NASA-STD-3001 Technical Brief

Relevant Technical 

Requirements

Habitable Atmosphere

OCHMO-TB-003

Rev A

NASA-STD-3001 Volume 1, Rev C

[V1 3003] In-Mission Preventive Health Care

[V1 3004] In-Mission Medical Care

[V1 5002] Crewmember Training 

NASA-STD-3001 Volume 2, Rev D

[V2 4015] Aerobic Capacity

[V2 6001] Trend Analysis of Environmental and 

Suit Data

[V2 6002] Inert Diluent Gas

[V2 6003] O2 Partial Pressure Range for Crew 

Exposure

[V2 6004] Nominal Vehicle/Habitat Carbon 

Dioxide Levels

[V2 6006] Total Pressure Tolerance Range for 

Indefinite Crew Exposure

[V2 6007] Rate of Pressure Change

[V2 6008] Decompression Sickness (DCS) Risk 

Identification

[V2 6009] Decompression Sickness Treatment 

Capability

[V2 6017] Atmospheric Control

[V1 6020] Atmospheric Data Recording

[V1 6021] Atmospheric Data Displaying

[V1 6022] Atmospheric Monitoring and Alerting 

Parameters

[V2 6107] Nominal Vehicle/Habitat Atmospheric 

Ventilation

[V1 6108] Off-Nominal Vehicle/Habitat 

Atmospheric Ventilation

[V1 6150] Barotrauma Prevention

[V1 7041] Environmental Control During Exercise

[V2 9053] Protective Equipment

[V2 11032] LEA Suited Decompression Sickness 

Prevention Capability

[V2 11100] Pressure Suits for Protection from 

Cabin Depressurization

Executive Summary

Space is characterized by hostile 

conditions including variable pressure, 

changing temperature and humidity, 

and the lack of a survivable 

atmosphere. Spaceflight vehicle 

designers must develop life support 

systems that provide a safe and 

comfortable living environment, 

facilitate safe extravehicular activities, 

adapt to changing environments 

(including vehicles, rovers, and suits), 

and maintain the integrity of the space 

vehicle. In addition to vehicle design, 

the crew must be protected from 

conditions induced by atmospheric 

changes including inadequate oxygen 

supply (hypoxia or hyperoxia) and 

pressure changes that can cause 

pressure-related illnesses or 

barotrauma. 
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Background

Air Constituents

A safe, breathable atmosphere is critical to crewmember health and performance. A spacecraft must 

provide and maintain atmospheric constituents in the proper quantities and pressure environments to 

mitigate risks of fire, decompression sickness (DCS), and hypoxia while maintaining performance and 

operational requirements during spaceflight. Extra vehicular activities (EVAs) and crewmember excursions 

on lunar and planetary surfaces must also be considered. 

11/30/2023

Rev A 2

Sea Level Air Composition

• 78.08% Nitrogen

• 20.95% Oxygen

• 0.93% Argon

• 0.038% Carbon Dioxide and other gases

. 

Source: NASA

Crewmember Consideration

Atmospheric concerns when considering human health must ensure optimal oxygen, diluent gas, and 

pressure environment to avoid Hypoxia, Hyperoxia, Decompression Sickness (DCS), and Barotrauma. 

Diluent Gasis a physiologically inert gas. Nitrogen is the most used diluent gas of the air constituents with Argon 

and other trace constituents used occasionally. The function of diluent gas is to prevent excessive levels of 

oxygen absorption during prolonged exposures which could increase risk of lung impairment (Atelectasis). 

Diluent gases are also used to reduce the atmosphere ignition/flammability threshold (see next page for related 

technical requirement).

Gas Laws – Atmospheric gas behavior is best explained by gas laws that govern it. 

Ideal Gas Law describes relation between Pressure(P), Volume (V), and Temperature (T) of gas. PV=nRT 

Boyles Law describes how gas volumes vary with changes in ambient pressure. P2/P1=V1/V2

Charles Law relates the volume and temperature of gas held at constant pressure. V1/V2=T1/T2

Daltons Law describes the relationship of partial pressures in mixture of gasses. PT=P1+P2+...Px

Sea Level Total Air Pressure 

1 ATM=760 mmHg=14.7psia=101.3 kPa

Standard Sea-Level Atmosphere

Source: NASA HIDH

Atmosphere Measurement 

• Total pressure includes all air 

components PT=PO2+PN+... (see 

Daltons law above).

• Partial Pressure (pp) is a measure of 

the proportion of a single gas in a 

mixture of gases. (i.e., ppO2, ppCO2, 

ppN).

• Pressure Units: Total pressure can be 

expressed as ATA (atmosphere 

absolute), mmHg (millimeters of 

mercury), psia (pounds per square 

inch), or absolute kPa (kilopascals).
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Total Pressure Limits

Total atmospheric pressure is the pressure of the surrounding gases applied equally to the entire body. 

When designing a vehicle, total pressure must prevent the vaporization of body fluids (ebullism), which 

occurs at 6.27 kPa (0.91 psi, 47.0 mmHg) at 37°C (98.6°F) (normal body temperature) while not exceeding 

human physiologic tolerances.

Spacecraft Cabin Design Total Pressure Considerations

• Ensure oxygen (O2) is at suitable partial pressure for metabolic use and low enough to prevent O2 toxicity.

• Maintain the ratio of cabin to spacesuit pressure to minimize the risk of DCS.

• Maintain cabin pressure, gas density for cooling, and gas concentration minimizing flammability.

• Provide physiologically inert gas at low pressures to prevent atelectasis for durations >2 weeks.

• All other atmospheric constituents must be inert or of low enough concentration to preclude toxic 

effects.

• The breathing atmosphere must have minimal flame or explosive hazard. 

• Crew are trained to don oxygen masks anytime symptoms of hypoxia are experienced. 

• Crewmember pure O2 exposure duration is limited to prevent risk of lung damage (atelectasis).

• Supplemental O2 may be insufficient to maintain tissue oxygenation with pressures <130mmHg/2.5psia, 

41,500 ft equivalent air altitude (EAA).

Application

3

[V2 6006] Total Pressure Tolerance Range for Indefinite Crew Exposure The system shall maintain the 

pressure to which the crew is exposed to between 34.5 kPa < pressure ≤ 103 kPa (5.0 psia < pressure ≤ 

15.0 psia) for indefinite human exposure without measurable impairments to health or performance. 

From: NASA-STD-3001 Volume 2, Rev D

[V2 6002] Inert Diluent Gas Cabin atmospheric composition shall contain at least 30% diluent gas 

(assuming balance oxygen). From: NASA-STD-3001 Volume 2, Rev D

Factoring the diluent gas standard of 30%, the following technical requirement applies:

Designers and physiologists must evaluate and trade off the various atmospheric combinations. A low total 

pressure is desirable because it allows simple transfer to a low-pressure EVA suit (low pressure EVA suits 

are less stiff and allow greater range of motion). Low total pressure requires a higher percentage of oxygen

in the atmosphere to provide an acceptable PI

O2. Oxygen-rich atmospheres, however, present safety 

hazards because of their ability to feed fires. The lowest pressure at which normoxia (PIO2 = 149 mmHg) is 

maintained at 100% O2 is 3.8 psia, however given the need to maintain a diluent gas at 30% ([V2 6002] 

Inert Diluent Gas) while maintaining normoxia and CO2 limits ([V2 6004] Nominal Vehicle/Habitat Carbon 

Dioxide Levels), the lowest achievable cabin pressure is 5.0 psia (see Figure 6.2-1—Atmospheric 

Composition). Although mild hypoxia is allowed per [V2 6004], there is not sufficient rationale for mild 

hypoxia at the lower end of this pressure because the beneficial aspects of DCS mitigation have already 

been achieved and the materials flammability preferences have already been exceeded. Total pressure 

must be considered in conjunction with O2 and CO2 requirements. Under certain spacesuit operations (e.g., 

DCS treatment, leak checks), the crewmember may be exposed to pressure above or below this range for a 

limited period.

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Background

Partial Pressure of Oxygen 

• Oxygenation of tissues is one of the most important processes that take place within the human body. O2

feeds tissues and ensures metabolic processes and cellular functions take place. As crewmembers travel 

in space it is imperative to maintain the partial pressure of O2 to keep crewmembers healthy and able to 

preform required functions. 

• Partial pressure of O2 considers O2 in a dry gas environment. This does not accurately reflect the partial 

pressure available for diffusion within the alveoli of the lung and able to supply the body. When air is 

inhaled, it is warmed by the pulmonary tract and exposed to humidity/water vapor that affects the O2

partial pressure called partial pressure of inspired oxygen (PIO2), as inhalation continues O2 partial 

pressure is also affected by CO2 during the alveolar gas exchange, further decreasing oxygen referred, to 

as alveolar partial pressure PAO2.

4

Source: Ragdona et al 2020

Partial Pressure of Oxygen (ppO2) the percentage 

proportion of dry oxygen in air.

Partial Pressure of Inspired Oxygen (PIO2) the partial 

pressure of O2 considering how the humidity (containing 

H2O) of inspired air can change the air mixture 

percentages. 

Alveolar Partial Pressure (PAO2) the partial pressure of O2

that fills the alveoli during inspiration, reduced from dry 

pressure by inspired humidity (H2O) and mixing with end 

alveolar gas (CO2) during alveolar air exchange.

ppO2 = Pb X FiO2 (Pb is barometric 

pressure, FiO2 is a fraction of inspired 

oxygen assuming a dry gas condition).

PIO2 = (Pb – PH2O) x FiO2 (PH2O is partial 

pressure of H2O), PH2O is partial 

pressure of water introduced during 

inspiration.

PAO2 = (Pb – PH2O) x FiO2 – (PaCO2 x 

1.25), PCO2 is partial pressure of CO2

introduced during alveolar gas exchange.

Example of inspired partial pressure at 

sea level: (Pb=760, PH20=47mmHg, FiO2

= .21 (21% of Atmospheric Atm)) PIO2= 

(PB – PH2O) x (FiO2), 760mmHg-47mmHg 

x (.21)=149.7mmHg

Partial pressure determines the availability of oxygen in lung 

tissues for dissolution into the blood, thus affecting the 

availability of blood O2 to body tissues (see O2 cascade below). 

Symptoms occur if partial pressure is not adequate to perfuse 

body tissues or support body functions.

Source: Burtscher et al 2022

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Background

5

Physiological Effects - Hypoxia

Acute hypoxia is associated with a variety of physical symptoms and decreases in ability to complete tasks. The

onset and severity of hypoxia depends on factors including altitude, barometric pressure, and the O2 content. In

subjects with normal ventilation and gas exchange, the PI

O2 in the lungs controls how much oxygen is delivered to

the tissues.

Hypoxia occurs when O2 is not sufficient to supply body tissues and maintain homeostasis. 

Hyperoxia is a state of excess supply of O2 in tissues and organs than normally exists. 

Normoxia sufficient O2 is supplied to provide body homeostasis.

Hypoxia-induced physiologic changes include:

• CNS: mental changes, at times undetectable to 

crew

• Vision: decreases in color, night, and peripheral 

vision

• Nonspecific: fatigue, nausea, headache, dizziness, 

mood alterations

• Sleep: decrease in quality

• Breathing: hyperventilation

See the OCHMO-TB-004 Carbon Dioxide (CO2) for additional atmospheric information regarding 

CO2 and other constituents.

The balance of O2 supply—energetic crisis versus oxygen toxicity

Source: Burtscher et al 2022

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Background

6

Physiological Effects – Hypoxia and Hyperoxia

One of the most difficult parts of treating hypoxia is being able to recognize symptoms and diagnose the 

problem. Hypoxia symptoms can be mistaken for fatigue or other causes and can be complicated with the fact 

that crew experiencing hypoxia may have loss of cognitive function and reasoning. It is important to prepare 

crewmembers with pre-flight hypoxia training, which allows them to experience hypoxia (while being monitored) 

and to recognize their own symptoms. Crewmembers are also trained to monitor fellow crewmembers for 

symptoms as it may be easier to recognize symptoms in others. 

Hypoxia Treatment

Once diagnosed, the main treatment for hypoxia is to supply 

crew with supplemental oxygen to resolve the O2 deficiency. 

Crewmembers can be trained to use quick don masks that 

provide additional O2.

Hyperoxia breathing in high concentrations of O2 can result in 

a decrease in ventilation and heart rate. In space, normobaric

hyperoxia occurs when O2 has increased but metabolic 

requirements for O2 have not. This can result in toxic oxygen 

levels, potentially leading to respiratory concerns or tissuelevel injury.

High altitude chamber training Source: NASA

Emergency quick don oxygen mask Source: NASA 

• Hyperoxia in space is usually encountered when using different gas mixtures during various phases of the 

mission. For example, if crew encountered a contingency situation requiring a breathing apparatus supplying 

100% O2 for a prolonged time, illness or injury may require crew to breathe supplemental O2 for longer periods, 

or similar.

• Hyperoxia treatment primarily consists of reducing the partial pressure of oxygen in atmospheric gas and 

supportive care of any toxic sequelae. Toxic effects of hyperoxia can resolve quickly when crewmembers return 

to more appropriate atmospheric gas mixes. 

• In addition to hyperoxia risk, an increased percentage of O2 in the atmospheric environment significantly 

increases fire risk, posing potentially life-threatening conditions for the crew. See OCHMO-TB-008 Fire 

Protection for additional information.

[V1 5002] Crewmember Training Beginning with the 

crewmember candidate year, general medical training, 

including but not limited to, first aid, cardiopulmonary 

resuscitation (CPR), altitude physiological training, carbon 

dioxide exposure training, familiarization with medical 

issues, procedures of spaceflight, psychological training, 

toxicology, medical equipment, and supervised physical 

conditioning training shall be provided to the astronaut 

corps. From NASA-STD-3001 Volume 1, Rev C

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Background

7

Altitude – as altitude increases, air 

pressure and O2 partial pressure 

decreases.

Pressure – as air pressure 

decrease, gas molecules become 

less dense and have increased 

mobility. 

[V2 6022] Atmospheric Monitoring and 

Alerting Parameters The system shall alert the 

crew locally and remotely when atmospheric 

parameters, including atmospheric pressure, 

humidity, temperature, ppO2, and ppCO2 are 

outside safe limits. From NASA-STD-3001 

Volume 2, Rev D

Relative humidity – as humidity increases, 

levels of water vapor increase and the partial 

pressures of O2 decreases.

Temperature – alone does not affect O2 partial 

pressures.

Altitude, Humidity and Oxygen Partial Pressure

When creating the vehicle habitat, programs try to accommodate for equivalent altitude, pressure, and 

environmental (humidity) conditions to maintain the atmosphere as close as possible to sea level 

conditions.

To maintain this, the vehicle has a pressure control system which monitors the total pressure as well as O2

levels. As air pressure falls for example due to pressure vessel specification leakage or some contingency, 

the pressure control system will attempt to maintain the desired cabin pressure through the addition of air, 

nitrogen and/or O2. Dispersion of atmosphere gases change depending on the surrounding 

pressure/atmosphere conditions including altitude, barometric air pressure (Pb), and climate (temperature, 

relative humidity).

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Background

8

Physiological Effects at Altitude and Partial Pressure

To better explain ranges for total and partial pressure, scientists relate the pressure environment to that 

which is experienced at a particular altitude, the equivalent air altitude (EAA), assuming similar 

physiological conditions between the identified pressures and altitudes. A significant concern with pressure 

or altitude is symptoms associated with hypoxia that can interfere with crew comfort, health, and ability to 

execute tasks. These symptoms drive what pressure and oxygen values are best used for the cabin 

environment, and ultimately what pressures are tolerable for specific periods of time. Generally, the 

average person acclimated to sea level may begin to experience subtle decrements near PaO2 of 80mmHg, 

(EAA 5,100 ft) and continue till loss of consciousness at 30mmHg ( EAA 42,000 ft). Currently, the ISS cabin 

pressure is typically maintained at 14.7 psia with 21% O2, which is equivalent to sea level. Source: Barrett et al 

Principles of Spaceflight Medicine

Relationship between Altitude and 

Pressure

(3) Pa = Barometric pressure

(4) (Pa-47) = (a) total pressure of dry 

gases after inspired gas saturated with 

water vapor at 37 degrees, (b) also equal 

to the inspired O2 tension PIO2 when 

pure O2 inspired

(5) .209 (Pa-47) = inspired O2 tension, 

Source: W.G.Brombacher 1935 PI

O2 when air is breathed

The relationship of decreased O2 availability to 

altitude illness and performance decrements 

provides a classification of altitude exposure based 

on arterial O2 content and physiologic effects. This 

graph (left) shows that with ascent of 

unacclimatized low altitude, residents ascending 

rapidly (<6 hours) to increasing altitudes risk 

developing altitude illness. Performance decrement 

is inversely related to partial pressure of O2. The 

physical work performance (road march left) begins 

to decline at altitudes > 1,000m (3,000 ft). 

Left graph shows relationship between altitude, 

arterial oxygen partial pressure and arterial oxygen 

saturation in unacclimatized personnel. Categorizing 

altitudes is based on physiologic responses to 

hypoxia. 

Source: U.S. Army Research Institute of Environmental 

Medicine 2004

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Background

9

U.S. Army describes the rapid 

ascent of non-acclimatized persons 

to altitudes above 6,000 ft 

increases risk of altitude illness and 

decreases physical and cognitive 

work performance. Altitude 

acclimatization training involving 

hypoxia exposure over time can 

decrease risk of performance 

decrements.

Hypoxia Can negatively affect senses, mood, psychomotor, 

and cognitive tasks, with dark adaptation vision beginning 

to be affected between 3,000 and 4,000 ft to mood and 

reaction time affected around 9,000 ft. Human physiology 

adapts upon high altitude and hypoxia exposure, including 

plasma volume and red blood cell adjustment to better 

facilitate O2 delivery. The decrements can improve with 

length of exposure and/or acclimatization training. 

Pharmacology The medication Acetazolamide can assist 

with altitude prophylaxis and decrease altitude symptoms. 

Acetazolamide induces metabolic acidosis, stimulates 

breathing, and increases arterial oxygen to decrease 

hypoxia symptoms, however this has been associated with 

potential increased intracranial pressure and its use is 

currently under study. 

Source: Army Altitude Acclimatization Guide

Altitude-related Complications Rapid ascent without acclimatization can increase risk of serious 

complications.

Acute Mountain Sickness (AMS) Short lived (2-7 days) symptoms of headache, nausea, fatigue, and 

lightheadedness varies in severity, prior AMS history increases AMS risk.

High Altitude Pulmonary Edema (HAPE) Life threatening form of non-cardiogenic pulmonary edema 

involving accumulation of fluid in lungs, worsens when rapid altitude ascent accompanied by exercise.

High Altitude Cerebral Edema (HACE) Life threatening swelling of the brain due to physiological effects of 

high-altitude including symptoms of disorientation, lethargy, and nausea. 

Physiological Effects at Altitude and Partial Pressure

Spaceflight atmosphere experience is similar to the physical experience of ascending to increased altitudes; 

this data is considered (pressure equivalent at specific altitudes) to predict pressure effects during 

spaceflight. Below is an example of altitude effects on task completion as well as improvement upon 1 

week acclimatization when exposed to various altitudes.

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Application

10

When designing a vehicle, alterations in the partial pressure of O2 are significant to crew experience. In 

addition to altitude and humidity influences, pressure may be lowered during spaceflight due to nominal 

cabin leakage, unplanned events, and during EVAs. When total pressure is lowered, partial pressure of O2 is 

lowered, risk of hypoxia increases, and it may be necessary for the relative percent of available O2 to be 

increased to sustain crew health. 

[V2 6003] O2 Partial Pressure Range for Crew Exposure The system shall maintain inspired oxygen partial 

pressure (PI

O2) in accordance with Table 6.2-1—Inspired Oxygen Partial Pressure Exposure Ranges. From NASASTD-3001 Volume 2, Rev D

The graph represents the range of hypoxia, normoxia, and hyperoxia in 

various spaceflight vehicle baseline cabin environments and during 

extravehicular activities (EVAs) when considering barometric pressure. 

Additionally, the green vertical line marks the limit of O2 content to prevent 

pulmonary atelectasis. This graph uses psia for pressure measurement.

PB – Ambient Barometric Pressure (mmHg) FI –

Fractional concentration of inspired oxygen FIO2 – The 

dry-gas decimal fraction of ambient O2

*From Johnson Procedural Requirements (JPR) 1830.6 

(Requirements Applicable to Personnel Participating in 

Diving, Hyper/Hypobaric Chambers, and Pressurized Suit 

Operations). Page 15, subsection 4.2: Limitations during 

Oxygen Breathing,” shows the limits for prebreathe in a 

spacesuit. The limit is nine hours when that is the only 

exposure to enriched O2 in a 48-hour period. The limit is 

six hours when it is the only exposure to enriched O2 in a 

24-hour period and also states that consecutive daily 

exposures are not to exceed five consecutive days.

**There is no opportunity to collect data in microgravity 

with PIO2 of 127 mmHg to cover the durations of 

Exploration Class missions, so a health monitoring and 

mitigation plan are required to implement this condition

***This PI

O2 may be exceeded during DCS treatment.

**** 1-hour time-weighted average with an absolute 

lower limit for the minimum hypoxia range of 122 

mmHg/2.36 psia

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Application

11

Pressure/Oxygen Challenges associated with mission pressure transitions

As missions evolve from Low Earth Orbit (LEO) to long duration, many atmospheric challenges and 

considerations will occur. Programs will have to plan, as they do for current EVAs, the best way to make the 

atmosphere/pressure transition while protecting the crewmembers and mission objectives. The image 

below is an example developed by NASA to plan for Constellation missions and shows the type of pressure 

transitions that can take place and need to be accounted for in future missions.

Source: 2010 NASA EWAG Atmosphere Report

Cross Vehicle Atmosphere Considerations

[V2 6007] Rate of Pressure Change For pressure changes >1.0 psi, the rate of change of total 

internal vehicle pressure shall not exceed 13.5 psi/min.

From: NASA-STD-3001 Volume 2, Rev D

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Background

12

Physiological Effects Decompression Sickness (DCS)

Rapid alterations in ambient pressure environments can lead to a series of illnesses or injuries, frequently from 

evolution of gases in body fluids. Decompression sickness is caused by the evolution of dissolved nitrogen into 

gaseous state, forming bubbles as pressure decreases.

Types of Decompression Sickness

• Type 1: joint pain, single extremity tingling or numbness, and mild skin symptoms.

• Type 2: central nervous system or cardiovascular symptoms. Symptoms can range from muscle weakness, 

confusion, impaired balance, and stroke. Injuries can be fatal.

• Gas emboli can be classified as venous gas emboli (VGE) or arterial gas emboli (AGE).

• The risk of DCS is can be raised by high levels of activity during exposure to altered pressure environments.

See OCHMO-TB-037 Decompression Sickness (DCS) for additional information.

Example of Spaceflight-related DCS: The ISS is pressurized to 14.7 psi, equivalent to the atmospheric 

pressure at sea level. During a spacewalk, astronauts wear an EVA suit that is pressurized to 4.3 psi. If 

crewmembers did not take preventive actions, the large pressure transition when moving from the vehicle 

to an EVA suit could lead to nitrogen evolution in the body. Bubble formation could cause physical pain 

(joint nitrogen displacement), and physical damage if lodged into the cerebral or pulmonary circulation.

• To prevent DCS, NASA astronauts complete a prebreathe protocol, including breathing oxygen-rich air to 

flush out nitrogen prior to EVA. This helps prevent nitrogen evolution and development of bubbles.

• If DCS occurs, current treatment options include recompression to sea level pressure or higher 

(hyperbaric therapy), breathing 100% O2, and supportive care of any injuries or sequelae. 

• When DCS symptoms occur in space, returning to normal cabin environments may be enough to resolve 

symptoms. Alternatively, use of the cabin or even an EVA suit to raise ambient pressure to hyperbaric 

conditions may be considered. 

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Background

13

Physiological Effects of Barotrauma

Barotrauma is injury to your body caused by changes in barometric (air) or water pressure. This most often occurs 

when people travel to high altitudes or dive underwater, but additionally can occur for crew when traveling in 

space. 

• Barotrauma is most often experienced in the ear and sinuses but is also experienced in the lungs or other 

major organs. Barotrauma injuries can cause pain or, in severe cases, can be life threatening.

• Barotrauma is caused by pressure changes between ambient pressure and the pressure of gases trapped 

within closed spaces in the body.

• Injury occurs when the change in ambient pressure overcomes the capacity of the enclosed space to equalize.

• Injury is more common when the trapped gas pressures are lower than the ambient pressure, but positive 

pressure injuries have also been reported. 

Examples of Barotrauma Ear/Sinus squeeze: failure to equalize pressure of trapped gas in the middle ear 

or sinuses cause pain, bleeding, or irritation of mucosa. Teeth (dental squeeze): expansion of gas within a 

tooth causes pain, fracture, or other injury. Lung: rapid expansion of tissue gas can cause pneumothorax 

or pulmonary tissue injury, can lead to arterial gas embolism.

Pulmonary 

Barotrauma (left) 

Source: YouTube 

Respiratory 

Therapy Zone

Ear Barotrauma 

(right) Source: 

NASA

Prevention includes gradually controlled 

pressure changes, crew training in equalizing 

techniques, and pharmaceuticals including 

decongestants, pain medication and 

supplemental or supportive care when required. 

Valsalva maneuver

Source: Wikipedia 

Underwater crew pressure training

Source: NASA

Barotrauma in Space

• Changes in pressure occur during spaceflight 

missions when the crewmember is transferring 

between habitable volumes of differing 

pressures, for example when going from vehicle 

atmosphere to EVA suit.

• Future missions potentially involve additional 

pressure change scenarios. For example, a lunar 

sortie mission scenario may include several 

circumstances where pressure is expected to 

change including transfers between a pressurized 

habitat, a pressurized rover, and pressurized suit. 

• Other scenarios may involve unplanned 

contingencies i.e., damage to the vehicle causing 

unintended pressure changes. 

Tympanic membrane rupture Source: NASA

[V2 6150] Barotrauma Prevention During a commanded pressure change, the system shall pause within 

1 psi of the pause command being issued by the unsuited or suited crewmember, with ability to increase 

or decrease pressure as needed after the pause. From: NASA-STD-3001 Volume 2, Rev D

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Application

14

Partial Pressure Extravehicular Activities (EVA) Considerations 

EVAs introduce many new challenges to the atmospheric pressure and oxygen balance:

• When crewmembers prepare for an EVA, they move from the cabin atmosphere to a space suit 

pressurized at much lower pressure. A too-rapid pressure transition can increase the risk of DCS. 

• The EVA preparation time must be minimized to accommodate the overall mission objectives and 

timeline.

• Crewmembers will complete a prebreathe protocol which involves O2 inspiration either via mask or insuit to flush nitrogen from the body decreasing risk of DCS. 

• 100% O2 environment introduces increased risk of flammability, thus imposing a requirement to 

maintain ppO2 below the cabin flammability limit while conducting mask prebreathe.

• For long duration flights, a variety of pressure transitions are possible within the mission based on 

lander, rover, EVA, and lunar or planetary surface activities. 

Alternative EVA Preparation Atmosphere Conditions

To optimize EVA preparation, NASA is studying options including changing the cabin pressure/oxygen 

balance, allowing for less cabin to suit pressure transition and shorter prebreathe times. See table below:

Example of Prebreathe atmosphere environments and equivalent Air Altitudes

Source: NASA

Advantages:

• Shorter prebreathe times allow crewmembers more time for rest and other mission activities. 

• Wessel study (2018) showed exposure to mildly hypoxic environment did not show mission limiting 

events, or effects carrying significant long term health consequences. No additional symptom reports 

related to mild hypoxia in microgravity on shuttle Equivalent Air Altitude of 4,000 feet.

Concerns:

• The O2 environment introduces a mildly hypoxic environment, which although it is well tolerated on 

earth, when in combination with microgravity there is a concern that the synergistic effects are unknown 

and have potential to adversely affect health and performance. For example, the acute change in ppO2

from normoxic to potentially hypoxic can result in AMS symptoms including headache, nausea, dizziness 

or fatigue which could potentially affect performance. 

• Flammability risks must always be factored in when considering higher percent oxygen.

11/30/2023

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NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Reference Data

15

Atmosphere Pressure History 

A variety of atmospheric conditions have been developed and operated to meet vehicle and spacesuit 

needs. The atmospheres have ranged from low-pressure, pure O2 environment (Mercury, Gemini, and 

Apollo), to the space shuttle and current ISS earth sea-level pressure and O2 content (14.7 psia/21% O2). 

Apollo preflight checkout procedures initially encompassed an over-pressurization of the Command Module 

using 100% O2. After the Apollo I fire, ground procedures were modified and a mixture of 60% O2 and 40% 

nitrogen was used to reduce the fire hazard.

Source: EAWG Report 2010

Development of Atmosphere Recommendations:

The Exploration Atmospheres Working Group (EAWG) was tasked by NASA to generate recommendations 

on characteristics of internal atmospheres for exploration spacecraft, including suits and planetary 

exploration vehicles

• Three EAWG subgroups: physiological and medical, mission operations, and vehicle and habitat systems, 

were formed to focus on specific risk areas.

• Analytical methods were combined to form systems engineering process. White papers were generated 

by EAWG members and NASA personnel to provide explanations of technical and design challenges.

• This group ultimately identified selection and validation of exploration spacecraft atmospheres.

• EAWG developed the Final Report of Recommendations for Exploration Spacecraft Internal Atmospheres 

(2010).

• EAWG developed Effects of the 8 psia/32% O2 Atmosphere on the Human in the spaceflight Environment 

(2013) report which gave additional atmosphere recommendations, including EVA considerations. 

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Lessons Learned

16

Acute Depressurization of Soyuz 11

A tragic mishap involving acute depressurization occurred on a Russian Soyouz vehicle on June 29, 1971. 

Cosmonauts Georgy Dobrovolsky, Vladislav Volkov, and Viktor Patsayev were returning from a successful mission 

as the first humans to crew a space station, the Salyut 1. Their mission involved multiple science experiments to 

determine the effects of space during the longest human endurance space mission to date. During reentry, the 

crewmembers did not wear spacesuits as this was felt at the time to be redundant in a pressurized vehicle 

environment. However, on descent, radio communications abruptly ended well before expected loss of signal; 

after landing, recovery personnel opened the hatch to find all crewmembers had perished. Resuscitation was 

attempted but was unsuccessful. The resulting investigation determined that vibrations occurring during jettison 

of the orbital module caused a pressure equalization valve to release a seal at approximately 168 km altitude 

rather than the nominal 4km opening altitude. This caused crew to be exposed to near-vacuum conditions, 

leading to loss of consciousness and severe decompression sickness including ebullism, the spontaneous evolution 

of body fluids to gaseous state. Resulting injuries were fatal. Following this tragic event, multiple design changes 

were made and the use of pressure suits for dynamic operational activities (including launch and reentry) became 

common to provide improved protection from sudden changes in vehicular pressure.

[V2 11100] Pressure Suits for Protection from Cabin Depressurization The system shall provide the 

capability for crewmembers to wear pressure suits for sufficient duration during launch, entry, descent 

(to/from Earth, or other celestial body) and any operation deemed high risk for loss of crew life due to 

loss of cabin pressurization (such as in mission dockings, operations during periods of high incidence 

of micrometeoroids and orbital debris (MMOD) or complex vehicle maneuvers).

Refer to OCHMO-TB-038 Decompression & LEA Suits Mishaps and OCHMO-TB-039 Entry-Landing 

Mishaps for additional information.

Cosmonauts Georgy Dobrovolsky, Vladislav 

Volkov, and Viktor Patsayev

Source: NASA

Memorial to the lost cosmonauts of Soyuz 11

Source: NASA

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Back-Up

17 11/30/2023

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NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Original  Rev A

• Updated information to be consistent with NASA-STD-3001 

Volume 1 Rev C and Volume 2 Rev D.

Major Changes Between Revisions

18 11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

NASA-STD-3001 Volume 1 Revision C

[V1 3003] In-Mission Preventive Health Care All programs shall provide training, in-mission capabilities, 

and resources to monitor physiological and psychosocial well-being and enable delivery of in-mission 

preventive health care, based on epidemiological evidence-based probabilistic risk assessment (PRA),

individual crewmember needs, clinical practice guidelines, flight surgeon expertise, historical review, 

mission parameters, and vehicle derived limitations. These analyses consider the needs and limitations

of each specific vehicle and design reference mission (DRM) with particular attention to parameters such as 

mission duration, expected return time to Earth, mission route and destination, expected radiation

profile, concept of operations, and more. In-mission preventive care includes, but is not limited to: (see 

NASA-STD-3001 Volume 1 Rev C for full technical requirement).

[V1 3004] In-Mission Medical Care All programs shall provide training, in-mission medical capabilities,

and resources to diagnose and treat potential medical conditions based on epidemiological evidence-based 

PRA, individual crewmember needs, clinical practice guidelines, flight surgeon expertise, historical

review, mission parameters, and vehicle-derived limitations. These analyses consider the needs and 

limitations of each specific vehicle and design reference mission (DRM) with particular attention to 

parameters such as mission duration, expected return time to Earth, mission route and destination, 

expected radiation profile, concept of operations, and more. In-mission capabilities (including hardware and 

software), resources (including consumables), and training to enable in-mission medical care, and 

behavioral care, are to include, but are not limited to: (see NASA-STD-3001 Volume 1 Rev C for full technical 

requirement).

[V1 5002] Crewmember Training Beginning with the astronaut candidate year, general medical training, 

including, but not limited to, first aid, cardiopulmonary resuscitation (CPR), altitude physiological training, 

carbon dioxide exposure training, familiarization with medical issues, procedures of space flight, 

psychological training, and supervised physical conditioning training shall be provided to the astronaut 

corps.

NASA-STD-3001 Volume 2 Revision D

[V2 4015] Aerobic Capacity The system shall be operable by crewmembers with the aerobic capacity as 

defined in NASA-STD-3001, Volume 1.

[V2 6001] Trend Analysis of Environmental and Suit Data The system shall provide environmental and suit 

monitoring data in formats compatible with performing temporal trend analyses.

[V2 6002] Inert Diluent Gas Cabin atmospheric composition shall contain at least 30% diluent gas 

(assuming balance oxygen). 

[V2 6006] Total Pressure Tolerance Range for Indefinite Crew Exposure The system shall maintain the 

pressure to which the crew is exposed to between 34.5 kPa < pressure ≤ 103 kPa (5.0 psia < pressure ≤ 15.0 

psia) for indefinite human exposure without measurable impairments to health or performance. 

[V2 6007] Rate of Pressure Change For pressure changes >1.0 psi, the rate of change of total internal 

vehicle pressure shall not exceed 13.5 psi/min. 

Referenced Technical Requirements

19

View the current versions of NASA-STD3001 Volume 1 & Volume 2 on the 

OCHMO Standards website

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

NASA-STD-3001 Volume 2 Revision D

[V2 6008] Decompression Sickness (DCS) Risk Identification Each program shall define mission unique DCS 

mitigation strategies to achieve the level of acceptable risk of DCS as defined below within 95% statistical 

confidence:

a. DCS ≤ 15% (includes Type I or isolated cutis marmorata). 

b. Grade IV venous gas emboli (VGE) ≤ 20%. 

c. Prevent Type II DCS. 

[V2 6009] Decompression Sickness Treatment Capability The system shall provide DCS treatment 

capability.

[V2 6017] Atmospheric Control The system shall allow for local and remote control of atmospheric 

pressure, humidity, temperature, ventilation, and ppO2.

[V2 6020] Atmospheric Data Recording For each isolatable, habitable compartment, the system shall 

automatically record pressure, humidity, temperature, ppO2, and ppCO2 data continuously. 

[V2 6021] Atmospheric Data Displaying The system shall display real-time values for pressure, humidity, 

temperature, ppO2, and ppCO2 data to the crew locally and remotely.

[V2 6022] Atmospheric Monitoring and Alerting Parameters The system shall alert the crew locally and 

remotely when atmospheric parameters, including atmospheric pressure, humidity, temperature, ppO2, and 

ppCO2 are outside safe limits.

[V2 6107] Nominal Vehicle/Habitat Atmospheric Ventilation The system shall maintain a ventilation rate 

within the internal atmosphere that is sufficient to provide circulation that prevents CO2 and thermal 

pockets from forming, except during suited operations, toxic cabin events, or when the crew is not 

inhabiting the vehicle.

[V2 6108] Off-Nominal Vehicle/Habitat Atmospheric Ventilation The system shall control for ppO2, ppCO2, 

and relative humidity during off-nominal operations, such as temporary maintenance activities in areas not 

in the normal habitable volume.

[V1 6150] Barotrauma Prevention During a commanded pressure change, the system shall pause within 1 

psi of the pause command being issued by the unsuited or suited crewmember, with ability to increase or 

decrease pressure as needed after the pause.

[V2 7041] Environmental Control During Exercise The system environmental control shall accommodate 

the increased O2 consumption and additional output of heat, CO2, perspiration droplets, odor, and 

particulates generated by the crew in an exercise area.

[V2 9053] Protective Equipment Protective Equipment shall be provided to protect the crew from expected 

hazards.

[V2 11032] LEA Suited Decompression Sickness Prevention Capability LEA spacesuits shall be capable of 

operating at sufficient pressure to protect against Type II decompression sickness in the event of a cabin 

depressurization.

[V2 11100] Pressure Suits for Protection from Cabin Depressurization The system shall provide the 

capability for crewmembers to wear pressure suits for sufficient duration during launch, entry, descent 

(to/from Earth, or other celestial body) and any operation deemed high risk for loss of crew life due to loss 

of cabin pressurization (such as in mission dockings, operations during periods of high incidence of 

Micrometeoroids and Orbital Debris (MMOD) or complex vehicle maneuvers).

Referenced Technical Requirements

20

View the current versions of NASA-STD3001 Volume 1 & Volume 2 on the 

OCHMO Standards website

11/30/2023

Rev A
NASA-STD-3001 Technical Brief

OCHMO-TB-003 Habitable Atmosphere

NASA Office of the Chief Health & Medical Officer (OCHMO)

This Technical Brief is derived from NASA-STD-3001 and is for reference only.

It does not supersede or waive existing Agency, Program, or Contract requirements.

Reference List

1. Abercromby et al. (2019). Suited ground vacuum chamber testing decompression sickness tiger team report. 

Houston, TX: NASA Johnson Space Center. NASA Technical Publication NASA/TP-2019-220343.

2. Abercromby AFJ, Conkin J, Gernhardt ML. (2015). Modeling a15-min extravehicular activity prebreathe protocol 

using NASA's exploration atmosphere (56.5kPa/34% O2). Acta Astronautica, 109: 76–87.

3. Conkin J. (2001). Evidence-based approach to the analysis of serious decompression sickness with application to 

EVA astronauts. NASA Technical Publication 2001-210196, Houston: Johnson Space Center.

4. Conkin J. (2011). Preventing decompression sickness over three decades of extravehicular activity. NASA 

Technical Publication NASA/TP-2011-216147, Johnson Space Center.

5. Conkin J. (2020). Probability of decompression sickness and venous gas emboli from 49 NASA hypobaric chamber 

tests with reference to Exploration Atmosphere. Houston, TX: NASA Johnson Space Center. NASA Technical 

Publication NASA/TP-2020-220529.

6. Conkin J, Abercromby AFJ, Dervay JP, Feiveson AH, Gernhardt ML, Norcross JR, Ploutz-Snyder R, Wessel JH, III. 

Hypobaric decompression sickness treatment model. (2015). Aerosp Med Hum Perform, 86: 508-17.

7. Conkin J, Abercromby AFJ, Dervay JP, Feiveson AH, Gernhardt ML, Norcross J, Ploutz-Snyder R, Wessel JH, III. 

(2014). Probabilistic assessment of treatment success for hypobaric decompression sickness. NASA Technical 

Publication NASA/TP-2014-218561. NASA Johnson Space Center, Houston, TX. 

8. Conkin J, Gernhardt ML, Powell MR, Pollock NW. (2004). A probability model of decompression sickness at 4.3 

psia after exercise prebreathe. NASA Technical Publication NASA/TP-2004-213158. Johnson Space Center, 

Houston, TX.

9. Conkin J, KV Kumar, MR Powell, PP Foster, JM Waligora. (1996). A probability model of hypobaric decompression 

sickness based on 66 chamber tests. Aviat Space Environ Med, 67: 176-83.

10. Conkin J, Pollock NW, Natoli MJ, Martina SD, Wessel JH, III, Gernhardt ML. (2017). Venous gas emboli and 

ambulation at 4.3 psia. Aerosp Med Hum Perform, 88: 370-76.

11. Conkin J, Powell MR. (2001). Lower body adynamia as a factor to reduce the risk of hypobaric decompression 

sickness. Aviat Space Environ Med, 72: 202-14.

12. Gernhardt ML, Dervay JP, Waligora JM, Fitzpatrick DT, Conkin J. Extravehicular Activities (Chap. 5.4). In: Risin D, 

Stepaniak PC, eds. Biomedical Results of the Space Shuttle Program. Washington, DC: U.S. Government Printing 

Office, NASA/SP-2013-607, 2013; 315-26.

13. Henninger D. (2010). NASA Exploration Atmospheres Working Group Recommendations for exploration 

spacecraft internal atmospheres: The final report of the NASA Exploration Atmospheres Working Group. NASA 

Technical Publication NASA/TP-2010-216134, . Johnson Space Center, Houston, TX. October 2010. 

14. Waligora JM, DJ Horrigan, Jr., J Conkin, AT Hadley, III. (1984). Verification of an altitude decompression sickness 

protocol for Shuttle operations utilizing a 10.2 psi pressure stage. NASA Technical Memorandum 58259, Johnson 

Space Center, Houston, TX.

15. Webb JT, Krock LP, Gernhardt ML. (2010). Oxygen Consumption at Altitude as a Risk Factor for Altitude 

Decompression Sickness. Aviation, Space, and Environmental Medicine, 81(11).

16. 17.Sharma,Sandeep, Hashmi, Muhammad. Partial Pressure of Oxygen. National Library of Medicine, National 

Center for Biotechnology Information September 26,2022.

17. Brombacher, W. G. “Altitude by Measurement of Air Pressure and Temperature.” Journal of the Washington 

Academy of Sciences, vol. 34, no. 9, 1944, pp. 277–99. JSTOR, http://www.jstor.org/stable/24531101. Accessed 

16 Feb. 2023 .

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