathing (the breathing waxes and wanes three or four times every minute) with the result that that the arterial PO2 falls to low levels following the periods of apnoea or reduced breathing.
Tolerance to high altitude varies greatly between individuals, and it is often very difficult to predict who is going to be intolerant of high altitude. A substantial number of people who would like to work at an altitude of 4,500 m find that they are unable to do so, or that the quality life is so poor that they refuse to remain at that altitude. Topics such as the selection of workers who are likely to tolerate high altitude, and the scheduling of their work between high altitude and the period with their families at sea level, are relatively new and not well understood.
In addition to the usual type of pre-employment examination, special attention should be given to the cardio-pulmonary system, because working at high altitude makes great demands on the respiratory and cardiovascular systems. Medical conditions such as early chronic obstructive pulmonary disease and asthma will be much more disabling at high altitude because of the high levels of ventilation, and should be specifically looked for. A heavy cigarette smoker with symptoms of early bronchitis is likely to have difficulty tolerating high altitude. Forced spirometry should be measured in addition to the usual chest examination including chest radiograph. If possible, an exercise test should be carried out because any exercise intolerance will be exaggerated at high altitude.
The cardiovascular system should be carefully examined, including an exercise electrocardiogram if that is feasible. Blood counts should be made to exclude workers with unusual degrees of anaemia or polycythaemia.
Living at high altitude increases the psychological stress in many people, and a careful history should be taken to exclude prospective workers with previous behavioural problems. Many modern mines at high altitude are dry (no alcohol permitted). Gastro-intestinal symptoms are common in some people at high altitude, and workers who have a history of dyspepsia may do poorly.
Selection of Workers to Tolerate High Altitude
In addition to excluding workers with lung or heart disease who are likely to do poorly at high altitude, it would be very valuable if tests could be carried out to determine who is likely to tolerate altitude well. Unfortunately little is known at the present time about predictors of tolerance to high altitude, though considerable work is being done on this at the present time.
The best predictor of tolerance to high altitude is probably previous experience at high altitude. If someone has been able to work at an altitude of 4,500 m for several weeks without appreciable problems, it is very likely that he or she will be able to do this again. By the same token, somebody who tried to work at high altitude and found that he or she could not tolerate it, is very likely to have the same problem next time. Therefore in selecting workers, a great deal of emphasis should be placed on successful previous employment at high altitude. However, clearly this criterion cannot be used for all workers because otherwise no new people would enter the high-altitude working pool.
Another possible predictor is the magnitude of the ventilatory response to hypoxia. This can be measured at sea level by giving the prospective worker a low concentration of oxygen to breathe and measuring the increase in ventilation. There is some evidence that people who have a relatively weak hypoxic ventilatory response tolerate high altitude poorly. For example, Schoene (1982) showed that 14 high-altitude climbers had significantly higher hypoxic ventilatory responses than ten controls. Further measurements were made on the 1981 American Medical Research Expedition to Everest, where it was shown that the hypoxic ventilatory response measured before and on the Expedition correlated well with performance high on the mountain (Schoene, Lahiri and Hackett 1984). Masuyama, Kimura and Sugita (1986) reported that five climbers who reached 8,000 m in Kanchenjunga had a higher hypoxic ventilatory response than five climbers who did not.
However, this correlation is by no means universal. In a prospective study of 128 climbers going to high altitudes, a measure of hypoxic ventilatory response did not correlate with the height reached, whereas a measurement of maximal oxygen uptake at sea level did correlate (Richalet, Kerome and Bersch 1988). This study also suggested that the heart rate response to acute hypoxia might be a useful predictor of performance at high altitude. There have been other studies showing a poor correlation between hypoxic ventilatory response and performance at extreme altitude (Ward, Milledge and West 1995).
The problem with many of these studies is that the results are chiefly applicable to much higher altitudes than of interest here. Also there are many examples of climbers with moderate values of hypoxic ventilatory response who do well at high altitude. Nevertheless, an abnormally low hypoxic ventilatory response is probably a risk factor for tolerating even medium altitudes such as 4,500 m.
One way of measuring the hypoxic ventilatory response at sea level is to have the subject rebreathe into a bag which is initially filled with 24% oxygen, 7% carbon dioxide, and the balance nitrogen. During rebreathing the PCO2 is monitored and held constant by means of a variable bypass and carbon dioxide absorber. Rebreathing can be continued until the inspired PO2 falls to about 40 mmHg (5.3 kPa). The arterial oxygen saturation is measured continually with a pulse oximeter, and the ventilation plotted against the saturation (Rebuck and Campbell 1974). Another way of measuring the hypoxic ventilatory response is to determine the inspiratory pressure during a brief period of airway occlusion while the subject is breathing a low-oxygen mixture (Whitelaw, Derenne and Milic-Emili 1975).
Another possible predictor of tolerance to high altitude is work capacity during acute hypoxia at sea level. The rationale here is that someone who is not able to tolerate acute hypoxia is more likely to be intolerant of chronic hypoxia. There is little evidence for or against this hypothesis. Soviet physiologists used tolerance to acute hypoxia as one of the criteria for selection of climbers for their successful 1982 Everest expedition (Gazenko 1987). On the other hand, the changes that occur with acclimatization are so profound that it would not be surprising if exercise performance during acute hypoxia were poorly correlated with the ability to work during chronic hypoxia.
Another possible predictor is the increase in pulmonary artery pressure during acute hypoxia at sea level. This can be measured non-invasively in many people by Doppler ultrasound. The main rationale for this test is the known correlation between the development of high-altitude pulmonary oedema and the degree of hypoxic pulmonary vasoconstriction (Ward, Milledge and West 1995). However, since high-altitude pulmonary oedema is uncommon in people working at an altitude of 4,500 m, the practical value of this test is questionable.
The only way to determine whether these tests for the selection of workers have practical value is a prospective study where the results of the tests done at sea level are correlated with subsequent assessment of tolerance to high altitude. This raises the question of how high-altitude tolerance will be measured. The usual way of doing this is by questionnaires such as the Lake Louise questionnaire (Hackett and Oelz 1992). However, questionnaires may be unreliable in this population because workers perceive that if they admit to altitude intolerance, they might lose their jobs. It is true that there are objective measures of altitude intolerance such as quitting work, rales in the lungs as indications of subclinical pulmonary oedema, and mild ataxia as an indication of subclinical high-altitude cerebral oedema. However, these features will be seen only in people with severe altitude intolerance, and a prospective study based solely on such measurements would be very insensitive.
It should be emphasized that the value of these possible tests for determining tolerance to working at high altitude has not been established. However, the economic implications of taking on a substantial number of workers who are unable to perform satisfactorily at high altitude are such that it would be very valuable to have useful predictors. Studies are presently underway to determine whether some of these predictors are valuable and feasible. Measurements such as the hypoxic ventilatory response to hypoxia, and work capacity during acute hypoxia at sea level, are not particularly difficult. However, they need to be done by a professional laboratory, and the cost of these investigations can be justified only if the predictive value of the measurements is substantial.
Scheduling between High Altitude and Sea Level
Again, this article is addressed to the specific problems which occur when commercial activities such as mines at altitudes of about 4,500 m employ workers who commute from sea level where their families live. Scheduling is obviously not an issue where people live permanently at high altitude.
Designing the optimal schedule for moving between high altitude and sea level is a challenging problem, and as yet there is little scientific basis for the schedules that have been employed so far. These have been based mainly on social factors such as how long the workers are willing to spend at high altitude before seeing their families again.
The main medical rationale for spending several days at a time at high altitude is the advantage gained from acclimatization. Many people who develop symptoms of acute mountain sickness after going to high altitude feel much better after two to four days. Therefore rapid acclimatization is occurring over this period. In addition it is known that the ventilatory response to hypoxia takes seven to ten days to reach a steady state (Lahiri 1972; Dempsey and Forster 1982). This increase in ventilation is one of the most important features of the acclimatization process, and therefore it is reasonable to recommend that the working period at high altitude be at least ten days.
Other features of high-altitude acclimatization probably take much longer to develop. One example is polycythaemia, which takes several weeks to reach a steady state. However, it should be added that the physiological value of polycythaemia is much less certain than was thought at one time. Indeed, Winslow and Monge (1987) have shown that the severe degrees of polycythaemia which are sometimes seen in permanent dwellers at altitudes of about 4,500 m are counterproductive in that work capacity can sometimes be increased if the haematocrit is lowered by removing blood over several weeks.
Another important issue is the rate of deacclimatization. Ideally the workers should not lose all the acclimatization that they have developed at high altitude during their period with their families at sea level. Unfortunately, there has been little work on the rate of deacclimatization, although some measurements suggest that the rate of change of the ventilatory response during deacclimatization is slower than during acclimatization (Lahiri 1972).
Another practical issue is the time required to move workers from sea level to high altitude and back again. In a new mine at Collahuasi in north Chile, it takes only a few hours to reach the mine by bus from the coastal town of Iquique, where most of the families are expected to live. However, if the worker resides in Santiago, the trip could take over a day. Under these circumstances, a short working period of three or four days at high altitude would clearly be inefficient because of the time wasted in travelling.
Social factors also play a critical role in any scheduling that involves time away from the family. Even if there are medical and physiological reasons why an acclimatization period of 14 days is optimal, the fact that the workers are unwilling to leave their families for more than seven or ten days may be an overriding factor. Experience so far shows that a schedule of seven days at high altitude followed by seven days at sea level, or ten days at high altitude followed by the same period at sea level are probably the most acceptable schedules.
Note that with this type of schedule, the worker never fully acclimatizes to high altitude, nor fully deacclimatizes while at sea level. He therefore spends his time oscillating between the two extremes, never receiving the full benefit of either state. In addition, some workers complain of extreme tiredness when they return to sea level, and spend the first two or three days recovering. Possibly this is related to the poor quality of sleep which is often a feature of living at high altitude. These problems highlight our ignorance of the factors that determine the best schedules, and more work is clearly needed in this area.
Whatever schedule is used, it is highly advantageous if the workers can sleep at a lower altitude than the workplace. Naturally whether this is feasible depends on the topography of the region. A lower altitude for sleeping is not feasible if it takes several hours to reach it because this cuts too much off the working day. However, if there is a location several hundred metres lower which can be reached within, say, one hour, setting up sleeping quarters at this lower altitude will improve sleep quality, workers’ comfort and sense of well-being, and productivity.
Oxygen Enrichment of Room Air to Reduce the Hypoxia of High Altitude
The deleterious effects of high altitude are caused by the low partial pressure of oxygen in the air. In turn, this results from the fact that while the oxygen concentration is the same as at sea level, the barometric pressure is low. Unfortunately there is little that can be done at high altitude to counter this “climatic aggression”, as it was dubbed by Carlos Monge, the father of high-altitude medicine in Peru (Monge 1948).
One possibility is to increase the barometric pressure in a small area, and this is the principle of the Gamow bag, which is sometimes used for the emergency treatment of mountain sickness. However, pressurizing large spaces such as rooms is difficult from a technical point of view, and there are also medical problems associated with entering and leaving a room with increased pressure. An example is middle ear discomfort if the Eustachian tube is blocked.
The alternative is to raise the oxygen concentration in some parts of the work facility, and this is a relatively new development that shows great promise (West 1995). As pointed out earlier, even after a period of acclimatization of seven to ten days at an altitude of 4,500 m, severe hypoxia continues to reduce work capacity, mental efficiency and sleep quality. It would therefore be highly advantageous to reduce the degree of hypoxia in some parts of the work facility if that were feasible.
This can be done by adding oxygen to the normal air ventilation of some rooms. The value of relatively minor degrees of oxygen enrichment of the room air is remarkable. It has been shown that every 1% increase in oxygen concentration (for example from 21 to 22%) reduces the equivalent altitude by 300 m. The equivalent altitude is that which has the same inspired PO2 during air breathing as in the oxygen-enriched room. Thus at an altitude of 4,500 m, raising the oxygen concentration of a room from 21 to 26% would reduce the equivalent altitude by 1,500 m. The result would be an equivalent altitude of 3,000 m, which is easily tolerated. The oxygen would be added to the normal room ventilation and therefore would be part of the air conditioning. We all expect that a room will provide a comfortable temperature and humidity. Control of the oxygen concentration can be regarded as a further logical step in humanity’s control of our environment.
Oxygen enrichment has become feasible because of the introduction of relatively inexpensive equipment for providing large quantities of nearly pure oxygen. The most promising is the oxygen concentrator that uses a molecular sieve. Such a device preferentially adsorbs nitrogen and thus produces an oxygen-enriched gas from air. It is difficult to produce pure oxygen with this type of concentrator, but large amounts of 90% oxygen in nitrogen are readily available, and these are just as useful for this application. These devices can work continuously. In practice, two molecular sieves are used in an alternating fashion, and one is purged while the other is actively adsorbing nitrogen. The only requirement is electrical power, which is normally in abundant supply at a modern mine. As a rough indication of the cost of oxygen enrichment, a small commercial device can be bought off the shelf, and this produces 300 litres per hour of 90% oxygen. It was developed to produce oxygen for treating patients with lung disease in their homes. The device has a power requirement of 350 watts and the initial cost is about US$2,000. Such a machine is sufficient to raise the oxygen concentration in a room by 3% for one person at a minimal though acceptable level of room ventilation. Very large oxygen concentrators are also available, and they are used in the paper pulp industry. It is also possible that liquid oxygen might be economical under some circumstances.
There are several areas in a mine, for example, where oxygen enrichment might be considered. One would be the director’s office or conference room, where important decisions are being made. For example, if there is a crisis in the mine such as a serious accident, such a facility would probably result in clearer thinking than the normal hypoxic environment. There is good evidence that an altitude of 4,500 m impairs brain function (Ward, Milledge and West 1995). Another place where oxygen enrichment would be beneficial is a laboratory where quality control measurements are being carried out. A further possibility is oxygen enrichment of sleeping quarters to improve sleep quality. Double blind trials of the effectiveness of oxygen enrichment at altitudes of about 4,500 m would be easy to design and should be carried out as soon as possible.
Possible complications of oxygen enrichment should be considered. Increased fire hazard is one issue that has been raised. However, increasing the oxygen concentration by 5% at an altitude of 4,500 m produces an atmosphere which has a lower flammability than air at sea level (West 1996). It should be borne in mind that although oxygen enrichment increases the PO2, this is still much lower than the sea-level value. Flammability of an atmosphere depends on two variables (Roth 1964):
- the partial pressure of oxygen, which is much lower in the enriched air at high altitude than at sea level
- the quenching effect of the inert components (i.e., nitrogen) of the atmosphere.
This quenching is slightly reduced at high altitude, but the net effect is still a lower flammability. Pure or nearly pure oxygen is dangerous, of course, and the normal precautions should be taken in piping the oxygen from the oxygen concentrator to the ventilation ducting.
Loss of acclimatization to high altitude is sometimes cited as a disadvantage of oxygen enrichment. However, there is no basic difference between entering a room with an oxygen-enriched atmosphere, and descending to a lower altitude. Everybody would sleep at a lower altitude if they could, and therefore this is hardly an argument against using oxygen enrichment. It is true that frequent exposure to a lower altitude will result in less acclimatization to the higher altitude, other things being equal. However, the ultimate objective is effective working at the high altitude of the mine, and this can presumably be enhanced using oxygen enrichment.
It is sometimes suggested that altering the atmosphere in this way might increase the legal liability of the facility if some kind of hypoxia-related illness developed. Actually, the opposite view seems more reasonable. It is possible that a worker who develops, say, a myocardial infarction while working at high altitude could claim that the altitude was a contributing factor. Any procedure which reduces the hypoxic stress makes altitude-induced illnesses less likely.
The various types of high-altitude sickness, including acute mountain sickness, high-altitude pulmonary oedema and high-altitude cerebral oedema, were discussed earlier in this chapter. Little needs to be added in the context of work at high altitude.
Anyone who develops a high-altitude illness should be allowed to rest. This may be sufficient for conditions such as acute mountain sickness. Oxygen should be given by mask if this is available. However, if the patient does not improve, or deteriorates, descent is by far the best treatment. Usually this is easily done in a large commercial facility, because transportation is always available. All the high-altitude-related illnesses usually respond rapidly to removal to lower altitude.
There may be a place in a commercial facility for a small pressurized container in which the patient can be placed, and the equivalent altitude reduced by pumping in air. In the field, this is commonly done using a strong bag. One design is known as the Gamow bag, after its inventor. However, the main advantage of the bag is its portability, and since this feature is not really essential in a commercial facility, it would probably be better to use a larger, rigid tank. This should be big enough for an attendant to be inside the facility with the patient. Of course adequate ventilation of such a container is essential. Interestingly, there is anecdotal evidence that raising the atmospheric pressure in this way is sometimes more efficacious in the treatment of high-altitude illness than giving the patient a high concentration of oxygen. It is not clear why this should be so.
Acute mountain sickness
This is usually self-limiting and the patient feels much better after a day or two. The incidence of acute mountain sickness can be reduced by taking acetazolamide (Diamox), one or two 250 mg tablets per day. These can be started before reaching high altitude or can be taken when symptoms develop. Even people with mild symptoms find that half a tablet at night often improves the quality of sleep. Aspirin or paracetamol is useful for headache. Severe acute mountain sickness can be treated with dexamethasone, 8 mg initially, followed by 4 mg every six hours. However, descent is by far the best treatment if the condition is severe.
High-altitude pulmonary oedema
This is a potentially serious complication of mountain sickness and must be treated. Again the best therapy is descent. While awaiting evacuation, or if evacuation is not possible, give oxygen or place in a high-pressure chamber. Nifedipine (a calcium channel blocker) should be given. The dose is 10 mg sublingually followed by 20 mg slow release. This results in a fall in pulmonary artery pressure and is often very effective. However, the patient should be taken down to a lower altitude.
High-altitude cerebral oedema
This is potentially a very serious complication and is an indication for immediate descent. While awaiting evacuation, or if evacuation is not possible, give oxygen or place in an increased pressure environment. Dexamethasone should be given, 8 mg initially, followed by 4 mg every six hours.
As indicated earlier, people who develop severe acute mountain sickness, high-altitude pulmonary oedema or high-altitude cerebral oedema are likely to have a recurrence if they return to high altitude. Therefore if a worker develops any of these conditions, attempts should be made to find employment at a lower altitude.