Does an SCBA (self-contained breathing apparatus) respirator protect wearers from oxygen-deficient atmospheres due to increasing altitude? Absolutely not. This answer probably surprises many readers, so let’s be certain we understand the question. In this situation, the respirator is a fully functional, NIOSH-approved, tight-fitting, full-facepiece, pressure-demand SCBA. This is the most protective respirator generally available for workplaces. The oxygen deficiency is due solely to a decrease in available oxygen as elevation above sea level increases. No airborne contaminants are present to displace or consume oxygen; there’s simply less oxygen to breathe when ascending to higher elevations. Given these conditions, wearers of SCBAs are not protected from oxygen deficiency. You might wonder whether a supplied air respirator (SAR) connected to a nearly unlimited supply of Grade D breathing air is protective in these conditions. Again, the answer may surprise you: absolutely not.
BACKGROUND Around the globe, regardless of where you go, the oxygen concentration in atmospheric air is 20.95 percent by volume. From sea level to Mount Everest, the percentage of oxygen and other gaseous components remains the same, notably nitrogen (78.09 percent), argon (0.92 percent), carbon dioxide (0.04 percent), and trace amounts of other gaseous components. Only water vapor varies due to changing meteorological conditions. However, for our purposes, we will ignore water vapor and assume dry air.  What does change with increasing altitude is barometric pressure. With increasing altitude there are fewer molecules within any given volume of air (that is, air becomes thinner). Consequently, the partial pressure of all gases, including oxygen partial pressure (PO2), decreases with increasing altitude. Partial pressure is the pressure a specific gas exerts within a mixture of gases and is calculated as the product of the fractional concentration of a gas and the atmospheric (barometric) pressure expressed in millimeters of mercury (mmHg):  Fractional Concentration • Barometric Pressure in mmHg = Partial Pressure For example, at sea level, average barometric pressure is 760 mmHg. Therefore, the partial pressure of oxygen is:  0.2095 • 760 mmHg = 159.2 mmHg At 7,500 feet, average barometric pressure is 570 mmHg. At this elevation the partial pressure of oxygen is: 0.2095 • 570 mmHg = 119.4 mmHg From a physiological standpoint, it’s the partial pressure of oxygen in air that becomes biologically important. Although the exact partial pressure of oxygen that causes various physiologic effects can be debated, the earliest effects of decreasing oxygen partial pressure are loss of visual acuity at night and a reduction in the ability to sustain maximal physical exertion (exercise). This becomes noticeable for persons who live at sea level and travel to Denver, Colo., where the elevation is 5,280 feet, the barometric pressure is 620 mmHg, and PO2 is 130 mmHg. As elevation increases above 6,000 feet, unacclimated individuals may begin to experience increased pulmonary ventilation and cardiac output, incoordination, and impaired attention and cognitive function, according to ACGIH’s 2017 TLVs and BEIs book. As altitude increases, persons with preexisting coronary heart disease, circulatory problems, or certain respiratory conditions, as well as older individuals or persons in poor physical condition, may notice shortness of breath earlier than persons who are healthy. RECOMMENDATIONS FOR RESPIRATORY PROTECTION Previous Synergist articles have discussed respirator selection for oxygen-deficient atmospheres; see "Respirator Selection for Oxygen-Deficient Atmospheres" and "Revisiting Respirators for Oxygen-Deficient Atmospheres." This article, intended as a complement to those articles, addresses a specific scenario—an oxygen deficiency caused solely by increased altitude. Oxygen deficiencies due to displacement by other gases or consumption of oxygen are different matters, not addressed in this article. The definition of oxygen-deficient atmospheres in ANSI/ASSE Z88.2-2015, Practices for Respiratory Protection, is based upon the partial pressure of oxygen. Section of Z88.2 defines an atmosphere that has an oxygen partial pressure less than 122 mmHg as oxygen-deficient IDLH (immediately dangerous to life and health). For these atmospheres, Z88.2 requires either a full-facepiece, pressure-demand SCBA or full-facepiece, pressure-demand SAR with self-contained auxiliary air supply (an escape bottle). Section states that atmospheres with partial pressures of oxygen between 122 and 148 mmHg are oxygen deficient non-IDLH. For these atmospheres, any atmosphere-supplying respirator is permitted.  Z88.2 includes a table that summarizes respirator selection for the effect of altitude alone (that is, when oxygen concentration is maintained at 20.9 percent) as well as the combined effects of altitude with reductions in oxygen concentration (for atmospheres with less than 19.5 percent oxygen as well as those with less than 16.0 percent oxygen). This table—designated as “Table 1” in Z88.2—is intended for persons not acclimated to altitude. Acclimation requires about four weeks’ residence for the body’s respiratory, cardiovascular, and hematopoietic systems to adapt. In all cases, Z88.2 requires that the source of any reduction in oxygen be understood and controlled. In addition, a footnote to Z88.2, Table 1, notes that “acclimated” workers can continue to perform their work without any atmosphere-supplying respirators up to altitudes of 14,000 feet, provided the oxygen concentration remains above 19.5 percent. Inspection of Z88.2, Table 1, column 1, reveals that in the absence of any airborne contaminants (in atmospheres where the oxygen concentration remains 20.9 percent), an SAR is recommended for unacclimated individuals above 2,000 feet. Above 7,000 feet, a pressure-demand, tight-fitting, full-facepiece SCBA or combination SAR/SCBA is recommended. 
ACGIH: 2017 TLVs and BEIs (2017).
American National Standards Institute: ANSI/ASSP Z88.2-2015, Practices for Respiratory Protection (2015). NIOSH: NIOSH Guide to Industrial Respiratory Protection (1987). NIOSH: NIOSH Respirator Decision Logic (1987). OSHA: Occupational Health and Safety Standards, Personal Protective Equipment, Respiratory Protection (1998).
Other organizations have similar recommendations to define oxygen-deficient atmospheres. For example, the NIOSH Respirator Decision Logic established an oxygen-deficient atmosphere based upon “alveolar” partial pressures of oxygen. When “alveolar” partial pressure falls below 60 torr (1 torr is essentially equal to 1 mmHg), adverse physiologic effects can occur. An alveolar partial pressure of 60 torr has an equivalent atmospheric partial pressure of approximately 132 mmHg. ACGIH also defines oxygen deficiency as an atmospheric oxygen partial pressure of less than 132 torr. To keep this in perspective, 132 torr is equivalent to an altitude slightly lower than Denver’s (5,280 feet).  SHORTNESS OF BREATH AT 6,000 FEET Given this information, let’s say a 55-year-old, mildly overweight but otherwise healthy employee who normally lives and works at an altitude of 1,000 feet is asked to respond to a work request at a site with an altitude of 6,000 feet. Shortly after arriving at the worksite, the employee experiences shortness of breath and compensates by working at a much slower pace. Prior to returning to the worksite on the second day, the employee informs his supervisor of the shortness of breath. The supervisor reaches out to both the safety department and employee health services for assistance. Together, they recognize that altitude decreases oxygen partial pressure, and that this can be responsible for the employee’s shortness of breath on exertion. They inspect Z88.2, Table 1, column 1 for unacclimated individuals and identify 6,000 feet as an oxygen-deficient atmosphere having an oxygen partial pressure of 127 mmHg. This table permits the use of a supplied-air respirator. They also read footnote number 3, which states: “For oxygen partial pressure equal to or greater than 122 and less than 148 mmHg, airline respirators may be worn if the source of the oxygen reduction is understood and controlled.” Feeling confident that the source of the oxygen reduction is understood and controlled, they issue an SAR. The employee, previously trained on and medically cleared for use of an SAR, returns to the worksite at 6,000 feet. After donning the SAR, the employee develops shortness of breath more quickly than the day before. Why? Because the SAR does not provide additional oxygen. Consequently, it does not protect from oxygen-deficient atmospheres due to increasing elevation. In this case, the respirator does not provide any benefit to the wearer. Furthermore, the SAR adds physiological stress (increased breathing resistance, weight, and workload due to the need to continuously pull a heavy hose, among other factors). Wearing the SAR either exacerbates intolerance to altitude, causes an adverse response to the use of the respirator itself, or both.  In this example, the safety department and employee health services mistakenly relied solely on the information in Z88.2 Table 1. HOW PRESSURE-DEMAND REGULATORS WORK To understand why SAR and SCBA respirators with breathing air provide no benefit to the wearer requires an understanding of respiratory physiology and how these respirators work. OSHA’s respiratory protection standard (29 Code of Federal Regulations, Part 1910.134) defines an SCBA as an atmosphere-supplying respirator for which the source of breathing air is designed to be carried by the user. The agency defines a pressure-demand respirator as a positive-pressure, atmosphere-supplying respirator that admits breathing air to the facepiece when the positive pressure is reduced inside the facepiece by inhalation. To maintain this positive pressure, the pressure-demand regulator utilizes a spring-like device to hold the admission valve in a slightly open position, theoretically allowing a continual flow of air into the facepiece. However, working in conjunction with the regulator is an additional spring-like exhalation valve on the facepiece. This spring on the exhalation valve prevents air from escaping out of the facepiece until the in-facepiece back pressure exceeds a critical positive pressure. When in-mask pressure decreases during inhalation, the admission valve fully opens, and air begins flowing into the facepiece. Working together, these two valves prevent air from continuously emptying from the facepiece (that is, the valves maintain a slight positive pressure inside the facepiece) and avoid premature emptying of the cylinder. (For a full explanation of pressure-demand respirators, see the NIOSH Guide to Industrial Respiratory Protection.) Although a pressure-demand, tight-fitting, full-facepiece SCBA is intended to maintain a positive pressure within the facepiece, these can go negative for short periods of time, such as during periods of strenuous activity when the wearer inhales faster than the rate of delivery or when there is damage or face-seal leakage due to poor fit. For pressure-demand SCBA certification, NIOSH conducts a series of performance tests. My understanding of these tests is based on conversations with NIOSH personnel and respirator manufacturers. One of the tests incorporates a breathing machine set to 40 liters per minute (lpm) using a cyclic breathing frequency of 24 breaths per minute (bpm). At the point of maximum inspiration, the in-mask pressure must not go negative. Typical pressures during maximum inspiration are approximately +0.10 inches of water, which are just slightly above ambient pressures. During maximum expiration, the in-mask pressure must not exceed +3.5 inches of water. Modern pressure-demand regulators typically achieve maximum pressures between +2.5 and +3.0 inches of water during expiration. During testing, in-mask pressures are measured in units of water. However, guidelines for oxygen-deficient atmospheres are expressed in units of mmHg. One inch water gauge is approximately equal to 1.868 mmHg. Therefore, in the best-case scenario, the maximum in-mask pressure during expiration is +6.5 mmHg above ambient pressure (3.5 inches H2O • 1.868 mmHg/inch H20). Since breathing is cyclic, the length of time in-mask pressure is near the maximum expiratory pressure is very short. For a nearly equal but short length of time during maximum inspiration, in-mask pressures are just above ambient. During the breathing cycle, most of the time in-mask pressures are between the minimum and maximum pressures.  PRESSURE-DEMAND REGULATORS AT ALTITUDE The performance of a pressure-demand regulator is relative to the atmosphere of its environment. Although not tested during the certification process, pressure- demand regulators have a linear response with atmospheric pressure. In other words, the in-mask pressures will not exceed ambient pressure by more than 6.5 mmHg; usually, the exceedance is less than 6.5 mmHg. Table A shows calculated ambient and in-mask pressures, if certification testing was conducted at two locations. The first location is Cincinnati, Ohio, which has an elevation of 460 feet above sea level and an average barometric pressure of 747 mmHg. The second location is a ski resort near Park City, Utah, with an elevation of 7,500 feet and a barometric pressure of 570 mmHg. Also shown in the table are the ambient and in-mask oxygen partial pressures for both locations. Results shown are for a pressure-demand SCBA. 
As altitude increases, persons with preexisting coronary heart disease, circulatory problems, or certain respiratory conditions, as well as older individuals or persons in poor physical condition, may notice shortness of breath earlier than persons who are healthy.
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Table A. Ambient and In-Mask Pressures for a Pressure Demand SCBA at Point of Maximum Expiration (Units of mmHg)*
When wearing a pressure-demand respirator, the in-mask pressures are just slightly above the atmospheric pressure, regardless of location. This is because the exhalation valve opens, which prevents in-mask pressures from becoming too high. This slight increase in positive pressure is not able to compensate for the much larger decrease in oxygen partial pressure with increasing altitude. Furthermore, in-mask pressures shown in Table A represent the most protective scenario. Because breathing is cyclic, maximum pressures are only achieved for a short fraction of time (that is, during exhalation). For the majority of the breathing cycle (inhalation and pauses between inhalation and exhalation), in-mask pressures and oxygen partial pressures are lower than the maximum values shown in Table A. In addition, typical pressure-demand regulators maintain pressures below the maximum values required during certification testing.  A MEDICAL DECISION
Atmosphere-supplying respirators (SARs and SCBAs) do not overcome oxygen-deficient atmospheres due to increasing altitude. These respirators only add physiologic stress to the wearer. In these cases, providing an atmosphere-supplying, positive-pressure industrial respirator is not helpful and may make things worse. A worker, whether acclimated or not, who experiences physiologic effects at altitude should undergo medical evaluation. (Again, note that displacement of oxygen by other gases or consumption of oxygen are not covered in this article.) A potential solution for oxygen deficiency due solely to altitude could be supplemental oxygen via a nasal cannula, which may resolve or mitigate reductions in oxygen partial pressure due to altitude. However, the decision to provide supplemental oxygen should be made and prescribed by a competent physician or other licensed healthcare provider. Prior to using supplemental oxygen, health and safety personnel would need to reassess the workplace to ensure no flammability or explosion potential exists or is introduced. Training of the worker will also be necessary. Substituting another worker who is acclimated to the altitude may be another option.  Ever wonder why high-altitude mountain climbers don’t carry industrial SCBA respirators? Now you know.    ROY T. MCKAY, PhD, is professor emeritus with the University of Cincinnati and a member of the ANSI/ASSP Z88.2 committee. Send feedback to The Synergist.
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Examining a Common Misunderstanding about Atmosphere-Supplying Respirators
Respirator Use at High Altitudes
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Disadvantages of being unacclimatized:
  • Readily show signs of heat stress when exposed to hot environments.
  • Difficulty replacing all of the water lost in sweat.
  • Failure to replace the water lost will slow or prevent acclimatization.
Benefits of acclimatization:
  • Increased sweating efficiency (earlier onset of sweating, greater sweat production, and reduced electrolyte loss in sweat).
  • Stabilization of the circulation.
  • Work is performed with lower core temperature and heart rate.
  • Increased skin blood flow at a given core temperature.
Acclimatization plan:
  • Gradually increase exposure time in hot environmental conditions over a period of 7 to 14 days.
  • For new workers, the schedule should be no more than 20% of the usual duration of work in the hot environment on day 1 and a no more than 20% increase on each additional day.
  • For workers who have had previous experience with the job, the acclimatization regimen should be no more than 50% of the usual duration of work in the hot environment on day 1, 60% on day 2, 80% on day 3, and 100% on day 4.
  • The time required for non–physically fit individuals to develop acclimatization is about 50% greater than for the physically fit.
Level of acclimatization:
  • Relative to the initial level of physical fitness and the total heat stress experienced by the individual.
Maintaining acclimatization:
  • Can be maintained for a few days of non-heat exposure.
  • Absence from work in the heat for a week or more results in a significant loss in the beneficial adaptations leading to an increase likelihood of acute dehydration, illness, or fatigue.
  • Can be regained in 2 to 3 days upon return to a hot job.
  • Appears to be better maintained by those who are physically fit.
  • Seasonal shifts in temperatures may result in difficulties.
  • Working in hot, humid environments provides adaptive benefits that also apply in hot, desert environments, and vice versa.
  • Air conditioning will not affect acclimatization.
Acclimatization in Workers