High-altitude medical and operations problems and solutions

Peter J. Napier^a and John B. West^b

aNational Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801

b Department of Medicine, School of Medicine, UCSD

The 5000 m altitude of the potential site for the Millimeter Array (MMA) in Northern Chile is so high that high- altitude problems for both the staff and equipment must be considered and included in planning for the facility. The very good accessability of the site, only one hour's drive from the nearest town at altitude 2440 m, makes it possible for MMA workers to sleep and perform much of their work at low altitude. Workers on the site will have 11% less oxygen available than workers at Mauna Kea Observatory. It is expected that the mental abilities and ability to do hard physical labor of workers on the high site will be reduced by 10% to 30% compared to sea-level. In-doors working areas on the MMA site will have their atmospheres oxygen enriched to provide an effective working altitude of 3500 m where loss of mental ability should be small. Tests of oxygen enrichment at high-altitude Chilean mines and at the University of California White Mountain Research Station show that it is feasible and economic. Problems of equipment operation at 5000 m altitude are expected to be manageable.

Keywords: high altitude, medical, physiology, oxygen

The Millimeter Array (MMA) is a major new radio telescope for millimeter and sub-millimeter wavelengths, the design and development phase of which is just beginning. The MMA is currently planned to be approximately 36 ten meter diameter radio telescopes operating as a synthesis array with maximum interferometer baselines of up to 10 km. The antennas will be transportable into a number of different array configurations of various diameters. The maximum frequency of operation of the instrument will be 950 GHz. This very high operating frequency requires that the array be placed at high altitude to reduce the attenuation of the short wavelength signals by the water vapor in the earth's atmosphere. Since the scale height of water vapor is about 2 km, placing the array at an altitude of several kilometers can significantly reduce signal attenuation. One of the sites being considered for the MMA is at an altitude of 5020 m in Northern Chile. This is so high so that considerations of the high-altitude problems for both the staff and equipment must be included in the planning. This paper provides a summary and update of an earlier study¹ of these problems. We first make an estimate of the magnitude of the problems expected at this altitude and then present the currently planned scenario which makes the high-altitude operation feasible. Details of the radio astronomical quality of the site² and other aspects of the MMA can be found in other papers^3,4,5 at this conference.

The location of the potential MMA site in Northern Chile is shown in Figure 1. The location of the site is longitude 67^o45' W, latitude 23^o01' S. The site is 50 km southeast of the small town of San Pedro de Atacama (pop 905, altitude 2440 m). The nearest large town to San Pedro is Calama (pop 90,000, altitude 2300 m) at a distance from San Pedro of 92 km on a paved road. San Pedro is 320 km from the coastal city of Antofagasta (pop 200,000). An important feature of the site, which makes it practical to consider using its high altitude for a major scientific instrument, is its easy access from San Pedro. The road from San Pedro to the site is well graded, as can be seen from Figure 2 which shows the altitude as a function of distance along the road from the planned Operations Support Base (OSB) in San Pedro. The first 35 km of the route are on the paved Jama Pass road which is an international highway leading to Bolivia and Argentina. The final 15 km are on an easily driven dirt road. The road from San Pedro to the MMA site can be driven in about 1 hour.

The site is a volcanic desert plateau on the Andean Altiplano. The site surface is mainly volcanic gravel covering a mixture of flat plain cut by shallow arroyos and gently rolling hills. The site is surrounded by old volcanic peaks with altitudes of about 5600 m. One of these, Co. Toco, can be driven to an altitude of about 5500 m on a mining road. Considering its altitude, the environmental conditions on the site are surprisingly benign. This is in large part due to the low annual precipitation resulting from the site's location on the edge of the Atacama Desert. Annual precipitation is about 10 cm, much of which falls in the summer months of January and February. Winter snows are sporadic and deep snow accumulations are not expected to be a problem. Anecdotal evidence suggests that the site does not experience severe ice storms of the kind that occur occasionally on the summit of Mauna Kea. The annual median temperature is -4 ^o C and the annual median wind velocity is 6 m/s.

The chief concerns with respect to the staff of the MMA are the medical and physiological problems which they might experience due to the reduced oxygen in the atmosphere at 5000 m altitude. Discussions of the medical and physiological problems associated with high altitude are available in the non-medical literature^6,7,8,9. In the recent medical literature a good review of the subject is available¹⁰ , as are detailed treatments^11,12. In addition, a comprehensive bibliography of high-altitude medicine references, containing more than 5600 entries, is available on the WWW¹³.

A critical quantity that determines the ability of workers to function on a high-altitude site is the partial pressure of oxygen, Po₂,

of the air inside the lung. PIo₂, the Po₂ of the moist inspired gas in the lung can be calculated as¹¹:

PIo₂ = 0.2094 (P_B-47) (1)

where P_B is the barometric pressure in Torr and 47 Torr is the saturated water vapor pressure at the temperature of the human body. Using a measured value of 419 Torr (55 % of sea-level) for P_B on the MMA site, equation (1) shows that PIo₂ is 78 Torr, or 52% of its sea-level value. As a comparison with a successful, existing high-altitude observatory, a similar calculation for the Mauna Kea Observatory (MKO) site at 4200 m altitude using a P_B value of 465 Torr (61% of sea-level) gives a PIo₂ value of 88 Torr. So workers on the MMA site will have 11% less oxygen available than MKO workers.

The hypoxia experienced by the MMA workers will give rise to two kinds of problems: a reduction in work ability, both mental and physical work and the possibility of developing one of the high altitude illnesses.

Mental performance: The hypoxia of altitude reduces a person's mental abilities because of the reduced oxygen supply to the brain, the effect often being described as similar to slight intoxication. The technical complexity of the MMA makes this effect of particular concern. There are a number of studies of the severity of the effect although researchers point out that it is difficult to obtain reliable results because of differences between individuals and because of the tendency of subjects to "try harder" during tests at altitude. Although anecdotal evidence of reduced mental ability due to the altitude at MKO abounds, limited measurements on UKIRT shift workers⁷ showed little quantitative reduction in ability. Numeric memory was reduced to 88% of its sea-level value on the first day of work at altitude with improvement over the next few days so that by the fifth day there was no significant difference from sea-level performance. Measurements of motor speed and information recording showed no statistically significant difference from sea-level performance on any day. McFarland ¹⁴ has compiled the results of several studies at various altitudes and uses these to quantify mental ability loss as a function of altitude. Table 1 below gives numbers taken from his detailed curves. The results are for unacclimatized individuals and are therefore presumably applicable to MMA workers at the beginning of their work week. The numbers for 3500 m are included to indicate the expected performance of workers in MMA buildings containing a 26 % oxygen atmosphere. Visual sensitivity is a measure of the lowest light level detectable to the eye and its reduction at altitude is caused by brain hypoxia. It must be emphasized that the error bars on these data are large, but they do serve to give an idea of how rapidly performance degrades with altitude.

Jimenez¹⁵ has tested 26 professional and technical level acclimatized Chilean mining workers who were working on approximately a week-on week-off basis at 4500 m. In the area of general cognitive aptitude, spacial aptitude and mathematical reasoning were reduced by 13% and 19% respectively whilst numeric aptitude and verbal aptitude did not give statistically significant results. In the area of mental performance, attention ability was reduced by 11% whilst concentration and memory did not give statistically significant results.

From the limited available data described above it seems reasonable to assume that the kinds of mental skills that workers on the MMA site will need to solve complex technical problems will be reduced by approximately 10% to 30%. It is common for workers to deny that any such reduction is in fact occurring so it will be important to train workers to accept its presence and prepare for it by increased use of plans and checklists prepared at low altitude and by extensive checking of work by partners and supervisors on the MMA site. The common experience of technical workers at high altitude, such as at MKO, is that accurate work can be done, particularly if detailed protocols are written out, but it takes longer and more concentration is required. Problem solving is particularly difficult and this is where errors are most likely. The principal way in which it is planned to respond to the problem of reduced mental ability is to provide an oxygen enriched atmosphere in those situations where it is required (see Section 5 below). As shown by the 3500 m entries in Table 1, workers breathing an atmosphere in which the oxygen concentration is increased from its natural 21% up to 26% should suffer very little mental impairment.

Physical Work: The extent to which the human body can do hard aerobic work is determined by the maximum rate at which the body can take up oxygen. This maximal oxygen consumption drops significantly at altitude with little improvement with acclimatization. A work ability reduction of 25 % on the MMA site can be expected based on measured test data (see Figures 10.7a and 10.7b of ¹¹ ). This compares with a 20% loss expected at MKO. It is clear that the ability of workers to perform hard labor will be significantly reduced at the MMA site and this reduction in efficiency must be allowed for. In rare instances it might be necessary to supply supplemental oxygen to workers outside the oxygen-enriched rooms and this could be done using a lightweight, shoulder mounted tank feeding an unobtrusive nasal cannula.

There are three principal high-altitude illnesses which could be experienced by MMA workers: Acute Mountain Sickness (AMS), High-Altitude Pulmonary Edema (HAPE) and High-Altitude Cerebral Edema (HACE).

AMS: This is the most common, and fortunately the least dangerous of the high-altitude illnesses. It is characterized by one or more of the following symptoms: headache, fatigue, insomnia, loss of appetite, dizziness, palpitations, nausea and shortness of breath on exertion. Symptoms begin a few hours after accent and typically disappear after a day or two when some acclimatization to the altitude has been achieved. Reports of its occurrence as a function of altitude¹⁰ predict that about 50% of workers will experience some symptoms at 5000 m. Tests made at UKIRT⁷ at the MKO showed that 80% of workers sleeping at 3000 m and working at 4200 m experienced some symptoms on the first day of work. By the fifth day of work only 40% of workers experienced some symptoms. No treatment for AMS is necessary for most people who will experience little more than a mild headache. Severe cases should be taken to lower altitude immediately because severe AMS can progress to HAPE or HACE. Acetazolamide (Diamox) is effective in reducing the incidence of AMS but has several side effects and should not be used routinely. The headache of mild to moderate AMS frequently responds well to aspirin or acetaminophen (Tylenol or Excedrin). It can be expected that a small fraction of workers will not adapt well to the 5000 m altitude and these employees must be used for tasks not requiring ascent to the high site.

HAPE: This a dangerous illness involving the accumulation of fluid in the lungs. The mechanism of HAPE is probably a large increase in pulmonary artery pressure which damages the walls of some capillaries. It occurs much less frequently than AMS and studies of its incidence¹⁰ suggest that occurrence in the MMA situation might be in the range .01% to .1%. The primary symptom is extreme shortness of breath and subjects may cough up pink frothy fluid.The primary treatment is to immediately take the patient to lower altitude (in the case of the MMA, down to San Pedro) while administering oxygen. The condition develops sufficiently slowly that there will be time to take the patient off the site before the situation becomes critical. To ensure that evacuation is always possible a "two man rule" and a "two vehicle rule" should be observed on the site at all times.

HACE: This is a dangerous illness involving increased pressure on the brain. It is rare and incidence can be expected to be much less than HAPE. It often begins like AMS with headache, nausea, loss of appetite, vomiting and photophobia, but progresses to ataxia (loss of muscle control e.g., difficulty with walking), irrationality, hallucinations and clouding of consciousness. The primary treatment is to take the patient to lower altitude as quickly as possible whilst administering oxygen. The condition develops sufficiently slowly so that there will be time to do this.

Since the site was first visited by NRAO in October, 1994, there have been in excess of 70 different individuals on the 5000 m site, or on the adjacent 5200 m site on Co. Toco, many of them having made several visits. These people have been installing, maintaining or visiting the NRAO atmospheric testing equipment or one of the other small astronomical projects starting to use the area. The visits range from a few hours to many work days in succession, sleeping in San Pedro at night. Reports of mild headaches and tiredness at the end of a work day are common. Anecdotal evidence of reduced mental ability exists, but the general reaction of most visitors is that the problems are much less than they expected. We are aware of only one case in which a visitor was unable to function effectively on the site. In this case a worker, who had previously successfully worked there for many days, had to be taken off the Co. Toco site with symptoms of HAPE. A subsequent investigation of the case suggests that the symptoms were initiated by, and recovery was slowed by, a viral chest infection which was exacerbated by the altitude. Blood arterial oxygen saturation measurements on two NRAO workers who spent 12 workdays on the 5000 m site gave first quartile, median and third quartile saturation values of 79%,82%,83% for a 39 year old male and 79%, 82%,86% for a 21 year old male. Saturation values were recorded at the beginning and end of each workday. These values are within the expected range for the altitude¹⁴.

Concerns with respect to the equipment on the MMA site include designing for the mountain climate and equipment performance losses due to the reduced cooling of the low pressure air or the effect of reduced oxygen on internal combustion machinery. As mentioned in Section 2 above, the climate is surprisingly benign considering the high altitude and is not expected to present major design challenges. No detailed estimates of equipment performance loss have yet been made for 5000 m altitude but the order of magnitude of the loss can be estimated from published data¹⁵ for somewhat lower altitudes shown in Table 2.

Table 2. Equipment size increases to achieve sea-level output (%), (from Jimenez ¹⁵).

The most challenging equipment problem, resulting from the reduced barometric pressure, is expected to be the issue of cooling the large digital correlator which will dissipate in excess of 100 kw. One solution would be to place the correlator in a pressurized room, but structural problems and the inconvenience of the airlock that would be required on the room make this approach unattractive. There would also be concerns about how quickly personnel could comfortably transit the airlock⁶. It currently appears feasible to provide a sufficiently high airflow through the correlator racks so that the heat can be extracted and there is an ample supply of cool air on the site.

Since human problems of performance and illness at high altitude result from the reduced partial pressure of oxygen of moist inspired gas, an attractive method for ameliorating the problems for high-altitude workers who can perform their tasks in-doors is to increase room Po₂ by increasing the concentration of oxygen in the room. The fundamentals of oxygen enrichment as a means of improving the efficiency of high altitude workers were summarized by Cudaback⁶ and discussed in more detail by West¹⁶. The key point is shown in Figure 3, taken from West¹⁶, which shows the equivalent altitude (as defined by the partial pressure of oxygen of inspired gas) as a function of the fractional oxygen enrichment. Figure 3 shows that at 5000 m, for each percentage point increase in oxygen concentration (increasing from 21% to 22% oxygen concentration is an increase of one percentage point), the effective altitude experienced by the body decreases by 300 m. Thus, if the oxygen concentration in the buildings on the MMA site is increased from its natural 21% up to 26%, the altitude effects on the workers in the buildings will be the effects expected for 3500 m, which should be acceptable. The choice of 26% oxygen is a compromise between competing requirements. Improved performance could be obtained with a higher concentration. However, a higher concentration would reduce the degree of acclimatization achieved by indoors workers who must also work outdoors and it would increase the cost and potential fire risk (see Section 5.2).

Figure 3: Reduction of equivalent altitude by oxygen enrichment. The equivalent altitude is that which has the same Po₂ of moist inspired gas as in air at that altitude. One percent oxygen enrichment means an increase from 21% to 22% oxygen. From West¹⁶. Curves are drawn for sites at 2, 3, 4, 5, and 6, km altitude, respectively.

5.1 Current experience with oxygen enrichment

To date there have been four studies of the effects of oxygen enrichment at high altitude. The first was a feasibility study at the Collahuasi mine in north Chile (4500-4600 m) where the feasibility of maintaining an oxygen-enriched atmosphere in two dormitory modules was demonstrated. The oxygen was provided from liquid oxygen tanks and it was shown that no special construction techniques were necessary to maintain an oxygen concentration of approximately 25%.

Subsequently, a study of the effects of oxygen enrichment on sleep quality was carried out at El Tambo mine, north Chile, altitude 4200 m, barometric pressure 464 torr. Sixteen dormitory rooms were supplied with 24-26% oxygen using a liquid oxygen source. Subjective measurements of sleep quality indicated that this was improved. There was also a tantalizing suggestion that the arterial oxygen saturation was higher when the subjects breathed air the day after nocturnal oxygen enrichment, but the design of the experiment did not allow this to be concluded with confidence.

An extensive double blind study of the effects of nocturnal oxygen enrichment was carried out at the University of California White Mountain Research Station, altitude 3800 m, in the summer of 1997. For this study, oxygen enrichment was provided from oxygen concentrators (AirSep New Life, AirSep Corp., Amherst NY) where the oxygen-enriched air is obtained by pumping air at high pressure through sieves composed of synthetic zeolite which preferentially adsorb the nitrogen. The sieve is regenerated by pumping air through it at low pressure so that the excess nitrogen is washed out. The dimensions of the room were 3.7 x 3.1 x 2.4 m and it contained two beds. The ventilation was set at 300 l/min in accordance with the 1975 ASHRAE recommendations (see below), and the carbon dioxide concentration in the room was kept between 0.2 and 0.3% with two people in the room. The oxygen concentration was set at 24% by injecting the output of two AirSep machines into the ventilation duct. This required two AirSep concentrators, initial cost about $1300 each, and each consuming 350 watts of power.

A double blind study was carried out on 18 subjects so that neither the subjects nor the person in charge of the experiment was aware of the composition of the room air. The results showed that the quality of sleep as determined from the number of periods of apnea (cessation of breathing) and total time spent in periodic breathing with apneas was greatly reduced (p < 0.01). Subjective assessment of mountain sickness symptoms was significantly less in subjects who had slept in oxygen enrichment as opposed to ambient air. A particularly interesting result was that the arterial oxygen saturation measured in the morning (with the subject breathing ambient air) after a night of oxygen enrichment was significantly higher than after a night of sleeping in ambient air. The reasons for this are not clear at the present time but one possibility is that there is a change in the control of ventilation as a result of the reduction of sleep disordered breathing by oxygen enrichment. It is known that patients at sea level whose sleep disordered breathing is treated by continuous positive airway pressure develop a change in the control of ventilation and therefore have higher arterial Po₂ values the following day. Another possibility is that the improvement in sleep fragmentation changed the control of ventilation. Again, studies at sea level show that sleep deprived patients can develop a change in the control of ventilation which reduces their arterial Po₂ the following day.

During the studies at the White Mountain Research Station, It was also possible to demonstrate the feasibility of testing the effect of oxygen enrichment on cognitive function at the altitude of the MMA site. Although the altitude of WMRS laboratory is only 3800 m, by nitrogen enriching the air in the test room, it is possible to simulate an altitude of 5020 m. Furthermore, it is also possible to simulate the atmosphere of the MMA site with 26% oxygen enrichment¹⁷. There are plans to carry out a double blind study of oxygen enrichment at a simulated altitude of 5020 m at the White Mountain Research Station during the summer of 1998.

Another study of nocturnal oxygen enrichment was carried out at the Collahuasi mine from April to July 1997 by the Centro Ergonomico y Estudios del Trabajo of Santiago, Chile. These professional industrial psychologists were contracted by the Collahuasi mine to determine whether nocturnal oxygen enrichment improved cognitive function the following day. The study was carried out on 37 mine workers who slept at an altitude of 3850 m in ambient air or 24-25% oxygen. The subjects spent 7 days at the mine, working altitude 4500-4600 m, and during that time spent every night in either oxygen enrichment or air. An extensive series of cognitive tests were carried out in including auditory memory, physiological attention, concentration, accuracy of visual perception, physiological attention, and short term memory. Several aspects of cognitive function showed significant improvement. The largest changes were seen in auditory memory, physiological attention, and concentration, though some improvement was also seen in accuracy of visual perception and psychological attention. The conclusion of this study which was done by an experienced professional group of industrial psychologists, was that some aspects of cognitive function are apparently improved as a result of sleeping in enriched oxygen.

The conclusions from these 4 studies are that oxygen enrichment of rooms is certainly feasible at high altitude, either by using oxygen concentrators, which require only electrical power, or liquid oxygen. Quality of sleep is clearly improved by nocturnal oxygen enrichment and there is evidence that this increases the arterial Po₂ the following day, and also some measurements of cognitive function. There is every reason to believe that oxygen enrichment at the altitude of the MMA site will improve some aspects of cognitive function but a double blind test of this has not yet been carried out.

Costs of oxygen enrichment. The pilot studies carried out to date allow some estimates of the costs of oxygen enrichment. At the White Mountain Research Station, one AirSep New Life oxygen concentrator was required for each subject to maintain the room oxygen concentration at 24%. At the MMA site, the desirable concentration is 26% so a conservative estimate is that each subject requires the output of two machines. The total capital cost is therefore about $3,000, and the running costs would be those of electrical power at 700 watts.

We have not yet tested liquid oxygen at the WMRS. However, from the reported experience at El Tambo mine, altitude 4200 m, where liquid oxygen was used to oxygenate 16 dormitory rooms, we estimate that oxygen enrichment of one room could be achieved by one tank containing 189 liters of liquid oxygen (50 gallons) for a period of 16 days if the oxygenation lasted 12 hours of each day. The cost of filling such a tank with liquid oxygen in San Diego is about $200. Of course to this should be added costs of transportation and rental of the tanks. Liquid oxygen is certainly available in north Chile because large amounts are being used in the Collahuasi mine.

Ventilation rates. These are discussed at length in West¹⁶. Briefly, U.S. building codes for room ventilation are based on the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) standards. These standards have changed considerably over the last 20 years. In 1975, the minimum standard of ventilation with outside air for an office, conference room, or dormitory was 8.5 cubic meters per person per hour which corresponds to 142 l/min/person. This minimal acceptable ventilation was calculated to maintain the carbon dioxide concentration in the room below 0.24%, and was based on a CO₂ production of 0.3 l/min per person. Note that substantially higher levels of carbon dioxide can exist without people being aware of them. However ASHRAE used the concentration of carbon dioxide as a marker because studies showed that lower degrees of ventilation were unacceptable because of body odor.

In 1989, ASHRAE increased the minimum standard of ventilation to 28-36 cubic meters per person per hour (ASHRAE Standard 62-1989, 1989), that is they increased the level nearly four-fold. This was a controversial decision and was partially based on the fact that there may be smokers in the room, there may be health variations among people, and some types of room furniture cause outgassing which may be injurious. Our attitude has been that in designing a facility for use at high altitude, it can be assumed that people will not be allowed to smoke in the room, there will not be chronically sick people present, and it will be possible to choose furniture which does not cause unusual outgassing hazards. Therefore it seems reasonable to provide the 1975 ventilation standard of 142 l/min per person. Of course the lower the level of ventilation, the smaller the amount of oxygen that has to be generated.

Incidentally, ASHRAE does not have special standards for buildings at high altitude. However because the CO₂ production rate (STPD) of humans is independent of altitude, there seems to be no reason to alter the recommended level of 142 l/min.

It is likely that some areas of the MMA facility will need higher levels of ventilation, for example where soldering is being carried out. Also, personnel who are dealing with potentially toxic liquids would carry out these procedures somewhere else. These considerations should not negate the advantages of having oxygen enrichment in areas such as control rooms and offices.

Effects on workers who leave the oxygen-enriched environment. It is unlikely that workers who move from an oxygen-enriched room to ambient air at the MMA site will suffer serious problems. Recall that oxygen enrichment is simply equivalent to reducing the altitude from 5000 m to 3500 m. In other words, a worker who moves from an oxygen-enriched area to the outside is simply moving rapidly along the road shown in Figure 2 from an altitude of 3500 m to 5000 m. Experience at the Collahuasi and El Tambo mines has indicated that this is not a problem, and workers do not complain of feeling faint or dizzy. The same is true at the WMRS although admittedly the altitude is lower there. If people do feel untoward effects as a result of moving into ambient air, they can easily use a little portable oxygen or simply return to the oxygen-enriched environment.

Another issue is whether acclimatization to high altitude will be slowed by working in an oxygen-enriched atmosphere. This is likely to be the case because, in effect, the worker is at an altitude of 3500 m. However, the important issue is not whether the worker is acclimatized to 5000 m, but whether his well being and productivity are improved by oxygen enrichment. This is the bottom line and all evidence to date suggests that this will be the case. Note that MMA workers who sleep in San Pedro and work at 5020 m will not fully acclimatize to the higher altitude irrespective of whether O₂ enrichment is used.

Fire hazard: One of the concerns often raised with respect to oxygen enrichment is the possibility of increased fire hazard. The important point here is that although with 26% O₂ enrichment, the Po₂ is increased, it is still far below its sea level value and in general risk of fire with solid materials is less than at sea level. The problems of fire hazard in a low pressure, oxygen enriched atmosphere such as that proposed for the MMA buildings are addressed by the National Fire Protection Association (NFPA) codes^18,19. The NFPA¹⁹ defines an oxygen enriched atmosphere to have increased fire hazard, in the sense that it will support increased burning rates of materials, if the percentage concentration of oxygen is greater than 23.45/(TP_atmos)^0.5 , where TP_atmos is the total barometric pressure expressed as a fraction of sea-level pressure. For the MMA site TP_atmos=0.55 so if the oxygen concentration is greater than 31.6% it would exceed the NFPA threshold. Our proposed oxygen concentration of 26% is well below this threshold. Tests show that the burning rate of paper in 26% oxygen at an altitude of 5000 m is about 70% of the sea level burning rate²⁰. The discussion above is concerned principally with the fire hazard of solid materials. Common sense requires that flammable gases and liquids must not be used in an oxygen enriched atmosphere and such a rule should not create significant operational inconvenience. Volatile liquids require careful handling even without oxygen enrichment. At MKO it has been found that volatile liquids ignite more easily on top of Mauna Kea and their vapors spread more readily than at sea-level²¹. This is due in part to the fact that the flash point of volatile liquids decreases as barometric pressure is reduced²².

Based on the information reviewed above we believe that the following scenario will make construction and operation of the MMA on the 5000 m site feasible. A basic philosophy in the design of the instrument will be to minimize the amount of time that workers must spend on the high site. This is feasible because of the excellent access to the site from San Pedro where a substantial Operations Support Base (OSB) can be built. All equipment will be highly modularized so that it can be rapidly installed or replaced on the high site. During the installation phase of the project all equipment will undergo final preparation and testing at the OSB, possibly even to the extent of completely assembling, outfitting and testing the antennas there then carrying them up the 50 km access road on the antenna transporter vehicle. During the operations phase the main control room for the instrument will be in the OSB and all modules will be repaired there. It is not planned that it will be necessary for observers to go to the OSB for their observations, and the need for an observer to go to the high site should be rare.

Workers will not routinely sleep on the high site, although a few oxygen enriched bedrooms will be available for special tests or activities requiring an overnight stay. The OSB is at an altitude easily tolerated by all workers and where they can sleep soundly. The altitude of the OSB will be high enough, however, so as to provide a useful degree of acclimatization for those workers who must spend some or all of their workday on the high site. Although an altitude higher than San Pedro's would provide more acclimatization for workers on the high site, sleep quality would be poorer and this is judged to be a poor tradeoff since, in normal operation, it is expected that no more than 20% of OSB workers will need to travel to the high site. On the high site, in enclosed work areas such as engineering offices, electronics equipment rooms, transporter operator's cabin and possibly antenna receiver cabins, the atmosphere will be oxygen enriched to 26% to relieve the effects of the hypoxia. Workers whose tasks are predominantly outdoors will acclimatize sufficiently to be able to work effectively, although with reduced efficiency. This has been demonstrated by our experience to date on the site. In special cases, where full efficiency is needed outdoors, portable oxygen could be provided but is not expected to be generally necessary. It can be expected that early in their workweeks many workers will experience mild symptoms of AMS, but these can be alleviated by taking Diamox or aspirin. In the unlikely event that a worker on the high site starts to develop one of the serious high-altitude illnesses he will be immediately taken down to San Pedro. San Pedro has a small medical clinic and NRAO will enhance its capabilities. There are good hospitals in Calama.

An engineering test interferometer will be built at a convenient location in the US which will be identical to the MMA in both its hardware and software. It is planned that the design and functionality of all hardware and software will be tested on this test bed before it is required to work on the high-altitude site. By this means the duration and difficulty of the commissioning phase of the MMA on a remote high-altitude site will be minimized.

At 5000 m altitude the potential site for the MMA in Northern Chile would be the highest permanent site in the world for a major astronomical observatory. The high-altitude problems for both the staff and equipment must be considered in the planning for the instrument.

The staff will have 11% less oxygen available than is available at MKO, the current highest major observatory site. Studies published in the high-altitude medical literature predict that workers on the MMA site will experience a 10 to 30% loss of mental and physical performance compared to sea-level. The mental performance of in-doors workers will be improved by enriching the oxygen in working areas to provide an equivalent working altitude of 3500 m. Tests of oxygen enrichment of high-altitude buildings demonstrate that it is feasible and economic. The location of a small town at an elevation of 2440 m only one hour's drive from the site means that workers will not have to sleep on the site but instead can sleep at an Operations Support Base (OSB) in the town . In the unlikely event that a worker begins to develop one of the serious high-altitude illnesses, he can quickly be taken to low elevation where the condition will correct itself. The close proximity of the OSB makes it feasible to do much of the construction preparation and maintenance of the MMA at low elevation.

The efficiency of most electrical and mechanical equipment will be significantly reduced on the high site and equipment sizes will have to be increased appropriately. The major high-altitude equipment problem is likely to be the cooling of the large digital correlator. It is expected that this problem can be solved by increasing the air flow through the correlator.

The construction and operation of the MMA at 5000 m altitude appears to be feasible. The measures needed to make high-altitude operation possible are acceptable considering the exceptional astronomical quality of the site.

We acknowledge helpful discussions with Harold Beeson (JFC WSTF), David Cudaback (UC Berkeley), Larry Olsen (Keck Observatory), Rob Roach (Copenhagen Muscle Research Center), Bob Brown (NRAO) and Simon Radford (NRAO)