Educational only — not for clinical decisions

These tools illustrate published standards and research. They are simplified, unvalidated and not reviewed. Do not use them to diagnose, treat, clear anyone to fly, or plan real operations. All calculations run in your browser; nothing you enter is stored or sent.

TLS-02 · Interactive tool · Tools

Spacewalk Oxygen Prebreathe Planner

Also: EVA prebreathe calculator · decompression tissue ratio · R-value calculator

Hostile/Closed Environment Pulmonary Musculoskeletal Neurological & Cognitive EVA & Surface Ops LEO long-duration Lunar surface Mars transit & surface Clinician Student Operator / Spaceflight participant Astronaut & candidate

draft AI-assisted Last reviewed: Pending review · Reviewer: Pending review

Bottom line

Breathing 100% oxygen before a spacewalk removes tissue nitrogen; NASA models this with a 360-minute half-time tissue and a tissue ratio (R) = tissue nitrogen ÷ suit pressure [1][2]. ◆◆◆◇ Moderate · A E
For the U.S. suit at 4.3 psia, R ≤ 1.65 has historically been accepted [1]; from sea-level air the model needs about 4 h 15 min of resting oxygen. ◆◆◆◇ Moderate · A E
Real protocols (campout, exercise, in-suit light exercise) are validated in chamber trials to below 4% decompression sickness probability with 95% confidence [1]. ◆◆◆◆ High · E
Educational model only: it ignores exercise, mask leaks and individual factors.

Proposed certainty (◆ High → ◇ Very low) and evidence-type tags: F flight · A analog · M model/animal · C computational · E expert/operational/standards. AI-proposed, pending reviewer confirmation. How to read the grades.

Planner · oxygen prebreathe & tissue ratio

Educational model · never use to plan a real decompression

Starting condition Values from the NASA evidence report [2]

Spacesuit Reference limits [1][2]

Prebreathe on 100% oxygen: 240 min0–8 hours, resting

Prebreathe start (Eastern Time)

Tissue ratio at suit pressure—reference limit —

Model minimum prebreathe—to reach the reference ratio

Prebreathe ends—

Tissue ratio (lower is less decompression stress)—

limit

1.01.52.02.53.0

Curve: R(t) = R(0) × e^{−ln2·t/360}, the 360-minute tissue model NASA uses [1][2]. Dashed line: reference limit for the suit.

Assumes perfect 100% oxygen with no break (any breath of cabin air restarts nitrogen uptake [1]), a person at rest, and an instant pressure drop. Real NASA protocols add exercise or staged pressure and are proven in chamber trials [1]. Nothing you enter leaves your browser.

How it works

Before a spacewalk (EVA), astronauts breathe pure oxygen so nitrogen leaves their tissues. Otherwise the drop to low suit pressure can form bubbles and cause decompression sickness (DCS) [1].

NASA summarizes decompression stress with the tissue ratio (R-value): nitrogen pressure in a model tissue divided by suit pressure [1][2].

Step	Formula or value	Source
Nitrogen washout	P1N2 = P0 + (Pa − P0)(1 − e^−kt); breathing 100% oxygen makes Pa ≈ 0	[2]
Rate constant	k = ln 2 ÷ 360 minutes (a 360-minute half-time tissue, chosen because it best matched NASA and U.S. Air Force chamber results)	[1][2]
Tissue ratio	R = P1N2 ÷ suit pressure	[1]
Start: sea-level air	11.6 psia tissue nitrogen at 14.7 psia	[2]
Start: 10.2 psia, 26.5% oxygen	Tissues equilibrate to about 7.5 psia nitrogen	[2]
U.S. EMU suit at 4.3 psia	R ≤ 1.65 historically accepted	[1][2]
Russian Orlan suit at 5.8 psia	R of about 1.85 gives similar risk to the EMU	[2]

Minimum prebreathe in the model = 360 × log₂(P0 ÷ (R limit × suit pressure)) minutes. For sea-level air and the EMU this is about 4 h 15 min.

NASA protocols for comparison

Protocol	What happens
In-suit 4-hour prebreathe	100% oxygen in the suit at 14.7 psia for 4 hours [1]
Campout	Night before the EVA in the airlock at 10.2 psia, 26.5% oxygen (limited to 8 h 40 min), which shortens in-suit prebreathe [1]
Exercise prebreathe	Short, intense exercise on 100% oxygen at 14.7 psia plus in-suit prebreathe at 10.2 psia [1]
In-suit light exercise (ISLE)	40 min of 100% oxygen by mask, 20-min depressurization to 10.2 psia, then mild exercise in the suit [1]

Exercise speeds nitrogen removal, which this calculator does not model [2]. Real protocols are accepted only after chamber testing shows below 4% DCS probability with 95% confidence [V2 6008] [1].

Clinical takeaways for any clinician

Most DCS symptoms appear 60–240 minutes after decompression [1]. Ask about joint pain, numbness, rash or breathing symptoms after a spacewalk, altitude chamber run or dive.
A breath of cabin air during prebreathe restarts nitrogen uptake; operationally it can add time or restart the protocol [1].
An R-value at the limit does not mean zero risk [2].

Evidence profile

Overall certainty: ◆◆◆◇ Moderate (proposed)
Evidence types: A · Terrestrial analogE · Expert / operational / standards
Literature through: 2026-10-14
Validation: Formulas checked against the cited sources; tool output not clinically validated
Credit: Educational only; no CME or other credit

Requirements referenced 1

Requirement	Title	Plain-language meaning
[V2 6008]	DCS Risk Identification	Each program's decompression plan must show below 4% chance of decompression sickness with 95% confidence in ground chamber tests, with no serious cases.

IDs as published in NASA-STD-3001 Vol. 1 Rev C (2023-09-15) and NASA-STD-3001 Vol. 2 Rev F (2026-07-14). NASA-STD-3001 binds NASA programs; it is not a regulation for commercial flights. Paraphrased. Standards quick reference.

References 2

NASA OCHMO. Decompression Sickness (DCS) Risk Mitigation. Technical Brief OCHMO-TB-037, Revision D (2026). www.nasa.gov/wp-content/uploads/2023/12/ochmo-tb-037-decompr… NASA E
NASA Human Research Program. Evidence Report: Risk of Hypobaric Hypoxia and Decompression Sickness (2024). Section "Tissue Ratio", Equations 2–3. humanresearchroadmap.nasa.gov/evidence/reports/Evidence%20Re… NASA A

Machine-checked on 2026-10-14: each URL was fetched, each DOI's title and year matched via Crossref, PubMed IDs matched to DOIs, and every number used in a calculation was checked against the source text (abstract, NASA document, regulation or official web page). Human verification against full text is still required before review.

Article info

Authors: Pending (AI-assisted draft)
Reviewer: Pending review
Last reviewed: Pending review
Literature through: 2026-10-14
AI-assisted: Yes (disclosed)
COI: Pending
License: CC BY-SA 4.0 (text); code MIT
Privacy: Runs entirely in your browser; no inputs stored or sent
Use: Educational only · not for clinical decisions · Disclaimer
Cite this version: Pending first edition

Educational use only · not medical advice · not for clinical decisions · Disclaimer · CC BY-SA 4.0