Chapter 3: Altitude & Oxygen | Human Performance & Limitations

§ 4Partial Pressure of Oxygen — Effects of Increasing Altitude

As you climb, the total barometric pressure drops, and with it the partial pressure of oxygen (pO₂). Because the diffusion of oxygen across the alveolar membrane depends on the pressure gradient, what matters to your blood is not the percentage of O₂ — it is the pO₂. Here are the standard values the DGCA expects you to know:

Standard barometric pressure & oxygen availability at altitude
Altitude	Standard Barometric Pressure	O₂ Available (% of Sea Level)	Pilot Significance
Sea Level (0000 ft)	101 kPa (760 mmHg)	100 %	Normal physiological baseline.
8,000 ft	77 kPa (574 mmHg)	76 %	Cabin pressure is maintained between 6,000 – 8,000 ft.
12,000 ft	66 kPa (496 mmHg)	65 %	Lower edge of the Physiological-Deficient Zone.
18,000 ft	53 kPa (395 mmHg)	52 % (≈ half)	Half the O₂ of sea level.
24,000 ft	41 kPa (311 mmHg)	41 %	Without supplemental O₂ — incapacitation imminent.
36,000 ft	25 kPa (187 mmHg)	25 % (one-fourth)	Only ¼ of sea-level oxygen.

Three deductions you must be able to state in the exam

The availability of oxygen and barometric pressure DECREASE with altitude.
The oxygen available is one-fourth at 36,000 ft and half at 18,000 ft of the oxygen available at sea level.
Atmospheric pressure drops faster at lower altitudes in comparison to the same altitude changes at higher altitudes (the pressure–altitude curve is exponential, not linear).

Oxygen available vs altitude — Figure 3.1 — The oxygen available falls non-linearly with altitude: half is gone by 18,000 ft, three-quarters by 36,000 ft.

§ 5The Physiological Zones of the Atmosphere

The atmosphere is divided — from the pilot's body's point of view — into four functional zones, each demanding a different protective strategy. You must know the boundaries and the SOP for each.

Figure 3.2 — The four physiological zones of the atmosphere and the protection each demands.

5.1 The Physiological Zone

SOP — Within this zone Normal healthy human beings used to living near sea level will need supplementary oxygen to function normally at altitude exceeding 10,000 to 12,000 ft. Pilots will normally begin breathing supplementary oxygen from 10,000 ft above sea level.

Oxygen supply regime by altitude — Figure 3.3 — The oxygen-supply regime by altitude: air only, oxygen+air, 100% oxygen, then oxygen under pressure.

5.2 The Physiological-Deficient Zone

Limits & symptoms — strict Exists from 12,000 ft to 50,000 ft. The body is not used to this environment. The adverse effects include:

Middle ear and sinus blockage
Shortness of breath
Dizziness
Headache

Compensatory threshold At 6,000 – 7,000 ft altitude the human organism starts with remarkable measures to compensate for the drop in pO₂ when climbing — this is the threshold for compensatory reactions. (Hyperventilation, raised heart rate, raised cardiac output, etc. — all later sections.)

Critical-threshold limits — non-pressurised flight

A pilot climbing in a non-pressurised aircraft and WITHOUT using supplemental oxygen will pass the "critical threshold" at approximately 22,000 ft.
Breathing 100 % oxygen will lift the pilot's physiological safe altitude to approximately 38,000 ft.

5.3 The Partial Space-Equivalent Zone

Definition & extent This zone extends from 50,000 ft to 120 nm (nautical miles). Even 100 % oxygen ceases to be enough at these heights — total pressure becomes the limiting factor.

5.4 The Total Space-Equivalent Zone

Definition The Total Space-Equivalent Zone extends outwards from 120 nm.

Mandatory limits in the upper two zones

100 % oxygen does NOT protect from hypoxia — pressure-breathing required.
Sealed cabins and pressure suits are a MUST.
Blood and body fluids BOIL above 63,000 ft (Armstrong's Line — vapour pressure of body fluids equals ambient pressure).
Gravitational changes affect the body (micro-G adaptation, fluid shift, motion-sickness, etc.).

Altitude versus pilot protection timeline — Figure 3.4 — Altitude-versus-protection timeline, from air-only at low level to pressure suits and Armstrong's Line.

§ 6Self-Check & Memory Aids

6.1 Numbers you MUST know by heart

Master cheat-sheet — Part 1 numbers
Parameter	Exact Value	Where it comes from
Top of troposphere (average)	13 km / 8.1 mi / 43,000 ft	§1 Atmospheric layers
Top of stratosphere (over)	100,000 ft	§1
Troposphere holds	75 % of atmospheric mass	§1
Nitrogen / Oxygen / Other	78.084 / 20.95 / ~1 %	§1.2
Water vapour content	1 – 4 % (variable)	§1.2
ISA sea-level pressure	1013.25 hPa = 760 mmHg = 29.92 inHg = 14.7 psi	§3.1
ISA lapse rate	1.98 °C / 1000 ft	§3.1
Tropopause altitude (ISA)	36,090 ft	§3.1
Isothermal temperature	−56.5 °C	§3.1
Cabin altitude maintained	6,000 – 8,000 ft	§4
O₂ available at 18,000 ft	52 % (half of SL)	§4
O₂ available at 36,000 ft	25 % (one-fourth of SL)	§4
Compensatory reactions begin	6,000 – 7,000 ft	§5.2
Pilot starts supplementary O₂	10,000 ft	§5.1
Physiological-Deficient Zone	12,000 – 50,000 ft	§5.2
Critical threshold (no O₂, unpressurised)	~22,000 ft	§5.2
Safe altitude on 100% O₂	~38,000 ft	§5.2
100% O₂ Under Pressure (PBA) from	Above 40,000 ft	§5.1 fig.
Partial Space-Equivalent Zone	50,000 ft – 120 nm	§5.3
Armstrong's Line (body fluids boil)	63,000 ft	§5.4
Total Space-Equivalent Zone	Outwards from 120 nm	§5.4

6.2 Common DGCA-style probe questions

Try answering these without looking back

State the percentage composition of nitrogen and oxygen in the atmosphere. Does it change with altitude?
Give the ISA sea-level values in five different units.
What is the ISA lapse rate, and up to what altitude does it apply?
Why does hypoxia occur with altitude, when the % of O₂ is unchanged? (Hint: Dalton's Law.)
At what altitude does a pilot normally start using supplementary oxygen?
State the four physiological zones with their altitude boundaries.
At what altitude does an unpressurised pilot, NOT using supplemental O₂, hit the critical threshold?
Why is a pressure suit mandatory above 50,000 ft? Quote Armstrong's Line value.
Explain Boyle's Law and give two flight-medical situations where it applies.
How does Henry's Law explain both hypoxia and decompression sickness?

Mnemonic — 5 Gas Laws"Bad Hens Cluck, Geese Dance" → Boyle · Henry · Charles · Graham · Dalton.

Mnemonic — Zones (low → high)P – P-D – PS – TS → Physiological · Physiological-Deficient · Partial Space-equivalent · Total Space-equivalent. (Or: "Pilots Probably Prefer Tea" — P, PD, PS, TS.)

Mnemonic — Halving rule"Half by Eighteen, Quarter by Thirty-Six" → Half of sea-level O₂ at 18,000 ft; one-quarter at 36,000 ft.