Left and center photos courtesy of MSA. Right photo courtesy of NIOSH/NPPTL.

Authors’ note: This article, an update to our article “
Respirator Selection for Oxygen-deficient Atmospheres
” in the March 2016
, explains the differences between the OSHA and ANSI/ASSE Z88.2 requirements for O2-
deficient atmospheres.
The O2 concentration in atmospheric air is 20.95 percent, from sea level to the highest mountain peak. However, since atmospheric pressure decreases with increasing altitude, fewer molecules exist within a given volume of air. Consequently, the partial pressure of air components, including oxygen (PO2), decreases with increasing altitude. (Partial pressure, measured in millimeters of mercury, or mmHg, refers to the pressure a particular gas exerts in a mixture of gases.) It therefore becomes important to consider the amount of O2 available for biological processes even though the percentage of O2 at higher altitudes remains the same. The amount of biologically available O2 for respiration decreases as the PO2 decreases. Thus, the PO2 level is one of several factors that play a role in the amount of O2 available for respiration and biological metabolism at any altitude. OSHA’s approach to evaluating O2 deficiency is to assess the percent O2 in a workplace atmosphere. However, OSHA’s approach does not directly assess the workplace PO2. In comparison, the Z88.2 hazard assessment approach assesses both the percent O2 and PO2. For this reason, and because it uses more conservative oxygen-deficiency limits, Z88.2 has more conservative respirator selection criteria than OSHA in some instances.
According to OSHA’s 1998 final rule on respiratory protection (29 CFR 1910.134), the OSHA respirator standard defines O2-deficient atmospheres as those whose O2 content is below 19.5 percent by volume. OSHA considers this as immediately dangerous to life or health (IDLH). The standard requires use of the most protective and reliable respirators for IDLH atmospheres. Per paragraphs (d)(2)(i)(A) and (B) of 29 CFR 1910.134, for entry into IDLH atmospheres, OSHA requires the use of a full facepiece pressure-demand self-contained breathing apparatus (SCBA) or a full facepiece pressure-demand combination supplied-air respirator (SAR) with an auxiliary self-contained air supply, which provide positive pressure inside the facepiece. OSHA outlines altitude-adjusted, non-IDLH oxygen percentages. OSHA also allows exceptions to IDLH respirator selections at altitudes up to 8,000 feet, provided the employer can demonstrate that under all foreseeable conditions the O2 concentration can be maintained within the ranges specified in Table II of paragraph (d)(2)(iii). This table is reproduced as Table 1 below. Per the preamble to the 1998 final rule, OSHA’s logic allows any atmosphere-supplying respirator (for example, an SAR) to be worn when employers understand, control, and demonstrate that workplace O2 is strictly controlled within O2 concentrations outlined in 29 CFR 1910.134, Table II. OSHA’s logic is largely based on the definition for O2-deficient IDLH found in ANSI Z88.2-1980. The Z88.2-1980 O2-deficient IDLH atmosphere cutoff point is “an oxygen partial pressure of 100 mmHg or less in the freshly inspired air in the upper portion of the lungs [trachea], which is saturated with water vapor.” PO2 ≤100 mmHg in the trachea equates to alveolar blood having only an estimated 90 percent oxygenated hemoglobin. O2 deficiency symptoms become increasingly noticeable below this blood oxygenation level. The lower limits in OSHA Table II correspond to PO2 <100 mmHg in the trachea. The 1998 preamble explains “that the language of paragraph (d)(2)(iii), along with the exception, reflects the same requirement as” OSHA’s 1994 proposed rule, “but avoids the potential confusion associated with having separate definitions and requirements for oxygen-deficient, and oxygen-deficient IDLH, atmospheres, as originally proposed.” During the 1994 proposed rulemaking, NIOSH pointed out
that in the presence of an oxygen concentration of less than 16% at sea level one can experience impaired attention, thinking and coordination. At 14% or below, NIOSH states the possibility of faulty judgment, poor muscular coordination, rapid fatigue that could cause permanent heart damage, and intermittent respiration. In an IDLH or escape situation all of the described effects could place a worker in serious jeopardy.
This explains why, although the calculated lower O2 range value at sea level is 14 percent, OSHA’s Table II shows 16 percent O2 as the IDLH limit for atmospheres from sea level up to 3,000 feet. However, from 3,001 to 8,000 feet, the calculated lower O2 percentage concentration in each of the allowable ranges in 29 CFR 1910.134, Table II, is equivalent to 14 percent O2 at sea level. This corresponds to the Z88.2-1980 O2-deficient IDLH atmosphere cutoff point of PO2 <100 mmHg in the trachea. OSHA allows acclimated workers to work without atmosphere-supplying respirators at altitudes up to 14,000 feet, if the ambient O2 content remains above 19.5 percent and the workers have no medical condition that requires using supplemental O2. This is also true for Z88.2.
The NIOSH publication
A Guide to Industrial Respiratory Protection
explains the importance of evaluating biologically available O2 based on workplace PO2, which supports the Z88.2 approach for assessing O2-deficient atmospheres. The publication also supports following the more conservative Z88.2 approach by stating, “If the oxygen deficiency level as legally defined is less than the O2 concentration you believe safe for human exposure, you must consider raising your minimum O2 level above the legal limit.” This exactly matches the Z88.2 approach to assessing O2 deficiency. Z88.2 requires evaluation of the legally required percent O2 and PO2. Therefore, unlike OSHA, Z88.2 classifies O2-deficient atmospheres as either IDLH or non-IDLH and primarily addresses oxygen deficiency in terms of PO2 directly. Using PO2 provides employers with O2 measurement units that work well for assessing O2-deficient atmospheres at sea level, at increased altitudes, and combinations of increased altitude and decreased O2 concentrations. Z88.2 requires that workers have the same amount of O2 available for biological processes at any altitude as they would receive when breathing ambient air at sea level.
Table 1.
A Reproduction of Table II from 29 CFR 1910.134
Per, any workspace that contains less than ambient oxygen concentration must be evaluated and the cause of the deficiency identified. Until the cause of the deficiency is understood and controlled, that workplace is considered IDLH. After this workplace oxygen deficiency is evaluated and characterized, it will then be classified as either O2-deficient non-IDLH or O2-deficient IDLH depending on the measured O2 concentration and PO2. In contrast, OSHA considers any atmosphere with less than 19.5 percent O2 as IDLH. Z88.2 is therefore more conservative by classifying all workplace atmospheres containing PO2 <159 mmHg (<20.9 percent oxygen at sea level) as O2-deficient IDLH unless the source of the O2 reduction is understood and controlled.
Section of Z88.2 defines atmospheres with PO2 less than 122 mmHg (<16 percent oxygen at sea level) as being O2-deficient IDLH. Section states that O2-deficient non-IDLH atmospheres have a PO2 range greater than or equal to 122 mmHg (≥16 percent oxygen at sea level) and less than 148 mmHg (<19.5 percent oxygen at sea level).
For O2-deficient IDLH atmospheres, Z88.2 respirator selection requires either a full facepiece pressure-demand SCBA or full facepiece combination pressure-demand SAR with self-contained auxiliary air supply. For O2-deficient non-IDLH atmospheres, Z88.2 respirator selection allows any atmosphere-supplying respirators. This requirement is similar to OSHA’s exception for allowing any atmosphere-supplying respirators in controlled O2-deficient atmospheres, but only if the source of the O2 reduction is understood and controlled (Z88.2 Section 7.2.4).
In addition, Z88.2 only permits the use of air-purifying respirators (APRs) for atmospheres with an O2 concentration range between PO2 159 to 148 mmHg (20.9 to 19.5 percent oxygen at sea level). However, the source of the O2 reduction must be understood and controlled and other inhalation hazards must be within such a range that the protection provided by APRs is adequate. OSHA indirectly has the same policy, although it is not written in the 1998 standard.
What is the significance of the difference between the OSHA and Z88.2 approaches to O2 deficiency?
Consider a workplace at the elevation of 9,000 feet in which no other airborne contaminants exist and the O2 concentration is 20.95 percent. In OSHA’s assessment, no respirator is required because the workplace O2 concentration is not below 19.5 percent. In contrast, Z88.2 assesses both percent O2 and PO2. At 9,000 feet, the atmospheric PO2 is 113 mmHg, which is below the Z88.2 O2-deficiency IDLH limit of <122 mmHg. This is equivalent to breathing the atmospheric PO2 of <16 percent oxygen at sea level.
The Z88.2 approach to assessing O2 deficiency requires the following respirator selection:
  • Individuals who are acclimated at this altitude are not required to wear respirators up to 14,000 feet as long as the ambient O2 content remains above 19.5 percent and the wearer has no medical condition that would require the use of supplemental O2.
  • For individuals not acclimated to this altitude, a full facepiece pressure-demand SCBA or full facepiece pressure-demand combination SAR/SCBA is required. The physician or other licensed health care professional (PLHCP) will determine if the employee has any medical conditions that would preclude the use of respirators, limitations on use, or other restrictions.
Thus, by using Z88.2 to assess O2 deficiency, respirator selection is more conservative than the OSHA approach. In Z88.2, the same precautions that apply for workers at sea level also apply for workers not acclimated to high altitudes. In other words, Z88.2 policy provides the same protection for workers not acclimated to high altitudes as it provides for workers at sea level in atmospheres with identical levels of PO2.
However, it is important to recognize that the cause for a reduction in PO2 plays an important role in health outcomes. Reduction in PO2 can result from an increase in altitude or displacement or consumption of oxygen. Displacement of O2 can occur with the introduction of simple or chemical asphyxiants or volatile chemical contaminants. Oxygen consumption can result in O2 deficiency due to the process of oxidation (rust). Fatalities have occurred from inert gases and chemical contaminants displacing oxygen.
For example, two causes of reduced PO2 are displacement of O2 due to the introduction of an airborne contaminant (such as a solvent) or increasing altitude without a contaminant. If both scenarios have an identical PO2 (for example, 113 mmHg), the health effects at sea level will likely be of a far greater magnitude despite the same PO2. In addition to the greater magnitude, additional health effects may be present. This occurs because at sea level, the reduction in PO2 can only be caused by the introduction of airborne contaminants (except for oxygen consumption and inert gases). These contaminants contribute to health effects (at least in an additive manner) and cause health outcomes to be of greater magnitude or effect. For example, solvents—especially at high concentrations, such as those necessary to reduce O2 concentration to levels near 16 percent (approximately 247,500 ppm)—have significant neurotoxic properties resulting in health effects far greater than those caused by equivalent reductions in PO2 with increasing altitude.
Z88.2 requirements include a medical evaluation (Section 4.5.3) by a PLHCP. In collaboration with the respirator program administrator, the PLHCP will determine if the employee has any medical conditions that would preclude the use of respirators, limitations on respirator use, or other restrictions. In some high-altitude workplace scenarios, the PLHCP may determine that a worker not acclimated to the altitude would be better served with supplemental oxygen (for example, nasal cannula) rather than a respirator, since a respirator may introduce additional safety hazards (such as tangling of airline hoses) and physiologic stress. Supplemental O2 may sometimes be provided in lieu of a respirator depending on the industrial complexity of the high-altitude workplace, the wearer’s medical condition, other equipment that may be worn, and the operations performed. Considering these factors, employers may decide to provide workers not acclimated to the altitude with supplemental O2 as an alternative to a respirator when a hazard assessment determines the absence of other inhalation hazards. However, there must be no potential for flammability hazards caused by the supplemental O2. The decision to equip a worker with supplementary O2 should be made by the employer and facility medical staff on a case-by-case basis. This is a choice for the employer in consultation with the PLHCP.
Providing employees supplemental O2 is under the cognizance of the medical department’s program and is not covered by the Z88.2 standard respirator programs. Section 1.1 of Z88.2 states, “The following are not covered by this standard: … medical inhalators and resuscitators.” Another reasonable alternative for inclusion in the respirator program is to provide individuals not acclimated to high altitudes with appropriate training to recognize early warning signs of high altitude illness and instruct them to avoid alcohol, drink plenty of water, and so on. These actions will gradually acclimate the worker to higher elevations.
The differences between OSHA and Z88.2 O2-deficient hazard assessments and respirator selection recommendations can, in some instances, result in Z88.2 having a more conservative respirator selection. It is the employer’s responsibility to follow the OSHA requirements or implement a more conservative alternate solution to protect workers. If the employer decides to follow Z88.2, the respirator program must be established and implemented in accordance with all Z88.2 requirements, including a medical evaluation by a PLHCP. However, employers may elect to implement a non-respirator alternative solution, such as providing employees supplemental O2, which is not covered by the Z88.2 or OSHA respirator programs. In this case, appropriate medical expertise should be consulted.
(1997–2015), served as the U.S. Navy’s respirator expert from 1987 to 2015 and was the Navy’s official liaison to the ANSI Respirator Committee and the ANSI respirator subcommittees. He can be reached at
a consultant based in Houston, Pa., is a past chair of the ANSI/ASSE Z88.2 subcommittee. He can be reached at
is a consultant with JSJ and Associates in Pleasanton, Calif., and chair of the ANSI/ASSE Z88 committee. He can be reached at
is a general engineer with the NIOSH National Personal Protective Technology Laboratory (NPPTL) and a member of the ANSI/ASSE Z88.2 subcommittee. He can be reached at
is professor emeritus with University of Cincinnati and a member of ANSI/ASSE Z88. He can be reached at
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A Comparison of OSHA and Z88.2 Approaches
to Respiratory Protection

Revisiting Respirators
for Oxygen-deficient Atmospheres
ANSI: American National Standard for Respiratory Protection, ANSI Z88.2-2015 and ANSI Z88.2-1980.
Federal Register
: OSHA 29 CFR Parts 1910 and 1926, Respiratory Protection Final Rule (
, January 1998).
Federal Register
: OSHA 29 CFR Parts 1910 and 1926,
Respiratory Protection Proposed Rule
(November 1994).
NIOSH: “A Guide to Industrial Respiratory Protection” (reprinted April 1979).
The Synergist
: “
Respirator Selection for Oxygen-deficient Atmospheres: Answers to Critical Questions about ANSI/ASSE Z88.2-2015
” (March 2016).