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. 

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.