FASTmagazine 38 - July 2006

HYPOXIA -

AN INVISIBLE ENEMY -

CABIN DEPRESSURIZATION EFFECTS ON HUMAN PHYSIOLOGY

Operating at high altitude without
adequate understanding, training
or equipment protection can be
dangerous as shown by the follow-
ing extracts from two accident
reports:

‘

One of the first encounters with

the dangers of high altitude flight
was reported in 1862 when a bal-
loon flight was made to study the
effects of low ambient pressure.
The balloon ascended to approxi-
mately 29,000ft and during the
flight a series of “strange” symp-
toms, notably loss of visual and
hearing capability, paralysis of
arms and legs, and finally, uncon-
sciousness occurred. The team
could have been lost, but was
saved by one member pulling the
balloon valve rope with his teeth
(his arms were already paral-
ysed), to descend the balloon. The
team recovered as the balloon
descended, but this marked for the
first time the risk of low ambient
pressure.’

‘In 1998 a decompression incident
occurred on an aircraft at
35,000ft. Both the captain and the
first officer had received altitude-
chamber training during their pre-
vious military careers and knew
about the effects of low cabin
pressure. The first officer attempt-
ed to control the cabin rate of
climb by switching to the standby
pressurization system. When use
of the standby system failed to
improve the situation, he donned
his oxygen mask. The captain, who
had been talking with a passenger
who was visiting the flight deck,
attempted to don his oxygen mask
too, but in doing so he knocked his
glasses to the floor. When trying to
retrieve them he lost conscious-
ness and slumped forward. The
first officer attempted to help the
captain but was unable to do this,
so initiated a descent to 25,000ft.
A short time later the first officer
asked the senior flight attendant
to assist the captain. To enter the
flight deck the flight attendant had
to remove her oxygen mask con-
nected to the fixed cabin oxygen

system. She decided not to use the
portable oxygen equipment and
went straight to the flight deck.
Before being able to assist the
captain she collapsed onto the
floor. Once again, the first officer
attempted to put on the oxygen
mask for the captain, this time
successfully. Soon afterward, the
captain regained consciousness
and was unaware he had been
unconscious, which is a typical
reaction from a victim of hypoxia.’

The hypoxia
effects of
a quick cabin
depressurization

During a quick depressurization
the partial pressure of oxygen in
the lungs/alveolae reduces rapidly
with the effect of reverse diffusion.
This means that once the oxygen
partial pressure in the alveolae has
reached a level that is below the
level in the blood, the blood oxy-
gen moves out of the body back
into the ambient air. This effect of
reverse diffusion unfortunately fur-
ther reduces the already very limit-
ed oxygen storing capability of
blood and supports hypoxia effects.
Holding of breath cannot stop the
reverse flow since the pulmonary
gas expansion would lead to seri-
ous lung injury.

Severe hypoxia caused by a signif-
icant reduction in cabin pressure is
very dangerous for flight crew
because:

•

The victims of hypoxia rarely

notice that they are about to
pass out.

•

Usually there is quickly a loss
of critical judgment

•

Most victims often experience a
mildly euphoric state

•

Thinking is slowed, muscular

coordination is impaired

The only effective means of
protection is the quick donning of
oxygen masks as the first action -
before troubleshooting!

Human physiology

Within the lungs the alveola provide the interface between
air and blood. The blood which is returned from the body
tissue into the alveolae has given away most of its oxygen
so that the oxygen partial pressure in the lungs is higher
than in the arriving blood. A process of diffusion then
drives oxygen through the thin alveolar wall into the blood.

The most important parameters for the oxygen diffusion
process are the oxygen percentage and barometric
ambient pressure. Changing these parameters changes
immediately the oxygen saturation level in blood and with
it the oxygen supply to the body tissue. Unfortunately,
there is no significant storage of oxygen in the human
body, unlike many other chemical substances necessary
to maintain life. The blood is the only storehouse for
oxygen, and its capacity is very limited. Hence, the
human body lives only a hand-to-mouth existence with
its oxygen supply.

As the pressure of air in the atmosphere decreases with
increasing altitude, the partial pressure of oxygen in the
air reduces and with it the diffusion of oxygen into the
body. Reduction of oxygen availability in the body results
in loss of functions ranging from slight impairment up to
death. It is the nervous system, in particular in the higher
centres of the brain, and the eyes which have a high
metabolism with no oxygen reserve. These are most
sensitive to oxygen depletion and therefore are the first
to be affected by a reduced oxygen supply.

For healthy persons altitude exposure up to 15,000ft is
usually not hazardous since cardiovascular and
respiratory compensatory mechanisms (faster breathing
and increased pulse rate/blood circulation) act to
maintain adequate oxygenation at the cellular level.

The effects of reduced oxygen supply to the body
(hypoxia) vary between persons, depending on health,
physical fitness, age, activity level and statistical scatter
with the population. Pilots and flight attendants usually
require more oxygen during an emergency than healthy,
seated passengers and might therefore suffer earlier
from hypoxia effects.

HYPOXIA -

AN INVISIBLE ENEMY -

CABIN DEPRESSURIZATION EFFECTS ON HUMAN PHYSIOLOGY

Hartwig Asshauer

Certification Manager

Hydro-Mechanical & Air Systems

Airbus Engineering

When public air transportation first became
commonly available, flights did not reach alti-
tudes that represented a significant risk of
reduced oxygen supply - called hypoxia - to
either passengers or crew. However, in the late
1940s and 1950s aircraft were developed that
allowed safe transport of the flying public at
altitudes around 40,000ft, which have remained
relatively constant since then.

DEFINITIONS OF HYPOXIA

Hypoxia is separated into four types:

•

Hypoxic hypoxia

is a condition caused by reduced

barometric pressure, affecting the body's ability to
transfer oxygen from the lungs to the
bloodstream.

•

Histotoxic hypoxia

can be induced by the

introduction of substances like alcohol or drugs
into tissue, reducing its ability to accept oxygen
from the bloodstream.

•

Hypaemic hypoxia

(or

anaemic hypoxia

) is a result

of the blood being unable to carry oxygen, e.g.
caused by exposure to carbon monoxide.

•

Stagnant hypoxia

results from the body's

inability to carry oxygen to the brain, which can
result from high gravity-forces causing blood to
pool in the lower extremities of the body.

Hypoxia

An invisible enemy

Cabin depressurization effects

on human physiology

GENERAL BLOOD CIRCULATION

Oxygen
partial pressure

The concentration of oxygen in the
atmosphere is constant at 20.95%
at altitudes up to 100,000ft, which
means that according to Dalton's
Law* the oxygen partial pressure
at sea level is 212mbar (20.95% of
1013mbar where 1013mbar is the
standard atmospheric pressure at
sea level).

As altitude increases above sea
level the partial pressure of the
component gases decreases consis-
tent with the decrease in total
atmospheric pressure. For exam-
ple, the partial pressure of oxygen
at 40,000ft is reduced to 39mbar
only, which is far too inadequate to
support human metabolism.

One means to increase oxygen par-
tial pressure is to increase the oxy-
gen concentration in breathing air.
At 40,000ft cabin altitude an oxy-
gen partial pressure of maximum
188mbar can be achieved by
breathing pure oxygen (100% oxy-
gen concentration without over-
pressure).

Another additional means for
hypoxia protection is positive
pressure breathing, which is usu-
ally found in modern crew oxy-
gen masks and means the delivery
of pure oxygen under pressure
into the respiratory tract. For civil
applications positive pressure
breathing is able to increase addi-
tionally the oxygen partial pres-
sure by around 20 to 30mbar
provided that the overpressure
condition is limited to some min-
utes only. This means that at
40,000ft it requires 100% oxygen
concentration of the breathing gas
combined with positive pressure
breathing to achieve sea level
equivalent conditions. Positive
pressure breathing requires some
training and is tiring and inconve-
nient, which is the rationale for
having so far provided this pro-
tection feature to flight crew only
(for short time use only).

HYPOXIA -

AN INVISIBLE ENEMY -

CABIN DEPRESSURIZATION EFFECTS ON HUMAN PHYSIOLOGY

100

* Dalton’s Law

(1766 -1844)
In 1801, the English astronomer
and chemist, John Dalton,
discovered the pressure
relationship among gases in a
mixture. Dalton's Law states that
the pressure exerted by a mixture
of gases is equal to the sum of
the pressures that each would
exert if it alone occupied the
space filled by the mixture.

Current oxygen mask for passengers

Early type of shaped
oxygen mask for passengers

Time of Useful
Consciousness

In the 'World of Hypoxia' the Time
of Useful Consciousness (TUC) is
a very important parameter. For
low ambient pressure conditions it
indicates the time available to per-
form purposeful activities, such as
oxygen mask donning or aircraft
control. Beyond this time frame
mental and physical capabilities
are dangerously impaired and
finally result in unconsciousness
and potentially death.

As shown in the table on the right,
TUC is negatively correlated with
altitude. It is important to note that
even if activities are performed
within the TUC time frame there is
a significant deterioration of work
rate and mental capability, which is
correlated with the time spent at low
pressure conditions (at the end of
the TUC time frame, performance is
much lower than at the beginning).

The TUC is the

'Window of

Opportunity'

for donning an oxy-

gen mask and can be very limited
so must take overriding precedence
over any other activities.

Time of Safe
Unconsciousness

Some experts believe that for pas-
sengers - in contradiction to the
flight crew - a short period of
unconsciousness during cabin
depressurization can be tolerated
since they are not performing an
operational task. Unconsciousness
is a clear sign of insufficient oxy-
gen supply to the brain and it is
obvious that this time can only be
very short before permanent brain
damage occurs. So far, it has not
been possible to associate a specif-
ic time frame for the safe time of
unconsciousness.

The uncertainties in extrapolation
of animal data and the wide vari-
ability in individual tolerances
have so far prevented determina-
tion of a commonly agreed value
for Time of Safe Unconsciousness
(TSU) among human physiology
experts. It is believed that a safe
time of unconsciousness is some-
where between 90 seconds and
4 minutes.

HYPOXIA -

AN INVISIBLE ENEMY -

CABIN DEPRESSURIZATION EFFECTS ON HUMAN PHYSIOLOGY

20,000ft All unacclimatized persons

lose useful consciousness
within 10 minutes

25,000ft Useful consciousness is lost

after 2.5 minutes or less

30,000ft TUC: approx. 30 seconds

37,000ft TUC: approx. 18 seconds

45,000ft TUC: approx.15 seconds

Mask straps inflated

These data on TUC are averaged
values based on tests with healthy
individuals when breathing ambient air
(no supplemental oxygen provided).
A large individual variation in the effects of
hypoxia has been found. There is evidence
that TUC is shorter for people exposed to
stress conditions.

Time of Useful Consciousness

Mask in place

* Manufacturer EROS

Flight crew oxygen mask *

HYPOXIA -

AN INVISIBLE ENEMY -

CABIN DEPRESSURIZATION EFFECTS ON HUMAN PHYSIOLOGY

The first step for any flight crew member
faced with cabin depressurization should
be the immediate donning of an oxygen
mask. Any delay in donning a mask will
significantly increase the risk of losing
consciousness before cabin pressure is
regained. Severe hypoxia leads usually to
the loss of critical judgement combined
with a mildly euphoric state, which makes
hypoxia very dangerous for flight crew.
This is highlighted also in the FAA Special
Certification Review that was issued some
years ago on the effects of cabin
depressurization.

Moreover, in case of rapid cabin
depressurization a quickly accomplished
emergency descent is often the only
means of fast re-oxygenation of

passengers that were unable to protect
themselves against hypoxia by using the
passenger oxygen masks provided.
Severe hypoxia is very dangerous for
unprotected passengers and requires a
quick return to an adequate cabin
pressure or where not possible (above
high terrain), it requires a check by the
flight attendants that the passenger
oxygen masks are correctly used.

For a long time transport aircraft have
been equipped with oxygen systems for
flight crew and passengers that provide an
adequate protection against hypoxia. As
long as these oxygen systems are used
according to their simple procedures the
invisible enemy hypoxia poses little
danger to flight crews and passengers.

CONTACT DETAILS

Hartwig Asshauer

Certification Manager
Hydro-Mechanical
& Air Systems
Airbus Engineering
Tel: +33 (0)5 62 11 04 98
Fax: +33 (0)5 61 93 31 55
hartwig.asshauer@airbus.com

Conclusion

Airworthiness
requirements

The Airworthiness authorities have
identified the risk of hypoxia and
have created requirements (see
table on the left).
Also, after an accident in the USA
the FAA initiated a Special
Certification Review (SCR) on
pressurization systems. The SCR
recommends that the aircraft flight
manual (for aircraft certified for
flights above 25,000ft) require in
the emergency procedures the
donning of oxygen masks as the
first crew action after a cabin
altitude warning.

This highlights again the impor-
tance of immediate donning of
oxygen masks when cabin depres-
surization occurs.

HYPOXIA -

AN INVISIBLE ENEMY -

CABIN DEPRESSURIZATION EFFECTS ON HUMAN PHYSIOLOGY

100

Oxygen equipment
on civil aircraft

On modern aircraft oxygen equip-
ment is installed to provide ade-
quate protection against the dam-
aging effects of hypoxia in case of
cabin depressurization:

For flight crew

there are usually

quick donning oxygen masks
installed, which can be donned
with one hand in less than 5 sec-
onds. The mask straps are com-
bined with elastic tubes that
inflate and stiffen when the mask
is taken from its stowage, allow-
ing the mask to be easily put over
the head with one hand. Once the
grip on the mask is released, the
tubes deflate and their elastic
characteristics ensure a perfect fit.
The required oxygen concentra-
tion of the breathing air is auto-
matically adapted to the cabin
pressure.

For the passenger

oxygen supply

the continuous flow concept is
used on all Airbus aircraft.
Oxygen is delivered continuously
to an expandable oxygen bag
where it is conserved during exha-
lation, so it is available during the
next inhalation to supplement the
steady oxygen flow.

It was decided at an early stage in
passenger oxygen mask develop-
ment that the untrained civilian
population should not be expected
to recognize the correct orienta-
tion for a shaped mask, and it was
required that a mask should be
operable in any position in which
it might be donned by the user.
A second basic requirement was
a universal size, which finally
defined the well-known cylindri-
cal mask body.

Effect on human physiology of moderate cabin altitude

Very large numbers of aircrew and passengers have been exposed to breathing air at cabin altitudes up
to 8,000ft over the last 60 years without significant deleterious effects. Although exposure to this altitude
reduces the oxygen partial pressure in the pulmonary tract the tissues of the body are maintained well
above the required level.

Some airlines still allow smoking in the aircraft cabin, which results in carbon monoxide inhalation with
the smoke. Carbon monoxide has a 240-times greater tendency than oxygen to attach to red blood
haemoglobin, thus inactivating a large amount of haemoglobin as an oxygen carrier. It has been found that
the hypoxia effects from carbon monoxide and altitude are additive; hence chronic smokers are at a
higher equivalent altitude than non-smokers in terms of blood oxygen supply.

Also, alcohol poisons body tissues in such a manner that they cannot use oxygen properly. Usually, it is
noticed by passengers that the physiological effect of alcohol consumed during flight is more intense than
at sea level, which is due to the additive hypoxia effects of alcohol and altitude.

GENERAL

• CS/FAR 25.841 (a): Maximum cabin pressure altitude under normal operation: 8,000ft
• CS/FAR 25.841 (a): Maximum cabin pressure altitude after any probable failure condition in the

pressurization system: 15,000ft

• FAR 25.841 (a) (2) (i): Maximum exposure time to cabin pressure altitude exceeding

25,000ft: 2 minutes

• FAR 25.841 (a) (2) (ii): Exposure to cabin pressure altitude that exceeds 40,000ft: Not allowed

CABIN OCCUPANTS

• CS/FAR 25.1443 (c): Provides oxygen system performance data on oxygen flow and

required partial pressure of oxygen

• CS/FAR 25.1447 (c) (1): The total number of masks in the cabin must exceed the number

of seats by at least 10%

• CS/FAR 25.1443 (d): Defines oxygen flow for first-aid oxygen equipment (for cabin

depressurization treatment)

• JAR OPS 1.760/FAR 121.333 (e) (3): Requires first-aid oxygen for at least 2% of

passengers

• JAR OPS 1.770 (b) (2) (i)/FAR 121.329 (c): Defines the percentage of passengers that need

to be provided with supplemental oxygen (cabin pressure altitude dependent)

FLIGHT CREW

• CS/FAR 25.1443 (a) & (b): Provides oxygen system performance data on oxygen flow and

required partial pressure of oxygen

• CS/FAR 25.1447 (c) (2) (i): For aircraft operating above 25,000ft quick donning oxygen

masks are required for the flight crew which can be donned with one hand within 5
seconds

• FAR 121.333 (c) (2) (i) (A): One flight crew member needs to wear permanently his oxygen

mask when the aircraft is operated above FL410

• FAR 121.333 (c) (3): In case one flight crew member leaves the controls the remaining pilot

needs to use his oxygen mask when the aircraft is operated above 25,000ft

Extract of the prime requirements