**1.2 Cabin Environment and EVA Environment:**

Barometric pressure (operational range, decompression hazards,
pressurization systems)

Temperature and humidity (physiological and psychological effects,
exposure limits, control methods, personal microclimate thermal control
systems)

Breathing gas composition, air circulation/ventilation, contaminant
removal (biological, chemical, particulates), atmosphere monitoring,
oxygen production {water electrolysis, molecular sieve system}

Chemical hazards (types of chemical hazards, biological effects, control
of hazards)

Fire hazards (predisposing factors, combustion toxicology, monitoring
and control of hazards, types of fire extinguishing equipment acceptable
for space use)

Electrical hazards (biological effects of exposure to electricity,
control of hazards)

Mechanical hazards (types of hazards, sources of hazards, control of
hazards)

Noise and vibration (sources, frequency, intensity, duration,
propagation, physiological and psychological effects, individual
tolerance, exposure limits, operational effects, countermeasures,
individual protection methods)

Cabin Environment and EVA Environment

Kira Bacal, MD, PhD, MPH

**CABIN ENVIRONMENT AND EVA ENVIRONMENT**

This chapter is designed to provide you with a basic understanding of
the cabin and extra-vehicular environments associated with spaceflight.
Emphasis is placed on the physical environment (pressure, temperature,
gases) as well as the environmental hazards (noise, vibration,
electrical, etc.). Included is a review of how the life support systems
of the spacecraft balances operational constraints (such as cost,
upmass, and safety concerns) with the physical needs and comfort of the
crew. Also, there will be a description of nominal vehicular
environments and the interaction of different parameters, such as oxygen
concentration and fire risk. Finally, a variety of the hazards
(mechanical, vibration, electrical) will be described.

Space exploration exposes travelers to arguably the most hostile
environment humans have ever experienced. The absence of air, heat,
pressure, and even gravity forces space travelers to bring along not
only their own food and water but also a protective atmosphere, similar
to that of earth. Environmental systems control atmospheric composition,
pressure, humidity, and temperature. Design of these systems is
remarkably complex and consumes a significant amount of budget and
resources, yet the life support systems are indispensable aspects of the
manned space program. The criticality of these systems is almost
impossible to underestimate. \[See Figure One.\] The effect of a loss of
the cabin environment was starkly illustrated by the 1971 deaths of the
entire Soyuz 11 crew (cosmonauts Georgi Dobrovolksy, Vladislav Volkov,
and Viktor Patsayev), when a pressure equalization valve opened too
early in the reentry phase and vented the cabin to space at an altitude
of over 100 miles. In order to prevent future tragedies of this nature,
it is important not only to understand the crew's physical needs, but
also the intricacies of the life support systems that supply them.

**Barometric Pressure**

**Operational Range**

A minimum barometric pressure of 0.9 psi (the saturated vapor pressure
of water at body temperature) is required to prevent ebullism, the
spontaneous boiling of body tissues. Armstrong's Line, or an altitude of
63,000 feet, is where this barometric pressure is reached in the Earth's
atmosphere.

However, while 0.9 psi is the minimum pressure required to avoid normal
body fluids from coming out of solution, normal physiological functions
require a still higher ambient pressure: cabin pressure must also permit
alveolar exchange of oxygen and carbon dioxide. In addition to the 0.9
psi of water pressure present in the alveoli, there is also 0.7 psi of
carbon dioxide, leading to a total alveolar gas pressure of 1.68 psi.
For any gas exchange to occur across the alveolar membrane, a gradient
must exist, so ambient pressure (i.e. pressure within the alveoli) must
be greater than 1.68 psi. The altitude corresponding to this pressure
level, 50,000 feet, serves as the threshold beyond which not only
supplemental oxygen (i.e. inspiration of 100% oxygen), but also pressure
suits, must be used to permit adequate oxygenation. As barometric
pressure continues to rise, less positive pressure is required to
prevent hypoxia, and above 3.46 psi ambient pressure (corresponding to
an altitude of 35,000 feet or below) pure oxygen can be breathed without
positive pressure.

Space suits, such as the American EMU (Extravehicular Mobility Unit) or
Russian Orlan, contain only a single gas, oxygen, and can thus be
pressurized to hypobaric levels (4.3 psi EMU, 5.7 psi Orlan) while still
maintaining alveolar pressures of oxygen that do not induce hypoxia
(adequate alveolar oxygen tensions are generally maintained by a oxygen
pressure in the lungs of approximately 3.1 psi). When the atmosphere
contains more than one gas, however, ambient pressure must rise further
in order to ensure that the partial pressure of oxygen at the alveoli is
high enough to support normal physiological processes.

As described by Dalton's Law, the pressure of a single component in a
mixture of gases is only a fraction of the total pressure.
Terrestrially, ambient ("breathing") air is a mixture of 21% oxygen, 78%
nitrogen, 1% argon, and other trace gases. As a result, at a barometric
pressure of 14.7 psi, the partial pressure of oxygen in breathing air is
only 3.1 psi. This is the value for which life support systems aim,
through manipulation of both barometric pressure and gaseous
composition. A terrestrial correlate relates to aviation regulations
concerning the use of supplemental oxygen. The USAF, for example,
requires supplemental oxygen for flight operations whenever cabin
altitude exceeds 10,000 feet, where the ambient atmosphere is 10.1 psi
and partial pressure of oxygen only 2.1 psi.

Both the US Space Shuttle Program and International Space Station make
use of a 14.7 psi normal cabin barometric pressure, as this requires the
least adaptation by crewmembers and avoids confounding of scientific
experiments that might be affected by atmospheric pressures different
from the terrestrial standard. The Apollo program, by contrast, used
100% oxygen at 5 psi. (This caused difficulties during the Apollo-Soyuz
program, as the Soyuz, then as now, used a nitrogen/oxygen environment
at 14.7 psi.) While the single gas environment simplified the
atmospheric control system for these early space vehicles, there were
concerns about the effects of prolonged hypobaric pressures on the
crewmembers as well as the increased danger of fire in oxygen-enriched
environments, particularly if the barometric pressure were raised above
the hypobaric level. This was the case in the Apollo 1 disaster, where a
100% oxygen cabin environment was temporarily raised to 16 psi for a
test on the launch pad. In the resulting fire, Ed White, Virgil "Gus"
Grissom, and Roger Chaffee died.

**Decompression Hazards**

The two main types of decompression hazards in spaceflight are
unexpected decompressions, as with the June 1997 collision which
ruptured one of the Mir space station's living modules, and scheduled
decompressions, as when extra-vehicular activities (spacewalks) occur.

Obviously, it is much harder to prevent against unintentional
decompressions, like puncture of a habitable module or spacesuit, but
extreme caution is taken during docking and undocking operations to
minimize the risks, particularly in light of the lessons learned during
the 1997 Mir event, and spacecraft designers consider micrometeorites
and other orbital debris (collectively referred to as "MMOD") when
developing proper shielding for the platforms.

Scheduled decompressions occur routinely, often several times in the
course of a mission, when space travelers move from the sea level
environment of Shuttle or Station to the lower pressure of a spacesuit.
Spacesuits of both American (EMU) and Russian (Orlan) design must
delicately balance their internal pressure against their functional
requirements. \[See Figures Two and Three.\] The lower the atmospheric
pressure within the suit, the more easily the astronaut or cosmonaut can
move around or manipulate the gloves. Further, because the suits are a
single gas environment, the ambient pressure required to provide an
adequate oxygen pressure at the alveoli is significantly lower than that
of the usual dual gas environment. However, since all of the currently
used spacecraft (Shuttle, Soyuz, Station) utilize a nominal dual gas
atmosphere at sea level pressure for reasons described above,
crewmembers face a risk of decompression sickness (DCS) whenever they
move from the cabin environment to the spacesuit.

With the exception of Michael Collins' self-reporting of a DCS event
during an Apollo 11 mission (which he described belatedly in his
autobiography "Carrying the Fire: An Astronaut's Journeys"), there have
been no cases of DCS during spaceflight. Whether this is due to proper
countermeasures, a great deal of luck, or a significant difference in
the pathological mechanisms between terrestrial DCS and DCS in
microgravity, is the subject of much current research. In the meantime,
however, it is assumed that DCS can occur in space, and preventative
measures are undertaken before a spacewalk to maximize nitrogen washout
in the EVA (extra-vehicular activity) crewmembers.

Among the procedures utilized are: pre-breathing 100% oxygen, exercising
during the oxygen prebreathe session, and (for the Shuttle only, whose
smaller habitable volume makes this a feasible option) temporarily
decreasing the cabin atmospheric pressure to 10.2 psi, so as to decrease
the gradient between the cabin and spacesuit environments. During this
decompression, if the oxygen concentration was held constant at 21%, the
alveolar oxygen tension would be significantly reduced, so it is
necessary to raise the concentration of oxygen in the cabin during this
decompression period to 23.8%. This reinforces the point that as
barometric pressure falls, the partial pressure of oxygen (which is
usually controlled by varying the oxygen concentration) must be raised
in order to maintain adequate alveolar oxygen tension. This is due to
the effects of the water vapor saturating the breathing air and the
dilution effects of alveolar carbon dioxide.

In the event of a suspected or confirmed DCS event, limited hyperbaric
treatment can be provided through the overpressurization of the EMU
spacesuit. When performed in a cabin pressurized to sea level, the
patient will experience a 100% oxygen environment at approximately 22
psi. For low earth orbit missions, a rapid medical evacuation to a
terrestrial medical facility would then occur. Exploration class
missions, by contrast, are unlikely to be able to return stricken space
travelers to Earth, and therefore discussions regarding a hyperbaric
chamber functionally equivalent to terrestrial models may be included in
medical systems for those vehicles.

**Pressurization Systems**

Air supplies for spacecraft repressurization and crew respiration can be
stored in high-pressure tanks as liquids in cryogenic storage or in
other forms. Nitrogen, for example, can be stored as hydrazine
(N~2~H~2~), which also is a spacecraft propellant. For platforms such as
the ISS, new supplies can be delivered either via the Shuttle or on
Russian "Progress" rockets, which have air tanks that can be filled with
nitrogen, oxygen, or "room air". The ISS can also generate oxygen
through several methods (see below).

Four high‑pressure gas tanks (two for nitrogen, two for oxygen) are
located on the exterior of the joint airlock, and can be either refilled
or replaced by the Shuttle. \[See Figure Four.\] The tanks are connected
to the ISS interior via a system of pipes that, in conjunction with a
"pressure control assembly", monitors the station pressure, introduces
gas(es) into the atmosphere when appropriate, and allows controlled
depressurization as needed.

Ambient cabin pressure is maintained, and atmospheric leakage countered,
by periodic injection of gas. The amount needed is calculated from
measurements of total cabin pressure and (in two-gas systems) the
partial pressure of oxygen. If the cabin pressure rises too much, for
example due to a leak of stored gas into the cabin, a controlled release
of some of the atmosphere is necessary before the ambient pressure
exceeds the structural limits of the spacecraft. In multi-module
spacecraft such as the ISS, pressure gradients between or among modules
are rectified through pressure equalization valves between compartments
of the ISS. Both ground control and on-board crew can control these
valves and thus regulate ambient pressure within the spacecraft.

In a dual-gas system, following repressurization, care must be taken to
diffuse the introduced gas so as to avoid local asphyxiation hazards
(areas of pure nitrogen), or fire danger (pockets of pure oxygen).
Systems are designed to circumvent this situation but if objects are
inadvertently placed in the path of gas flow, expected diffusion
patterns can be disrupted. In addition to the introduction and diffusion
of appropriate amounts of breathable gases, adequate ventilation is also
vital to maintaining atmospheric homogeneity (see below for more
detail).

**Breathing Gas Composition**

**Gaseous Components**

The nominal atmospheres of the Space Shuttle, the Soyuz, and the ISS are
dual-gas oxygen-nitrogen (21% oxygen and the balance nitrogen),
maintained at sea level pressures as discussed above. However, pure
oxygen is breathed during extravehicular activities or during
contingencies when the crewmembers are directed to don portable
breathing apparatuses (PBAs). Examples of such an emergency would
include fire (or other atmospheric contamination) or depressurization
(to maximize the alveolar partial pressure of oxygen and delay
development of hypoxia). In addition, the ISS ventilator (AutoVent 2000)
uses 100% oxygen exclusively. \[See Figure Five.\] As a result,
medically compromised patients aboard the ISS can receive 100% oxygen or
room air, but there is currently no means of providing supplemental
oxygen between 21-100%. This may create difficulties in seriously ill or
injured crewmembers who require mechanical ventilation or supplemental
oxygen for prolonged periods, as studies have shown deleterious effects
from breathing 100% oxygen at sea level pressures occur in as little as
a day.

A single gas environment has several benefits: it permits a lower
habitable atmospheric pressure, thus decreasing the risk of
decompression sickness during EVA; it minimizes the required strength of
the spacecraft hull since it will not need to withstand a higher
internal pressure; and it requires smaller stores of gas. However, it is
a highly artificial environment in which certain experiments cannot be
performed at all, while others are impossible to generalize from because
they were not performed in the standard sea-level, dual gas atmosphere.
In addition, the higher oxygen concentration (albeit at a lower
pressure) could pose a higher fire risk.

Atmospheric gases are stored on spacecraft either as "breathing air"
(i.e. a nitrogen/oxygen mix) or as the individual gases. In the case of
the former, the life support system does not need to mix gases, but
merely supplements the breathing air with small quantities of oxygen to
make up for the amount inspired by the crew. Separate gases, however,
provide additional flexibility to the crew, which can be important in
contingency operations, such as a leaking module (as recently occurred
on the ISS), or during routine activities on the Shuttle, such as the
partial decompression of the atmosphere in preparation for a spacewalk.

The ISS has both tanked air stores and separate supplies of oxygen and
nitrogen, while the Shuttle uses cryogenically stored oxygen and
nitrogen. The gases are added to the atmosphere to replace leaks (no
spacecraft is entirely free from leaks) or consumed oxygen. Obviously
care must be taken to ensure that the proper amount of gas is added and
that the individual components are present in their proper proportions
-- if too much nitrogen is added to the environment, hypoxia or anoxia
can result. If too much oxygen is introduced, then the fire hazard
increases. Even if the right proportions are maintained, the
introduction of too much gas may lead to overpressurization of the
module. If not enough gas is added, or if the addition is not timely
enough, hypobaric conditions, potentially leading to hypoxia, could
result.

**Air Circulation and Ventilation**

Hypoxia and hypercarbia are very real dangers during spaceflight, not
only from malfunctions in the life support system, but also because in
microgravity gases do not diffuse away from the face as they do
terrestrially. Carbon dioxide is heavier than breathing air, so on Earth
it falls away from the face once it is exhaled. In addition, breezes,
local airflows, and weather- or gravity-related convective forces also
tend to remove carbon dioxide from around the nose and mouth. These
forces are absent in microgravity, and, in the absence of adequate
ventilation systems to move the air from in front of the face, pockets
of carbon dioxide can accumulate, leading to hypercarbia. Similarly,
when the crew must operate in small, contained spaces where the
ventilation system is less effective, such as behind a rack, inside an
unventilated sleep station, or when numerous large items are out in the
cabin, extra caution must be taken to prevent buildup of carbon dioxide.

During the 1985 recovery mission of the Salyut 7 space station,
cosmonauts Dzhanibekov and Savinikh had to work in a module that lacked
power and was thus both cold and unventilated. They reported headaches,
lethargy, and sluggishness associated with the hypercarbia produced by
their own exhalations. Providing active ventilation as soon as possible
through portable fans and the resumption of powered systems mitigated
this effect.

Flight rules for the ISS and Space Shuttle programs define the length of
time which crews may remain in unventilated modules without the use of
oxygen masks, though the times vary based upon module volume and number
of crew present. If portable fans are available, they may be able to
provide the necessary amount of ventilation until the central
ventilation system can resume. Even manually fanning the air with hands
or checklists have been found to lower local carbon dioxide levels.

Forced ventilation is thus a vital component of any atmospheric system
for space travel. The prevention of hypoxia and hypercarbia is not,
however, the only reason air circulation is so critical. Without
adequate blending of the cabin atmosphere, heterogeneous
micro-environments can occur, containing undesirable levels of carbon
dioxide, particulates, water, or heat. Air movement also enables the
surveillance of the environment for toxins (such as smoke), permits the
extraction of undesirable components (such as carbon dioxide and trace
contaminants), and allows control of both heat and moisture content of
the atmosphere.

In order to achieve these necessary objectives, the air must be mixed at
a certain rate. Too slow a flow rate could allow buildup of toxins or
heterogeneous "pockets", while too rapid a rate would consume resources
unnecessarily. One of the major factors controlling ease of ventilation
is ambient pressure: more dense environments require more energy to move
the air and achieve the same mixing/cooling effects. For sea level dual
gas environments, ventilation systems usually operate at flow rates of
0.08-0.2 m/s, though they must have the capacity to regulate this level
depending upon the crew activities. If a number of crew occupy the same
module, then temperature and carbon dioxide levels will rise more
rapidly, necessitating greater ventilation.

There are three types of air circulation: within an individual rack,
within an individual module, and between two or more modules. The
avionics air assembly accomplishes the first, cooling the air within a
rack by a fan and non-condensing heat exchanger. The system is also
linked with smoke detectors from the fire detection and suppression
subsystem. Ventilation within a module occurs via the common cabin air
assembly. It draws cabin air through a HEPA filter then sends it through
a condensing heat exchanger to remove the moisture. This water is then
sent to the water recovery system for reclamation, while the cooled, dry
air goes first to the carbon dioxide removal system (see below) and then
back into the cabin. Ventilation between modules is accomplished by
drag-through flexible ducting and open hatches in the Russian segments
of the ISS and via a series of fans, valves, and hard-plumbed ducts on
the US side.

**Oxygen Production (Water Electrolysis, Molecular Sieve)**

There is a great deal of ongoing research into onboard oxygen generating
systems (OBOGS) including molecular sieves, electrolysis, various forms
of air separation, membrane technologies, carbon dioxide reduction (such
as a Sabatier reactor), chemical absorption, and biological systems.
However, only a few of these have actually been used in space up to this
point.

Molecular sieves, which were first used during the Skylab missions, are
also used in some military aircraft. They make use of technologies in
which ambient air is forced over a zeolite crystal molecular sieve that
separates the oxygen; the gas is then further concentrated and released
back into the atmosphere as needed.

Since the Mir space station, Russian life support systems have generated
onboard oxygen through the Elektron oxygen system, in which oxygen is
removed from water via electrolysis (2 H~2~O + electrical current 
2H~2~ + O~2~). The water used in this way can come from stored water or
even urine. On the ISS, electrical current is provided from the
station's solar panels. The hydrogen is currently vented into space,
though eventually it may be combined with carbon dioxide to create
additional oxygen and/or water in the (planned) Russian Sabatier
reactor. In this reactor, hydrogen from the Elektron oxygen system is
mixed with carbon dioxide from the carbon dioxide scrubbing system at
relatively high temperatures (900°F--1,200^o^ F) to create first water,
then oxygen.

Another OBOGS used since Mir is the solid fuel oxygen generator (SFOG)
system, which employs a chemical reaction (heating lithium perchlorate)
to create oxygen. \[This is the same technology used in the emergency
oxygen masks of commercial aircraft.\] Each 2.2 kg perchlorate "candle"
produces 600 liters of oxygen, a supply sufficient for one person for
one day. Although other solid chemical oxygen generating systems exist
(including one which uses potassium superoxide), the perchlorate system
has the smallest weight/volume penalty. The SFOG system's hazards
(notably fire and the need for a supply of the perchlorate candles) have
restricted its use on the ISS to that of a backup device.

For long duration exploration missions, however, biological systems are
likely to be more heavily employed. In 1995, a bioregenerative system
was successfully tested, supporting one person for 15 days in a closed
system. Eleven square meters of wheat in the chamber provided the
subject oxygen while consuming the carbon dioxide which he exhaled.
However, currently the plant-based systems are larger, more
labor-intensive, and (as any farmer or gardener knows) less reliable
than the chemical-mechanical systems. Given the criticality of the
atmospheric system, the current level of uncertainty associated with
biological systems is unacceptable, and additional research is needed
before it can be considered a viable technology for spaceflight.

**Atmosphere Monitoring**

Spacecraft life support systems must monitor several atmospheric
parameters, including ambient pressure, partial pressure of each gas
(including carbon dioxide), ventilation, temperature, and humidity. The
Russian modules of the ISS, for example, contain gas analyzers to
monitor oxygen and carbon dioxide partial pressures, as well as
humidity; in the US modules, air is checked with a mass spectrometer
after removal of carbon dioxide and other contaminants.

The goal of the oxygen monitoring system is to ensure that an adequate
alveolar oxygen tension is maintained at all times. As described above,
oxygenation of the lungs relies upon both atmospheric pressure and
gaseous composition. As a result, clinicians must have access to both
values in order to draw conclusions about the adequacy of the
environment. Simply knowing one parameter is insufficient: for example,
a 100% oxygen atmosphere at 4.3 psi is fine, while one at 1.4 psi is
not.

On the ISS, flight rules require that a partial pressure of oxygen must
be maintained between 2.82 - 3.44 psi. This would correlate to an oxygen
concentration of approximately 21% in an ambient pressure of 14.7 psi.
Of course, in an emergency, these limits can be relaxed somewhat and/or
alternate methods employed to ensure adequate oxygenation (such as
increasing the oxygen concentration in ambient air or using oxygen
masks).

While the oxygen monitoring system maintains the partial pressure of
oxygen relatively close to that experienced on Earth, spacecraft carbon
dioxide monitoring systems deal with levels significantly higher than
what is seen terrestrially: 7.6 mm Hg of carbon dioxide is the upper
limit of normal for both ISS and Shuttle life support (33 times
Earth-normal). Current technology does not permit an efficient method to
reduce these levels to terrestrial norms, but to date, no evidence
exists that these elevated levels (which are still well below the stage
where physiological effects are seen) have long- or short-term effects
on space traveler health.

Carbon dioxide detection systems are usually based upon measured levels,
rather than crew symptomatology, as most people will not detect a
problem with the environment until the carbon dioxide levels exceed 23
mm Hg, although physiological effects of hypercarbia first become
discernible around 12 mm Hg. However, spacesuits pose a particular
challenge for environmental monitoring, as the oxygen and carbon dioxide
sensor telemetry may not accurately reflect values around the face. As a
result, more emphasis is placed on reported symptomatology, even though
this is admittedly less sensitive. Although spacewalking crewmembers
receive additional training to recognize their own hypercarbic symptoms,
during EVA operations, the space clinician must maintain a heightened
index of suspicion for hypercarbia among the crew.

**Contaminant Removal (Biological, Chemical, Particulate)**

Contaminants fall under two general categories: particulate and gaseous.
The latter includes both biological (such as those created during food
preparation or by the crew) and chemical (as can be seen from
off-gassing of cabin materials, cleaning supplies, or certain scientific
experiments). Carbon dioxide is by far the most important contaminant to
be removed from the environment; accordingly, it is described in detail
in the section below.

Prevention of buildup is naturally preferred to removal, however, and
spacecraft designers create their plans accordingly. For example,
judicious selection of spacecraft cabin materials can minimize the
sources of off-gassing toxins.

However, not all contaminants can be avoided. Submarine and chamber
studies showed as early as the 1970's that in a contained environment,
even trace contaminants can build up to hazardous levels. Prior to the
Skylab program, US spacecraft had neither monitored nor analyzed trace
contaminants (with the exception of carbon monoxide). From Skylab on,
however, numerous methods have been used to monitor the cabin
environment, including gas chromatography, mass spectrometry,
ultraviolet and infrared spectroscopy, and sensors for individual
contaminants (such as carbon monoxide or hydrogen cyanide). Exploration
class missions must consider additional potential contaminants such as
Martian soil or lunar dust.

Current flight rules establish the maximum allowable concentrations for
a variety of chemical contaminants, from ammonia to xylene, as well as
for microbial contamination (measured in colony forming units) and
particulates (measured in particles per cubic foot). Removal of these
substances usually occurs through the use of various physical barriers,
such as charcoal filters and high efficiency particulate air (HEPA)
filters that screen out 97% of particles larger than 3 microns, or
through catalytic oxidizers. When dealing with newer hazards, such as
soil from other planetary bodies, new methods for removal and
decontamination may be required.

In addition to these "normal" contaminants, during contingencies (such
as a fire) additional hazards can be introduced into the atmosphere,
from cyanide gas to molten metal particles (see below for description of
the Mir fire). The removal of many of these substances can be achieved
through the use of filters, though not all (such as toxic byproducts of
the Halon used as a fire suppressant on the Shuttle).

An example of how contaminants are handled under nominal conditions is
as follows: On the Shuttle, cabin air is drawn through a 300-µm filter
to remove particulate matter, then sent through two lithium
hydroxide-activated charcoal canisters which absorb carbon dioxide and
odors. \[See Figure Six.\] The air then travels to the heat exchanger
and is cooled by water coolant loops; humidity condensate is removed (up
to four lbs/hour), separated from the air, and sent to the waste water
tank. Most of the revitalized air is then ducted back into the cabin,
with a small fraction of the air sent to the carbon monoxide removal
unit. This device converts carbon monoxide into carbon dioxide, which
can then be removed by the lithium hydroxide canisters. The entire
volume of cabin air travels through the system in this way approximately
8.5 times every hour (330 ft^3^/minute).

**Carbon Dioxide Removal**

In addition to oxygen and nitrogen, carbon dioxide is the other gas
which the life support system of a spacecraft must handle. The average
human exhales approximately 600 liters per day, and so the cabin
environment system must not only sustain adequate levels of oxygen but
also remove the carbon dioxide before it accumulates to dangerous
levels. Currently, this is most often accomplished by chemical
"scrubbing" of the recirculated atmosphere via one of several methods:
absorption (using a chemical reagent such as lithium hydroxide),
adsorption (using a physical reagent such as zeolite), or biological
reactions (using plant material to consume the carbon dioxide). Other
methods involve molecular "sieve" technology, electrochemical reactions,
or organic amines (as are often used in submarine life support systems).

Short-duration spacecraft with missions on the order of several days
have tended to use absorption reactions with lithium hydroxide. Lithium
hydroxide is an attractive choice for spaceflight because it has a high
absorption capacity (2 kg can remove one person's daily carbon dioxide
production from the cabin environment), and the reaction proceeds with a
relatively small amount of heat production; however, there are
disadvantages associated with the use of lithium hydroxide. The
irreversible chemical reaction requires not only periodic replacement of
the canisters (a chore which consumes crew time) but also requires
significant stowage of replacement supplies.

In addition, the lithium hydroxide is itself quite toxic and poses a
significant potential hazard. For these reasons, longer-term mission
planners prefer other means to remove carbon dioxide from their
spacecraft environments.

Some long-duration platforms, such as Skylab and Mir, have successfully
used active regenerable sorbent beds in order to avoid the resupply
problem of lithium hydroxide. Skylab's molecular sieve technology for
carbon dioxide removal used a regenerable crystalline zeolite matrix
with a very large surface area. The US segment of the ISS uses a similar
sieve, while EMU designers chose silver oxide (also known as metal oxide
or "metox") as their method for carbon dioxide removal. On the ISS, a
series of reusable sorbent beds are used to remove carbon dioxide which
is then vented to space. Cool, dry air is needed for maximum efficiency
of the system, and so it receives processed air directly from the
temperature and humidity control subsystem. Lithium hydroxide canisters
(such as those on the Shuttle) are also available on the ISS as a backup
system.

Despite these systems, carbon dioxide levels in spacecraft environments
are routinely 20-50 times higher than in Earth's environment. As a
result, further research into this area continues, particularly for the
design of atmosphere control systems for exploration class missions or
permanently manned colonies.

**Temperature and Humidity**

**Physiological and Psychological Effects**

The life support system of a spacecraft must not only protect its
occupants from the intense cold outside (as during the repair of the
Salyut 7 station mentioned above or the return of Apollo 13, when cabin
temperatures fell to 49-55^o^F due to the low power levels), but also
from the buildup of heat within it. Equipment such as avionics produce
high thermal loads and, in most cases, hyperthermia is more of a danger
to space travelers than is hypothermia. The literature clearly documents
decrements in task performance as crew comfort declines. Ambient
temperatures on current spacecraft are usually kept at "shirtsleeve"
conditions, i.e. around 75^o^F, though launch and landing temperatures
are usually 5^o^ cooler, to improve comfort when wearing Launch and
Entry System pressure suits. \[See Figure Seven.\]

Humidity is a key component in physical comfort aboard the spacecraft
and is integrally linked with temperature control. Although it is less
noticeable at comfortable temperatures, as either extreme is approached,
humidity levels become more important. To maintain crew comfort,
humidity must fall to promote evaporative cooling during increases in
temperature, while higher humidity levels reduce evaporation and
facilitate heat retention at cooler temperatures. The classic example of
the importance of humidity control is desert vs. jungle conditions.

Although absolute temperatures may be very similar in the two
environments, the latter is significantly more uncomfortable due to the
high humidity and lack of evaporative cooling. The cabin environment is
usually maintained at about 60% relative humidity (corresponding to
approximately 0.2 psi of water vapor pressure).

**Exposure Limits**

Just as ambient pressure affected the ventilation system (more dense
atmosphere requires more energy to circulate and cool), it also affects
the control of temperature and humidity. At low cabin pressures, the
ability to cool the spacecraft is impaired. There are also multiple
sources of heat within the spacecraft which can each affect thermal
control: electronics, lighting, solar heating, and metabolic heat (as
produced by exercise). Long duration Soviet/Russian spaceflights suggest
that ideal temperatures for a working environment are 72°--75°F.

When temperatures drop below 66°F, with a relative humidity around 70%,
the crews complain of cold. Higher temperatures are not only perceived
as unpleasant by the crew, but may also lead to increased sweating and
dehydration (thus additionally burdening the life support system), as
well as to decrements in equipment function.

If the relative humidity rises above 70%, there is an increased risk of
microorganism growth, corrosion, and/or spontaneous condensation. This
was seen on the Mir space station when use of the onboard shower
resulted in elevated humidity levels. Before the life support system
could remove the moisture from the air, the wall coverings and other
materials in the cabin did so. This resulted in a water shortage (less
of the shower's water was available for reclamation than had been
planned), decreased temperature control (the wet materials lost their
insulation properties), and increased microorganism growth (which in
turn contaminated the environment, attacked adhesive materials, and
produced odors). Under lower humidity, other problems exist; crew
comfort can be compromised and chapped lips, dry skin, and increased
upper respiratory infections may be seen.

**Control Methods**

Circulation of cabin air is the mainstay of temperature and humidity
control. As air is recirculated, moisture can be removed from it either
by a desiccant (as is usually the case in spacesuits) or through
condensation of the vapor and removal of the water from the gaseous air.
This latter method is used more often in spacecraft, since it allows
reuse of the water thus produced. In general terms, condensation occurs
when the majority of cabin air is cooled on heat exchangers and the
resultant water is then removed by separators (either acting via
capillary action or hydrophobic-hydrophilic surfaces). The water can
then be reclaimed for various uses (see the next chapter), while the
cooler, drier air is sent back into the cabin. (Some cabin air bypasses
the heat exchangers and is mixed with the cool, dry air to maintain
proper temperature.)

More specifically, thermal control systems use Freon-21 coolant loops,
cold plate networks, heat exchangers, and heat sinks to regulate the
temperature within a narrow range (65°F--80°F). Different systems are
used at different stages of flight and on different spacecraft. For
example, during launch and landing of the Space Shuttle, a "water flash
evaporator" is used to cool the cabin. When the Space Shuttle reaches
orbit, this system is deactivated once the payload bay doors open and
heat is lost from radiator panels on the doors' undersides. The same
system cools the avionics units and other equipment, although it makes
use of loops independent from the cabin air.

Currently, space travelers use personal items of clothing in order to
fine tune the environment to their comfort level. In the future,
however, particularly for exploration class missions when extended
extra-vehicular activities may be routine, other systems of personal
climate control may be developed. In this way, a "micro-climate",
ideally suited to that individual, may be created and controlled. The
liquid cooling garments and G-suits worn by Shuttle astronauts during
launch and landing may be the precursor of more advanced systems in
years to come.

**Chemical Hazards**

**Types**

As described above, there are several potential sources for chemical
contamination of the spacecraft environment. Propellants (such as
hydrazine) are notoriously dangerous, coolants (such as ammonia or
Freon) can leak into the atmosphere. Cleaning materials may also contain
hazardous chemicals. Over time, volatile organic compounds (from polymer
materials) can leach into the cabin environment, or toxic chemicals can
be abruptly created, as in a fire.

In addition, the chemicals that are associated with the payloads and
scientific research usually performed in spacecraft can pose hazards. As
Jay Perry, a life support engineer at NASA Marshall Space Flight Center,
explained, \"There could be any number of different types of
experimental facilities on board that could have any number of chemical
reagents. Some of these chemicals are likely to be hazardous,
particularly if they\'re allowed to combine in unforeseen ways."

It must also be understood that our understanding of the chemicals
present in the spacecraft environment is only as good as the systems and
sensors available to detect those chemicals. One problem following the
1997 Mir fire, for example, was that there was no onboard means to
analyze air samples and by the time samples were returned to the Earth
for analysis, additional breakdown of the substances could have
occurred. As a result, a great deal remains unknown about the actual
hazards experienced in flight.

However, there have been several chemical exposures during past missions
that provide an opportunity for study. In 1975 Apollo-Soyuz astronauts
Slayton, Stafford, and Brand were exposed to toxic fumes during their
descent. \[See Figure Eight.\] At approximately 24,000 feet, an
inadvertent firing of the capsule's jets allowed nitrogen tetroxide
(which served as an oxidizer for the capsule's thrusters) to be sucked
from a thruster outlet into the spacecraft through a nearby cabin
pressure relief valve. The crew described it as "the cabin seemed to
flood with a noxious gas... It irritated the skin... and the eyes... We
started coughing." Nitrogen tetroxide is highly toxic; it can be lethal
in sufficient quantities and can lead to both short term (pneumonitis)
and long term (heightened vulnerability to respiratory illnesses)
effects. The crew was forced to break out the oxygen masks, but in the
interim before Stafford could retrieve and distribute the masks, Brand
(who was sitting closest to the valve) blacked out. Stafford reported,
"I knew that I had a toxic hypoxia and I started to grunt-breathe to
make sure I got pressure in my lungs to keep my head clear. I looked
over at Vance \[Brand\] and he was just hanging in his straps. He was
unconscious." With an oxygen mask in place, Brand regained consciousness
within a minute, but all three subsequently developed a pneumonitis, for
which they were eventually hospitalized, but fully recovered.

More recently, on STS-98 in 2001, an EVA crewmember was covered in
ammonia from coolant lines during connection of the US lab module to the
ISS. Although the spacewalker was unaffected, the ammonia crystals posed
a contamination hazard to the cabin environment when he reentered the
Shuttle. His EVA partner assisted him in brushing off as much of the
ammonia as they could during the spacewalk, and he then stood in the
sunlight so that more could sublimate off the EMU. Following return to
the Shuttle airlock, several procedures to safeguard the intravehicular
crew were taken, including donning of oxygen masks by those inside the
vehicle, wiping down the contaminated EMU with wet towels, and cycling
the airlock. Eventually, after the EMU had been cleaned and the entire
cabin air volume had been circulated and scrubbed, the astronauts were
allowed to remove their oxygen masks and test the air. No ammonia odor
was detectable and the hazard control procedures were deemed successful;
however, this demonstrates that unexpected chemical hazards may be
encountered even during normal operations.

**Biological Effects**

The biological effects of some chemical hazards are not well understood,
particularly those which are present at low concentrations and in
mixtures with other compounds. While the Spacecraft Maximum Allowable
Concentrations (SMACs), which were first developed in the mid-1960's,
provide an excellent starting point for the evaluation of these hazards,
they should not be mistaken as a completely understood or comprehensive
listing. Approximately 200 contaminants are presently listed in the
SMACs, but there are an unknown number that are not listed, while still
more are under review. In addition, mixtures and potential synergistic
effects are largely unstudied.

The known toxins vary from those with acute impact (such as carbon
monoxide, hydrazine, or hydrogen cyanide) to those with long-term
effects (such as carcinogens). While careful scrutiny of scientific
payloads and spacecraft design is intended to identify these hazards and
mitigate them wherever possible, the spectrum of effects is very broad
and relies heavily upon the specific hazard involved.

**Hazard Control**

Providing a single type of hazard control system that would cover the
wide variety of chemicals required by years of basic research on
multiple space platforms is impractical. Instead, every experiment is
required to provide its own system of hazard control and to track its
chemicals (and byproducts) at all times (a "cradle to grave" approach).
On the ISS, for example, \"each experimental facility on board the lab
module \[has\] its own containment of its (chemicals)." Review by the
Safety Office determines what containment level each rack-mounted
experiment facility must have, as well as providing explicit
instructions for the crew on proper cleanup procedures in the event of
an accidental release.

Prevention is always the most effective method of hazard control, though
operational and mission objectives sometimes require dangerous
substances to be flown, as with the above example of ammonia on STS-98.
Safety review and engineering are also routinely used to minimize crew
exposures -- for example, a safety review might suggest a less hazardous
substitute or engineering controls be put in place to minimize the risk
(such as redesign of the Apollo thruster nozzle/pressure valve
placement). A variety of methods, such as Drager tubes to sense the
presence of specific chemicals, carbon monoxide detectors, and the
compound specific analyzer-combustion products (CSA-CP) device, are all
used to alert the crew when protective measures must be taken. \[See
Figure Nine.\]

When these preventive measures fail, however, more active measures must
be employed, including the use of oxygen masks (as were used on Mir or
Apollo-Soyuz) or decontamination procedures (such as during the STS-98
event).

**Fire Hazards**

**Predisposing Factors**

The closed environment of a spacecraft, like a submarine or airplane, is
a deadly place for a fire. The confined space, limited atmosphere, and
severely constrained evacuation routes all contribute to a fire's
lethality. As a result, preventive measures are considered the first and
most important line of defense. However, despite the best attempts,
fires have still occurred, including a "serious electrical fire" on
Salyut 1 (June 27, 1971) and another on the Mir space station in
February 1997, believed to have been caused by a crack in the SFOG
casing. Jerry Leninger, the US astronaut on the Mir at the time of the
fire, described it as a "very large flame bursting out of the canister
\[with\] smoke billowing out... \[Molten\] metal was flying out and
splattering on the opposite bulkhead." The dense black smoke not only
posed a choking hazard but also obscured the environment, making it easy
for the crew to become disoriented and causing difficulty in finding
things, including the escape route. As another Mir crew member remarked,
"\[In space,\] you can't escape the smoke. You can't just open a window
to ventilate the room."

As described above, oxygen-enriched environments pose a fire hazard,
particularly when ambient pressures are at or above sea level (14.7
psi). Following the Apollo 1 tragedy, NASA established strict rules
governing the oxygen concentration of the cabin environment in order to
minimize this risk.

**Combustion Toxicology**

The pyrolytic byproducts of a fire can be more lethal than the thermal
effects, particularly for a smoldering blaze in a confined space.
Following a fire, exponential increases in atmospheric contaminants
occur. Both the Salyut 1 and Mir space stations experienced fires; among
the contaminants detected were: smoke, particulate matter, benzene,
carbon monoxide, hydrogen cyanide, hydrogen sulfide, and carbon dioxide.
In addition to the atmospheric contamination, there was also significant
deposition on the cabin surfaces, which required additional cleaning.
Following the 1997 Mir fire described above, after removing their
portable oxygen masks, crewmembers wore surgical masks during the
cleanup and for several days thereafter to protect themselves from the
particulate matter and "grime" in the atmosphere.

Another combustion event aboard Mir (February 1998) was much less
spectacular in terms of fire and smoke -- paper filters within a low
temperature catalytic oxidizer ignited -- yet it caused high levels of
carbon monoxide (one of the monitors read 400 ppm) in the cabin
environment for several days and the crew reported symptoms ranging from
headaches to nausea. In many ways, a smoldering fire of this sort poses
a more insidious hazard to the crew, as it can produce large quantities
of carbon monoxide which, colorless and odorless as it is, could well go
unnoticed until serious exposures have occurred.

**Monitoring and Control of Hazards**

The ventilation system is a critical component of the fire detection
apparatus. Circulation of the air not only ensures homogeneous gaseous
composition and temperature (as described above) but also enables early
detection of particulate matter, including smoke. Of course, in the
event of a fire, the ventilation system can also spread the smoke
throughout the vehicle -- as a result, fans are shut off when a fire is
detected in order to minimize the spread, though (as was demonstrated on
the Mir) that may be only partially successful. Jerry Linenger, MD,
described the situation in an onboard interview shortly after the 1997
Mir fire, "smoke filled the station... everyone immediately went to the
\[portable oxygen masks, which\] worked very good and protected us.
Afterwards of course, being a physician, I was very concerned with crew
health. We set up a station for any respiratory problems that might take
place, we had all the emergency gear in place, and from my assessment, I
don't see where anyone had any serious inhalation damage, and it was due
to good action on the crew\['s part\] to get into the oxygen masks
quickly."

Fire detection and suppression on board spacecraft must make use not
only of crewmembers' senses but also automated systems, particularly in
those unoccupied areas such as avionics bays. Detectors which sense
carbon monoxide as well as smoke are necessary, and independent
suppression systems must be located in those areas of the spacecraft
which the crew cannot reach.

**Types of Fire Extinguishers**

Fire extinguishers may make use of water, Halon, or carbon dioxide to
extinguish a blaze in space. NASA prefers to avoid water as it can be
less effective in electrical fires. The Shuttle uses a Halon 1301 agent
that is highly effective and not overtly toxic in its original form.
However, when combusted, cardiotoxic byproducts are produced that are
difficult for the life support system to remove from the atmosphere.
Although carbon dioxide-based extinguishers require a higher
concentration than Halon to suppress a fire, the atmospheric control
systems can much more easily remove carbon dioxide from the cabin
thereafter, and no additional hazards are created.

Russian fire extinguishers generally use water in foam or spray form.
This avoids the problem of added toxins, but can be messy to clean up.
It is also possible to extinguish a fire by denying it needed oxygen,
either by smothering it with wet towels or by sealing off and venting
the involved module. Naturally, this latter option depends upon the
severity and extent of the fire, as well as the availability of a safe
haven for the crew, and subsequent recovery of the decompressed module
may be difficult.

During the 1997 Mir fire, crewmember Sasha Lazutkin initially attempted
to smother the flame with a wet towel, but it was instantly consumed by
the rapidly growing conflagration. \[A smaller fire in 1994 that began
with an identical oxygen generator had been successfully smothered by
cosmonaut Valery Polyakov, using an extra uniform.\] Commander Valery
Korzun then used a water-containing extinguisher: "When I started
spraying foam on the hot canister, the foam didn't stick and had little
effect. So I switched to water \[spray\] and started using that." This
created steam which, added to the already dense smoke, further obscured
and contaminated the atmosphere. The fire was finally extinguished only
after Korzun had directed the contents of three fire extinguishers into
and over it.

**Electrical Hazards**

**Biological Effects**

The effects of an electrical shock can range from an unpleasant tingle
to cardiac defibrillation and death. On the Shuttle, most risks of
electrocution have been "engineered out", but spacecraft construction,
whether of the ISS or, eventually, of lunar and Martian habitats, poses
greater risks. Whenever space travelers are called upon to build, rather
than inhabit, there is unavoidable exposure to wiring and other hazards,
making injuries more likely.

For non-construction activities, internal vehicle systems and payloads
are carefully screened by numerous groups, most notably the Safety,
Reliability and Quality Assurance Office, to ensure that electrical
hazards are noted and avoided wherever possible.

**Hazard Control**

Risk can be mitigated by careful preplanning and extensive practice of
any procedures in which electrical hazards exist. In some cases, this
attention may enable alternate methods to be used, thus eliminating or
minimizing the risk. If the hazard cannot be avoided, proper technique
and attention to safety may provide additional safeguards. As tertiary
prevention strategies, the medical system should be designed with these
hazards in mind, so that proper care may be provided in a worst case
scenario.

Currently, on the ISS a defibrillator is part of the Health Maintenance
System and could be used in the event of an electrically induced cardiac
arrest. \[See Figure Ten.\] However, in the absence of a crew return
vehicle that can safely evacuate a severely compromised casualty, doubts
remain about the ability of the current ISS medical system to provide
the necessary follow-on care.

**Mechanical Hazards**

**Types of Hazards**

The mechanical hazards on board spacecraft are significantly more
limited than those faced on Earth. Falls, motor vehicle-related
accidents, and other high energy-related sources of injuries are largely
absent. With the exception of collisions between spacecraft or with
micrometeorites and orbital debris, the energy for most
microgravity-related injuries is relatively low.

A space traveler can push off from a bulkhead with a fair amount of
speed and thus could inflict or suffer an injury should an unintentional
collision happen soon thereafter, but it would require a willful
disregard of safe practices. While astronauts have a long history of
enjoying a certain amount of horseplay in microgravity, they are also
well aware of both the mission impact and the publicity should their
antics get out of hand. In addition, one of the major contributing
factors seen in similar avoidable injuries on Earth -- alcohol -- is
absent from the space environment.

As a result, most mechanical hazards in space do not reach
life-threatening levels; pinch or crush forces exist and could pose
hazards, particularly to hands or digits. Torque forces could cause
injuries to joints, especially during EVA's when footholds are routinely
used. Sharp edges or recoiling bungee cords can also pose hazards. The
latter caused a problem for one astronaut, who narrowly escaped a
significant injury to the eye when a bungee cord snapped unexpectedly
and struck him in the face.

**Sources of Hazards**

As described above, most mechanical hazards are associated with either
the vehicle itself (sharp edges, hot/cold spots), crew activities
(horseplay, twisting while restrained in footholds), or routinely used
equipment (pinch points, bungee cords).

**Hazard Control**

All payloads and systems are carefully examined at several phases along
the design and flight certification pathway to identify any potential
hazards to the crew. Wherever possible, designs are adapted to eradicate
the hazard or workarounds suggested. In addition, safety procedures are
developed for any unavoidable hazards, such as when the Expedition One
ISS and STS-98 Shuttle crews were instructed to don eye protection to
guard against any floating debris or metal shavings prior to entering
the newly attached US Lab module for the first time.

**Noise and Vibration**

**Sources**

Although a significant issue in terrestrial aviation, vibrations are not
a significant source of health concern in microgravity. The aircraft's
propulsion system and air turbulence are the primary sources of
vibration experienced by terrestrial aircrews, but they are not present
in current spacecraft, which (following launch) rely more on inertial
forces rather than powered flight, and (by definition) are unaffected by
atmospheric conditions.

Although vibration can have effects -- often significant ones -- for
certain payloads, they are not at a level to create difficulties for
manned operations. Noise, on the other hand, is a major concern in
space, just as it is in terrestrial flight.

A major source of both noise and vibration in the spaceflight
environment is the avionics and other equipment: pumps, fans,
compressors, and so on. Although they are required to function
constantly in order to maintain a livable environment, they are often
loud enough to interfere with sleep, work, and even communication. On
long duration missions with long-term, chronic exposures, hearing loss
is a source of significant concern for the space clinician.

Vibrations, outside of the launch or landing phase where they are
associated with the stresses of flight, are usually associated with
either equipment or crew activity. Certain payloads can also cause
vibrations and may require special scheduling in order to prevent their
inappropriately influencing other experiments.

**Frequency**

Audio frequencies are typically 15 Hz -20,000 Hz, with spoken words
between 200-6000. While loud chronic ambient noise can eventually damage
hearing, short-burst noise, if loud enough, can also be dangerous. Even
if not at a level that will lead to auditory damage, unpredictable,
short duration noise can still pose a hazard to crew concentration and
could cause distraction at a dangerous time.

Low frequency (0.1-0.63 Hz) whole body vibration frequently provokes
motion sickness, while higher frequencies (1-30 Hz) cover the resonant
frequencies of the human body. However, as mentioned above, once orbit
has been achieved, spacecrafts do not experience much vibration other
than that produced by the crew or various payload devices. During
launch, however, structural vibration in the Shuttle occurs primarily in
the 2-15 Hz range.

**Intensity**

On Apollo spacecraft, ventilation fans were so loud that they had to be
turned off frequently and could not run continuously. The designs were
improved, and the Space Shuttle, by contrast, is relatively quiet,
though ISS modules vary in their ambient noise. The module with the
lowest ambient noise is the US Lab, where ambient noise is approximately
60 dB, while the Russian Service Module is the loudest (due mostly to
its fans and ventilation system), with a background noise level around
76 dB and peaks above 80.

**Duration**

The longer the mission, the greater the potential hazard posed by noise.
ISS crewmembers currently undergo hearing tests not only pre- and
post-flight, but also during the course of the mission. In this way, if
a threshold shift is noted, additional hearing protection measures can
be adopted, thereby detecting the damage as early as possible and
(hopefully) preventing its progession.

**Physiological and Psychological Effects**

Whole body noise and vibration for extended periods can induce
significant physiological and psychological stress. Significant
alterations to numerous organ systems, including renal, cardiovascular,
nervous, and musculoskeletal, have been reported in the literature. In
addition, noise and vibration increase fatigue and performance
decrements.

A high level of background noise can not only damage hearing, but it
also affects the crew's ability to sleep, lowers morale, and poses
problems for communication both on-board and between the spacecraft and
ground control. For these reasons, work is ongoing to lower the ambient
noise on the ISS, and designers of the next generation spacecraft -- for
exploration missions to the moon and Mars -- are continuing to
investigate new methods for noise reduction.

**Individual Tolerance**

Although there is individual variability in tolerance levels, some
generalizations can be made. At noise levels above 50 dB, most people
must raise their voices to be heard and find the ambient noise annoying.
Above 65 dB, temporary threshold shifts are seen and sleep is impaired.
Above 75 dB, performance degradation is seen. Obviously, for space
travelers -- especially those on long duration missions -- noise and
vibration control mechanisms can be very important for long term health
and mood.

**Exposure Limits**

Most workplace noise and vibration limits are designed for 8 hour
shifts, with 16 hours of recovery between exposures. During space
missions, by contrast, exposure is 24 hours a day, 7 days a week, and
standards need appropriate adjustments to more conservative levels. NASA
has developed its own standards based on the US Occupational Safety and
Health Act but extended to the continuous exposure experienced by space
travelers.

**Operational Effects**

Vibration and noise have deleterious effects not only on crew health but
also on payloads. There are numerous scientific payloads, such as
protein crystallography, that require a vibration-free microgravity
environment. However, recent studies also suggest that barely
perceptible vibrations may stimulate bone growth and thus be an
effective countermeasure for the microgravity-associated bone
demineralization. Vibration could thus be seen as both a positive and
negative for long duration flights.

The operational effect of noise is perhaps more easily appreciated: as a
distraction from mission-critical communication and alarms, in addition
to being a source of increased fatigue and stress, high noise levels are
an unmitigated negative.

**Countermeasures**

There are a number of ways to minimize mechanical noise and vibration,
from selecting different materials during system construction to
altering the equipment's hours of operation. Insulation can often be
used to muffle equipment noise and vibration, but care must be taken not
to interfere with either the function or the cooling of the machines.
Generally speaking, noise and vibration can be controlled at the source,
can be absorbed along the transmission path, or can be attenuated
through personal protective equipment.

Vibration isolation systems (VIS) have been developed for use with ISS
exercise equipment, in order to minimize the vibration associated with
their use. In some cases, without the VIS, the entire station can move
in response to astronaut activity.

On future spacecraft, wall coverings may help to reduce the acoustic
burden within the metal vehicle, and could provide noise muffling as
well as insulation, but as mentioned above. Care must be taken, however,
to avoid microbial growth or a potential fire hazard.

**Individual Protection Methods**

Personal protective equipment in the form of earplugs can be used to
counteract loud ambient noises. However, many crewmembers find them
uncomfortable or a nuisance to wear and as a result do not wear them as
often as they should. This can lead to problems not only for the space
traveler's long term hearing, but also for their ability to hear caution
and warning alarms during the course of the mission.

**[References and Suggested Readings:]{.underline}**

1\) DeHart, RL and Davis, JR. (Eds.). (2002). [Fundamentals of Aerospace
Medicine, 3^rd^ Edition.]{.underline} Philadelphia: Lippincott Williams
& Wilkins.

2\) Barratt MR and Pool SL (Eds.). (in press). [Principles of Clinical
Medicine for Space Flight.]{.underline} New York: Springer Verlag.

3\) Clement, G. (2003). [Fundamentals of Space Medicine. Space
Technology Library.]{.underline} El Segundo: Microcosm Press.

4\) Ball, JR and Evans, CH. (Eds.). (2001). [Safe Passage: Astronaut
Care for Exploration Missions.]{.underline} Washington DC: National
Academy Press.

5\) Harding, R. (1989). [Survival in Space.]{.underline} London:
Routlege.

6\) Busby, DE. (1968). [Space Clinical Medicine.]{.underline} Dordrecht:
D. Reidel Publishing Company.

7\) Parker, JF and West, VR. (1973). [Bioastonautics Data Book, 2^nd^
Edition.]{.underline} Washington DC: NASA Scientific and Technical
Information Office.

8\) Nicogossian AE, Huntoon CL, Pool SL. (Eds.) [Space Physiology and
Medicine*,* 3^rd^ Edition.]{.underline} Philadelphia: Lea & Febiger;
1994.

9\) Larson WJ and Pranke LK (Eds.). (1999). [Human Spaceflight: Mission
Analysis and Design.]{.underline} New York: McGraw Hill.

10\) Stine GH. (1997). [Living in Space: A Handbook for Work and
Exploration Stations Beyond the Earth's Atmosphere.]{.underline} New
York: M. Evans and Company.

11\) Newkirk D. (1990). [Almanac of Soviet Manned Space
Flight.]{.underline} Houston: Gulf Publishing Company.

12\) Burrough B. (1998). [Dragonfly: NASA and the Crisis Aboard
Mir.]{.underline} New York: Harper Collins.

13\) Shayler DJ. (2000). [Disasters and Accidents in Manned
Spaceflight.]{.underline} Chichester: Springer-Praxis.

14\) "Breathing Easy on the Space Station", a Science@NASA story,
<http://spaceflight.nasa.gov/living/factsheets/breathing5.html>