NASA-STD-3001 Technical Brief

Relevant Technical 

Requirements

Decompression 

Sickness (DCS) Risk 

Mitigation

OCHMO-TB-037

Rev C 

NASA-STD-3001 Volume 1, Rev C

[V1 3003] In-Mission Preventive Health Care

[V1 3004] In-Mission Medical Care

[V1 5001] Medical Training

[V1 5002] Crewmember Training

[V1 5003] Crew Medical Officer Medical 

Training

NASA-STD-3001 Volume 2, Rev D

[V2 6002] Inert Diluent Gas

[V2 6003] O2 Partial Pressure Range for Crew 

Exposure

[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

[V2 6022] Atmospheric Monitoring and 

Alerting Parameters

[V2 7043] Medical Capability

[V2 9053] Protective Equipment

[V2 11100] Pressure Suits for Protection 

from Cabin Depressurization

[V2 11032] LEA Suited Decompression 

Sickness Prevention Capability

[V2 11036] Suited Atmospheric Monitoring 

and Alerting

Executive Summary

Denitrogenation protocols (removing nitrogen from 

the body) are employed to minimize the risk of 

decompression sickness (DCS), which is the 

formulation of gas emboli (bubbles) during suited 

extravehicular activities (EVAs). Moving from a higher 

pressure to a lower pressure too quickly and without 

adequate denitrogenation can cause bubbles to form 

in the body. DCS should also be minimized during offnominal events such as cabin depressurization. If a 

suit (LEA-Launch, Entry, Abort) is utilized during a 

cabin depressurization event, it requires sufficient 

pressure relative to the initial cabin pressure to be 

effective. DCS mitigation protocols are implemented 

through the combination of habitat, EVA suit pressure, 

and breathing gas procedures to achieve nominal 

mission operations. 

Houston We Have a Podcast: 

The NASA research into preventing 

decompression sickness for astronauts who will 

walk on the Moon in future Artemis missions.

Ep269: Locked In, Pressure Low | NASA
NASA-STD-3001 Technical Brief

OCHMO-TB-037

Decompression Sickness (DCS) Risk 

Mitigation

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

Decompression Sickness Mitigation/Prevention/Treatment

• DCS is a major concern in spaceflight, as well as in certain terrestrial settings, due to the 

negative consequences during and after DCS occurrence.

• DCS is the result of nitrogen bubbles (or other gas emboli) causing damage to tissue.

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

• VGE are removed from circulation by the lungs, whereas AGE compromises tissue 

oxygenation and therefore is more serious.

• Conkin, et al. (3) describes DCS as, “[When an] astronaut travels to a hypobaric 

environment, the amount of inert gas in excess of what can be held in solution at the 

new lower pressure has the potential to come out of solution to form gas spaces that 

can displace or otherwise damage tissues.”

• DCS is typically mitigated with prebreathe protocols – denitrogenation of the body with 

oxygen.

• The risk of DCS is shown to increase during ambulation and periods of increased lower body 

activity on Earth.

Risks of Decompression Sickness

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

• Type II: central nervous system or cardiovascular symptoms (potentially fatal). Symptoms can 

range from muscle weakness, confusion, impaired balance, and stroke.

10/11/2023

Rev C 2

1. Central DCS is when 

bubbles get into the 

arterial blood stream 

and move to the brain, 

which can cause an 

arterial gas embolism.

2. Pulmonary DCS is very 

rare and only occurs 

when bubbles are in the 

lungs.

3. Neurological DCS is 

when nitrogen bubbles 

affect the nervous 

system and causes 

problems in the body.

The goal is to limit DCS risk 

to within acceptable levels 

through validated 

prebreathe protocols.

1

2 3
NASA-STD-3001 Technical Brief

OCHMO-TB-037

Decompression Sickness (DCS) Risk 

Mitigation

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.

Decompression Sickness Mitigation/Prevention/Treatment

Reference Data

3

Past NASA Decompression Protocols for Prevention of DCS

NASA has never experienced a Type II event in spaceflight

In-Suit 4-Hour Prebreathe

Astronaut breathes 100% O2 in the spacesuit at 14.7 psia for 4 hours.

Campout Protocol

Significantly reduces the required in-suit prebreathe duration by having EVA 

crewmembers “camp out” in the ISS airlock at 10.2 psia, 26.5% O2 during the 

night prior to their EVA. For various operational reasons, the time at 10.2 psia

is limited to 8 hours and 40 minutes.

Exercise Protocol

Intense, short exercise regimen at 14.7 psia while breathing 100% O2 combined 

with in-suit prebreathe at 10.2 psia.

In-Suit Light Exercise (ISLE) Protocol

A longer period of mild exercise in the EMU. The ISLE protocol shares many 

steps with the exercise prebreathe protocol but differs in that 40 minutes are 

spent breathing 100% O2 by mask followed by a 20-minute depressurization to 

10.2 psia.

Notes:

• DCS risk is significantly increased by ambulation and physical activity during 

altitude exposure, meaning ISS microgravity prebreathe protocols are not 

effective for planetary EVA.

• Apollo used a 100% O2 cabin environment which eliminated DCS risk during 

EVA.

Past prebreathe protocols were between 3-5 hours. Utilizing a lower cabin 

pressure/higher oxygen concentration such as 8.2 psia with 34% O2 can minimize 

the time to denitrogenate the body, which saves time and consumables. 

The following pages detail a comparison of prebreathe durations and atmosphere 

compositions.

10/11/2023

Rev C
NASA-STD-3001 Technical Brief

OCHMO-TB-037

Decompression Sickness (DCS) Risk 

Mitigation

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

Design Guidance

• The key to preventing DCS is to denitrogenate the body by breathing higher levels of oxygen and 

optimizing the pressures of the cabin and the suit.

• DCS requires cross discipline activities to provide effective mitigation:

• Habitat pressure and oxygen concentration

• Suit pressure and oxygen concentration

• Duration of prebreathe is determined by the pressure, percent oxygen, and nitrogen of the 

atmosphere. See the graph on the following page for prebreathe estimates based on 2.48 psia PI

O2

and 4.3 psia space suit operating pressure.

• Ineffective designs can consume larger amounts of oxygen and take more time prior to initiating an 

EVA (from many hours to minutes).

• Treatment is performed using the total pressures from the vehicle and suit atmospheres.

• When DCS symptoms occur, the suit and cabin pressure may be combined to resolve 

symptoms, or the cabin pressure needs to be increased. A minimum of 16.4 psia is 

recommended but is scenario dependent.

• Type II DCS prevention during a cabin depressurization may require the use of a suit (e.g., LEA suit).

• The suit should be able to reach sufficient pressure to lessen DCS symptoms, if they cannot be 

prevented altogether.

• An LEA suit pressure of 5.8 psia has been shown to reduce the risk of Type II DCS to <15% for a 

rapid depressurization. For more information, reference OCHMO-TB-038 Decompression and 

LEA Suit Mishaps.

4 10/11/2023

Rev C

Astronaut Joe Acaba assisting 

crewmates Randy Bresnik and Mark 

Vande Hei before a spacewalk. 

Bresnik and Vande Hei were 

performing the In-Suit Light Exercise 

protocol where the astronauts 

perform light exercise and prebreathe pure oxygen before beginning 

a spacewalk. The technique helps 

purge nitrogen bubbles from their 

bodies to prevent them from 

experiencing “the bends” at the lower 

pressure inside the space suit.
NASA-STD-3001 Technical Brief

OCHMO-TB-037

Decompression Sickness (DCS) Risk 

Mitigation

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.

Prebreathe Estimates vs. O2 Concentration & Cabin Pressure

The graph below is an estimate of prebreathe times vs. oxygen levels and cabin pressure at 

a hypoxic limit of 2.48 PI

O2 for EVAs with a suit pressure of 4.3 psia. At 8.2 psia and 34% O2 

the prebreathe would be minimal. These values need to be validated prior to 

implementation.

Reference Data

5 10/11/2023

Rev C

Prebreathe Estimates based on 2.48 psia PIO2 and 4.3 psia space suit operating pressure

Note: The Apollo program used a 5.0 psia 100% O2 cabin atmosphere, thereby eliminating DCS risk and 

the need for prebreathe. 
NASA-STD-3001 Technical Brief

OCHMO-TB-037

Decompression Sickness (DCS) Risk 

Mitigation

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

6 10/11/2023

Rev C
NASA-STD-3001 Technical Brief

OCHMO-TB-037

Decompression Sickness (DCS) Risk 

Mitigation

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.

Rev B  Rev C

• Updated information to be consistent with NASA-STD-3001 Volume 1 Rev 

C and Volume 2 Rev D

Rev A  Rev B

• Updated information to be consistent with NASA-STD-3001 Volume 1 Rev 

B and Volume 2 Rev C

Original  Rev A

Slide 1

• Minor update to Executive Summary

Slide 2

• Updated Background information 

Slide 3 

• Past Protocols updated with additional information 

• Additional notes added 

• Table and references removed and updated on additional slides 

Application Notes slide removed completely 

Slide 5 

• Added slide to capture new pre-breathe table 

Slide 7

• Added slide detailing Referenced Standards 

Slide 8

• Added slide with additional references

Major Changes Between Revisions

7 10/11/2023

Rev C
NASA-STD-3001 Technical Brief

OCHMO-TB-037

Decompression Sickness (DCS) Risk 

Mitigation

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 NASASTD-3001, Volume 1 Rev C for full standard).

[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 evidencebased 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 

standard).

[V1 5001] Medical Training Medical training shall be provided to crewmembers, flight surgeons

(FSs), mission control support staff, and other ground support personnel (GSP).

[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.

[V1 5003] Crew Medical Officer Medical Training Crewmembers who have received a mission assignment 

as a Crew Medical Officer (CMO) shall be provided with detailed and specific medical training, including 

but not limited to, health issues, space physiology, behavioral health, medical procedures, medical 

equipment, toxicology, and countermeasures.

NASA-STD-3001 Volume 2 Revision D

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

(assuming balance oxygen). 

[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.

[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 psia < pressure ≤ 15.0 

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

Referenced Technical Requirements

8 10/11/2023

Rev C

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

OCHMO Standards website
NASA-STD-3001 Technical Brief

OCHMO-TB-037

Decompression Sickness (DCS) Risk 

Mitigation

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 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. 

[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 6022] Atmospheric Monitoring and Alerting Parameters The system shall display real-time values for 

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

[V2 7043] Medical Capability A medical system shall be provided to the crew to meet the medical

requirements of NASA-STD-3001, Volume 1.

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

expected hazards.

[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).

[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 11036] Suited Atmospheric Monitoring and Alerting Suits shall monitor suit pressure, ppO2, and 

ppCO2 and alert the crewmember when they are outside safe limits.

Referenced Technical Requirements

9 10/11/2023

Rev C

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

OCHMO Standards website
NASA-STD-3001 Technical Brief

OCHMO-TB-037

Decompression Sickness (DCS) Risk 

Mitigation

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 AFJ, Anchondo C, Blanco RA, Cerimele MP, Conkin J, Counts CA, Cox SR, Dervay JP, Gernhardt ML, 

Moynihan S, Ross SD, Sanders RW. (2019). Suited ground vacuum chamber testing decompression sickness tiger 

team report. Houston, TX: NASA Johnson Space Center. NASA Technical Publication NASA/TP-2019220343. 

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 adynamiaas 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. 

14. 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. 

15. 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. 

16. Webb JT, KrockLP, GernhardtML. (2010). Oxygen Consumption at Altitude as a Risk Factor for Altitude 

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

10 10/11/2023

Rev C