vision and eye protection
Of the many tasks required of pilots and controllers, nearly all use
vision as a critical source of information input and assessment of
outcome. Indeed, visual input is the most critical source for
pilot/controller judgment and decision making. What can they do to protect
and enhance their vision? This article offers tips in several areas.
Many hazards to vision exist. All can jeopardize a pilot or controller's
career. Some hazards can rob a pilot/controller of vision in one eye very
suddenly, while others may affect both eyes very gradually. This section
is divided into vision protection suggestions for physical hazards,
radiation hazards, health hazards and nutritional hazards. Visual scanning
techniques for day and night flying are reviewed. Sunglasses for pilots
have very specific requirements. Eye strain and fatigue avoidance will
improve vision. FAA policy concerning waivers for vision problems are
outlined. Please see the VFS articles on Vision and FAA Standards,
Cataracts, Eyes and Physiology of Vision, LASIK,PRK, RK and Vision
Many people lose their vision to preventable eye injuries each year. These
injuries occur at the work place, during participation in sports, in motor
vehicle accidents and in casual activities around the house. Nearly 90%
are preventable. Traumatic injuries tend to occur suddenly with potential
total loss of vision in one or both eyes.
Active sports participants should use eye protection if available.
Polycarbonate "sports" lenses are shatter resistant and can stop a .22
calibre bullet. Vision protection is absolutely necessary in indoor
racquet sports, such as squash and racquetball. High contact sports such
as hockey and lacrosse also put the eye at risk for serious injury. Many
football players are now using face shields to prevent career ending
injuries. Hunters, trap shooters and target marksmen often wear protective
lenses that enhance contrast and avoid eye injuries. Baseball players may
wear polycarbonate lenses to protect their eyes, even if they do not need
glasses for better vision. The American Academy of Ophthalmology has
specific recommendations for athletic eye protection for many sports.
American Family Physician has published an article, Prevention and
Treatment of Common Eye Injuries in Sports and a patient information
handout, "Eye Injuries in Sports" on this important subject.
Eye injuries on the worksite may affect even full-time pilots and
controllers. Certainly those who do their own maintenance have experienced
sprays of hydraulic fluid, metallic chips or (hopefully not!) flakes of
rust near or in their eyes. Rust particles can permanently stain the
cornea (clear part of the front of the eyes) while a metal splinter coming
off a hammer may penetrate the globe of the eye. Simple and inexpensive
soft plastic goggles can prevent all of these injuries. For those working
with welding equipment, additional protection is required.
Many permanent eye injuries occur in the home or with recreation. Wood
chips and splinters thrown from power saws, rust and petroleum products
from working underneath a car and playful pets with claws all cause
serious injuries. Trimming bushes that reach to eye height often leads to
individuals focusing on one branch and walking into another. For the free
spirits who ride motorcycles without helmets, goggles or glasses, a bug in
the wind stream or a rock thrown from a tire can be disastrous. Having
goggles or wrap around glasses can prevent each of these injuries.
There are many forms of electromagnetic radiation. The primary forms of
concern to pilots and controllers include both ionizing and non-ionizing.
Non-ionizing radiation includes visible, near-visible (ultraviolet and
infrared) and radiofrequency. In general, aircraft cockpit windscreens,
fuselages, and tower cab windows block any significant ionizing radiation
that may affect the eye. Visible light and ultraviolet (UV) light require
sunglasses for comfort and improved visual acuity depending on the
intensity, but UV light does not present hazards when in an aircraft.
Sunglasses worn on bright days also improve night vision if a pilot is
flying from daylight into darkness. Outside an aircraft in direct or
reflected sunlight, protection from UV light is desirable (see sunglasses
for pilots below). UV-B light less than 315 nanometres in length, the same
type that causes sunburns and skin cancer, can induce cataracts, but is
blocked by plain glass and polycarbonate. It will penetrate acrylics and
soft plastics found in cheap sunglasses. It also contributes to macular
Infrared (IR) radiation is perceived primarily as heat. It is not a hazard
to vision. Microwave radiation from radar's may accelerate cataract
formation if an individual repeatedly stands in front of a radar that is
on. The microwave radiation is converted into heat energy when it is
absorbed by the lens. The heat causes the proteins of the lens to "clump"
and form a focus for a growing cataract which will obscure vision.
Weather radar's are generally lower power and will not cause as
significant visual problems as will military search radar's. Incidental
exposures, such as performing a pre-flight on an airliner with the weather
radar powered on a single time, is unlikely to cause any damage. Although
microwave ovens are supposed to be shielded, it would be wise not to stare
closely at food cooking in a microwave oven to avoid possible IR damage to
the lens of the eye.
Eye Strain and Vision Fatigue
As we move into the computer age, eye strain at work and at home is a more
frequent phenomenon. The eye focuses by using muscles to change the shape
of the lens, thereby adjusting for different working distances. As we age,
the lens becomes less pliable and focusing at near becomes more difficult.
This condition is called presbyopia.
When we stare at a computer monitor for extended periods of time, we are
asking the ciliary muscles of the eye to constantly contract a fixed
amount to keep the screen in focus. Just as a biceps muscle fatigues if
someone holds an object in a constant position (remember your student
pilot days holding the yoke for a straight and level attitude without
proper trim), the eye muscles fatigue if focusing at a fixed distance. As
a result, the eyes may get more blood flow (turn red), get dried from not
blinking (increased tears to compensate) and not be able to rapidly change
The key to prevention of eye strain and fatigue is to take frequent breaks
and focus the eyes at a distant object. This allows the ciliary muscles to
relax. Computers do not emit enough radiation to be a hazard to vision.
Likewise, reading for long periods of time or reading in the dark will
cause eye fatigue. Neither will cause any permanent damage, but both will
lead to temporary problems focusing, redness, tearing and headaches.
Overly bright conditions or large amounts of reflected light cause the
pupils to constrict, also using eye muscles to constantly adjust the light
reaching the retina. Although these are different muscles than those
changing the shape of the lens, the same symptoms of fatigue occur. The
key to preventing this type of fatigue is to read in a comfortable level
of light and take frequent breaks.
Sunglasses for Pilots
There are many types of sunglasses available, but no single type is ideal
for every pilot. Needs change based on age, light sensitivity, ambient
lighting conditions and type of flying. Some sunglasses are not right for
any pilot at any time. Valid reasons for wearing sunglasses in the
aviation environment include improved night vision adaptation, enhanced
contrast in the visual field, reduced glare, decreased UV exposure and
avoidance of eye fatigue. Though style and appearance may be a
consideration, the safety conscious pilot should focus on the proper
selection of lens features rather than frame styles with cheap lenses.
Visual acuity varies with the light available and the sensitivity of an
individual to various degrees of brightness. The pupil controls the amount
of light reaching the retina. Older individual's eyes do not transmit as
much light through the eye as younger people do. Therefore, many older
individuals need more light for optimum acuity. They may want to use
sunglasses that transmit more light. On high glare days, such as over snow
or sand, the pupils contract to protect the eye from the glare. Sunglasses
will reduce glare and allow the pupil to let more light on to the retina,
thus enhancing vision.
Sunglasses for Pilots - Glare
Glare can also be caused by indirect blue wavelength light and UV light.
UV light increases by 4% for every 1,000 feet of altitude and contributes
to the blue colour of the sky. Some researchers feel this can cause a
haziness on the retina decreasing visual acuity even when indirectly
viewed. Fortunately, most windscreens eliminate much of this wavelength.
Near sunrise and sunset, the atmosphere filters out this wavelength giving
the sky its characteristic red-orange colour. Yellow lenses, often called
"blue blockers" will block out this wavelength also and may improve vision
on a hazy day. They may present decreased perception of some cockpit
Sunglasses for Pilots - Tinted lenses
Tinted lenses distort colours to some extent. The yellow shaded "blue
blockers" will alter colour perception if tinted enough to block out 30%
of the light. Thus, these lenses should be used only by aviators on
bright, hazy days and avoided in low light situations. Green and grey
lenses have the least distorting effect on colour vision. Brown distorts
colours slightly more, but can block some of the blue light blurring in
Sunglasses for Pilots -
Darkness or degree of light reduction is calculated in percentage and
listed by numbers. A #1 lens blocks only 20% of the incoming light and has
little value for aviators. The exception may be the #1 Yellow lenses for
hazy or smog filled days. The #2 lens blocks 70% of light and are useful
for most aviation situations. It provides a balance of glare protection,
luminescence reduction and UV protection without significantly reducing
visual acuity. The light reduction of #3 (85%) may be useful for those
pilots particularly sensitive to bright light while others may find the #3
lens reduces visual acuity. The #4 lens blocks out 95% of incoming light
and significantly reduces visual acuity because the macula, where sharpest
vision is found on the retina, requires light to activate the cones of the
retina. Aviators wearing these lenses in flight will not meet FAA minimum
distant visual acuity standards.
Sunglasses for Pilots -
Mirrored glasses use metal particles to reflect images. They scratch
easily and can cause distortion or blind spots. While popular for
Hollywood movie pilots, professional pilots should leave them to actors,
policemen and other imitators.
Sunglasses for Pilots -
Photochromatic lenses increase darkness when exposed to UV light. Because
aircraft windscreens block most UV light, the lenses will not darken
substantially inside an airplane or car. The military prohibits its pilots
from using these sunglasses for good reason. Pilots flying open cockpit
airplanes are the only ones who may benefit from this feature.
Sunglasses for Pilots - Gradient lenses
Gradient lenses usually have a darker tint on the upper portion of the
lens and a gradually lightening color near the bottom. This may be useful
when trying to view instruments on a very bright day. The lighter tinting
below allows more light from the relatively dark instrument panel to reach
the retina and improve visual acuity while blocking out the glare from the
Sunglasses for Pilots - UV protection
UV protection is desirable for lenses worn outdoors, although it is not as
important for glasses worn inside the aircraft since this wavelength is
already blocked by the windscreen. Glass and polycarbonate block nearly
all of the UV-B light. Soft plastic lenses may block visible light but not
block any UV wavelengths. The hazard for cataract formation in the
individual using soft plastic lenses is increased because the pupil
dilates in response to decreased visible light. The dilated pupil allows
more UV light to enter and penetrate the lens increasing the risk of
Sunglasses for Pilots - Scratch resistant coating
Scratch resistant coating may increase the life of polycarbonate lenses
and plastic lenses. Ironically, polycarbonate will withstand direct hammer
strikes without breaking, but scratches relatively easily. Glass will
shatter, but is more resistant to scratching. Polycarbonate lenses are
thinner and lighter than glass lenses.
Sunglasses for Pilots - Polarized lenses
Polarized lenses should not be worn by pilots in the cockpit. Glare from
flat surfaces is blocked by polarized lenses which are oriented in
parallel lines like closely spaced prison bars. Light parallel to the
lines is transmitted while non-parallel light (glare) is blocked.
Unfortunately, if the windscreen is polarized and the lenses are not
precisely oriented the same as the windscreen, all light may be blocked.
Changing bank angle and head position could create blind spots. For those
who are boating and need glare protection from light reflected off the
water, polarized lenses are excellent choices.
Sunglasses for Pilots - What we recommend
What we recommend: On bright days, consider using a neutral tint (green or
grey) glass or polycarbonate lens that blocks 70-90% of the incoming
light, possibly with a gradient that lightens on the lower portion of the
lens. On a hazy or smog filled day, consider a yellow or brown lens that
blocks 20% of the light, but avoid using it if colour perception (IFR
flight), as opposed to visual acquisition (VFR flight), is important.
The military has found that some fighter pilots prefer the "high contrast"
yellow visors for their helmets to enhance target acquisition, while
others prefer not to wear yellow visors. At dusk or in lighting that is
comfortable without sunglasses, remove them to increase visual acuity.
Don't use polarized or photochoromatic lenses in the cockpit. Don't waste
your money on soft plastic lenses or mirrored lenses. Scratch resistant
coating may increase the life of polycarbonate lenses.
For additional information, please see the July-August 2002 Flight Safety
Foundation article on "For Pilots, Sunglasses Are Essential in Vision
Protection" and the Fall 2003 article from the Federal Air Surgeon's
Medical Bulletin on "Sunglasses in Aviation: A Primer for Pilots" by Mr.
Montgomery and Dr. Nakagawara of the FAA Vision Research Institute.
Nutrition for Optimum Vision
Many people think of the role of nutrition in lowering the risk of heart
disease, reaching ideal body weight, controlling diabetes and other
medical conditions. Few people consider nutrition in optimizing their
vision, a critical function for pilots and controllers. Current research
supports the role of several nutrients in visual health.
Next to errors in visual refraction, cataracts are the most important
cause of degraded vision in pilots and many controllers. This is
particularly true after age 50 years. Over 1.5 million cataract surgeries
are performed in the U.S. each year, costing $6 billion. It is the largest
single item on the Medicare budget. Over 40,000 Americans are legally
blind from cataracts. The hazards of radiation in causing cataracts are
discussed above. Cataracts are a surgically treatable cause of vision
loss. Several nutrients have been demonstrated to lower the risk of
Vitamin C is a powerful antioxidant that may protect the lens of the eye
from the free radical formation caused by radiation and aging. The lens
concentrates vitamin C sixty times greater than in the blood. Because the
lens does not have any direct blood supply, dosages of 1,000 mg per day
may be necessary to raise vitamin C levels in the lens. This dose
significantly reduced the requirement for cataract surgery over 11 years
in 450 patients with cataracts compared to those not taking vitamin C in
an older study in Archives of Internal Medicine.
A 1992 study in the British Medical Journal of 50,000 nurses demonstrated
a 45% lower risk of cataracts over 10 years for those taking vitamin C.
Harvard University's Physicians' Health Study of over 22,000 doctors
showed a 27% reduction in the incidence of cataracts in those doctors
taking multivitamin supplements. Research at Tufts University's USDA Human
Nutrition Research Center on Aging by Jacques demonstrated women on
vitamin C supplements for 10 or more years had 77% reductions in early
lens opacities, the beginning of cataracts. Jacques also showed people
with low vitamin C levels in the blood had 11 times the risk of cataracts
as those with the highest blood levels of vitamin C.
Vitamin E may also lower the risk of cataracts. The May 1998 issue of
Ophthalmology studied the growth of cataracts in 754 individuals. Those
using a multivitamin supplement containing vitamin E reduced the rate of
cataract formation. In those taking a regular vitamin E supplement and
having higher blood levels of vitamin E, cataract formation was reduced by
half. Doses of 400-800 IU per day are recommended. In individuals taking
either 300 mg of vitamin C or more than 400 IU of vitamin E had less than
half the amount of cataracts as those who did not use supplements.
Selenium is an essential cofactor in the breaking down of hydrogen
peroxide, a free radical, in the liquid filled chamber in the front of the
eye called the aqueous humor. The enzyme that does this, glutathione
peroxidase, depends on selenium. One study showed reduced levels of
selenium in the blood and aqueous humor of people with cataracts, but no
difference in the selenium levels in the lens itself. N-acetyl-cysteine is
a precursor to glutathione and helps maintain vitamin C levels in the eye.
Zinc may also play a role in reducing the risk of ARMD and studies are
ongoing, but definitive evidence is lacking.
A second significant cause of vision deterioration is Age Related Macular
Degeneration (ARMD). It is the leading cause of blindness over age 65.
ARMD is generally not treatable and results in permanent blindness. Over
11 million Americans have ARMD, including up to 20% of those aged 65 and
greater. The macula, as described below, contains the rods of the retina
responsible for our sharpest vision. People affected with ARMD lose the
ability to drive, read and even recognize faces. Obviously, a pilot and
controller's medical certificate is jeopardized even in early ARMD.
Risk factors for macular degeneration include smoking, blue eyes,
long-standing exposure to UV radiation, family history of vision loss,
female gender and obesity. Recent evidence indicates poor nutrition may
increase the risk of ARMD. A review article in the New England Journal of
Medicine discusses Age Related Macular Degeneration. Another article in
American Family Physician provides a well illustrated description and
explanation of ARMD. Also see Flight Safety Foundation article on "Early
Detection is the Key in Correcting Age-Related Vision Problems Among
When your physician looks in your eye, the macula appears yellow because
of the high concentration of carotinoids, especially lutein and zeaxanthin.
Many of these carotenoids are building blocks of vitamin A and
beta-carotene. As well as improving night vision, these nutrients may
exert an antioxidant effect that protects against long term UV exposure
contributing to ARMD and stabilize capillary (blood vessel) membranes in
The National Eye Institute's Eye Disease Case Control Study showed
individuals with higher levels of carotenoids had lower risks of ARMD.
Lutein, zeaxanthin, alpha and beta-carotene, lycopene and cryptoxanthine
all contributed to carotenoid levels. Vitamins C and E and selenium did
not affect the risk of ARMD.
Lutein is a well studied carotenoid that does not form vitamin A. Humans
deposit lutein and zeaxanthin in the macula. They protect the macula from
oxidative damage caused by free radicals triggered by UV and visible blue
light. Lutein is 10 times more effective as an antioxidant in the eye as
is vitamin E. Studies in the Archives of Ophthalmology show that the
higher the concentration of lutein, the lower the risk of ARMD. The
Journal of the American Medical Association reported in 11/94 that 6 mg of
lutein per day lowered the risk of ARMD by 43%. Several other studies
support the role of lutein in reducing the risk of ARMD.
Bilberry is a plant related to the blueberry and cranberry. It contains a
high concentration of flavinoids and anthrocyanosides. They exert an
antioxidant effect and improve ocular circulation. Bilberry also improves
night vision by stimulating rhodopsin production, an essential pigment in
the rods used in low light condition vision. Anecdotally, there is a story
of one RAF unit during the Battle of Britain had much higher kill rates at
night than other nearby units. The flight surgeon researched differences
in the units and found the only notable difference was that the higher
scoring squadron regularly used bilberry jam at tea time since bilberries
were found in the wild near the base. Although the explanation was not
apparent at the time, current research supports this observation.
Glaucoma - Regular Check-ups Save Vision
Age Related Macular Degeneration affects the central vision primarily and
is readily apparent to those afflicted with it. Glaucoma is a much more
common condition that is not readily apparent to its victims. Over 2
million Americans have glaucoma, but only half have been diagnosed. Nearly
80,000 are blind from this entirely preventable condition. Regular
check-ups by your eye professional will detect early warning signs of
glaucoma. See an article and a handout in American Family Physician, May
15, 2003, on glaucoma.
The Intraocular Pressure (IOP) of the fluid in the eye is usually measured
by a machine that gives a puff of air against the cornea. This pressure is
compared to "normal" pressures, similar to the way an individual's blood
pressure is compared to normal levels. Borderline levels, called ocular
hypertension, are observed and higher levels are treated with medication.
Glaucoma can also be treated with laser surgery.
In glaucoma, chronic elevations of IOP are thought to cause damage to the
optic nerve at the back of the eye. There is no pain associated with most
forms of glaucoma and no early symptoms. If left untreated, a person
begins to lose peripheral vision. This vision loss is permanent. Because
the peripheral visual fields are lost initially, many victims do not
notice this visual loss until the vision is restricted to a view similar
to looking through a tube or round window. Obviously, a pilot's ability to
clear for other aircraft is restricted by this condition.
The FAA medical examination does not include a check of IOPs. Visual
fields are noted on the exam, but even if performed correctly, only
detects late changes. The FAA will waiver pilots to fly and controllers to
control on most anti-glaucoma medications or after surgery for glaucoma.
Prior to returning to safety sensitive duty with a diagnosis of or
treatment for glaucoma, a pilot/controller must have an FAA Form 8500-14
completed by an eye care specialist with complete visual field testing
done. Contact Virtual Flight Surgeons for aeromedical assistance in
obtaining waivers to fly or control with this condition.
Diabetes is the leading cause of blindness in working age Americans.
Diabetes leads to blindness by causing a condition know as diabetic
retinopathy. Blood vessels in the back of the eye grow rapidly and leak
blood into the back chamber of the eye called the vitreous. These blood
leaks block vision. The retina can also detach which can lead to sudden
blindness. Poor blood sugar control can lead to leaking of serum into the
retina called exudates. Cataract formation is also increased in diabetes.
Diabetes and its implications for pilots and controllers is discussed in
the VFS article on Diabetes in the Information Resource section. This is a
very serious medical condition that requires careful evaluation,
monitoring and treatment. Vision changes are late finding with diabetes
and indicate that other organ systems are probably affected also.
Visual Scanning Techniques
The "see and avoid" rules for VFR flying (and IFR in some cases) are
essential for safe aviation. Effective visual scanning techniques depend
on many factors. These factors include good vision, a current lens
prescription if needed, ocular health and nutrition, appropriate
sunglasses and a very effective visual scanning technique. Visual scanning
techniques in the daytime and at night vary significantly, if fact, they
are nearly opposite from each other. While some of the other factors
affecting visual clearing for other aircraft are dependent on natural
visual health and available equipment, visual scanning techniques can be
learned (and forgotten). The FAA Office of Aviation Medicine's Aeromedical
Education Division has an excellent safety brochure on Pilot Vision for an
illustrated discussion of scanning techniques and eye anatomy. Caution!
The information on the location of the macula in the optic disc and the
absence of rods and cones in the macula is not correct in the safety
The portion of the eye that contributes to focusing images and sending
those images to the brain is called the retina. The retina is found on the
inner lining of the back portion of the eyeball. The retina contains rods
Rods are critical in low light and night time (scotopic) vision, but do
not have very good visual acuity (focusing ability). Experts estimate that
the best visual acuity using inputs from rods varies from 20/60 to 20/200
or worse. Rods are located in the periphery of the retina and are about
100,000 times more sensitive to light than cones. Rods are also critical
in detecting motion and provide peripheral vision. They do not detect
colours, but perceive objects in shades of grey.
Cones are found in the central portion of the retina in a small area of
about 10 degrees diameter called the macula. A condition known as Age
Related Macular Degeneration severely degrades vision on this area. The
central 1-2 degrees of the macula is called the fovea, which detects the
finest details. The cones are responsible for vision in bright light
conditions (photopic vision). The cones provide the best visual acuity and
colour vision. Outside this area, visual acuity drops off by 90% as the
concentration of cones decreases and the concentration of rods increases.
Because fovea contains no rods for night vision, it is also called the
"night blind spot."
Vision in twilight conditions is call mesopic vision and uses a
combination of rods and cones. The eye also contains a "physiological
blind spot" where the optic nerve enters the eyeball and the retina is
devoid of any rods or cones. This area is about 15 degrees off the central
visual axis and fovea. With different parts of the retina providing
critical inputs in different lighting conditions, you can see that visual
scanning techniques vary depending on the level of available light in the
area scanned and the contrast between the target and its background.
Visual Scanning - Day Time
Because the best visual acuity comes from the central portion of the
retina called the fovea, only a small cone of vision directly in front of
the eye about 10 degrees in diameter actual focuses on objects and
provides details with colour vision. Recall that areas outside this
central cone of best visual acuity detect motion in shades of grey. In day
time, the best scanning technique involves focusing on segments of the sky
in 1-3 second intervals. This allows you to detect small objects directly
in the area you are focusing in, while letting your peripheral vision
detect motion. After a brief search of one section of sky, shift your
focus to another section about 15-20 degrees offset from the original
area. Repeat the search in a stop-turn-stop method. If you detect motion
in your peripheral vision, shift your focus to that area to pick out
details and colours.
A sweeping technique across the horizon in daytime is not effective. Your
eyes do not have time to focus on any one section of sky for object
detection and the constant motion of the eye negates the ability of the
peripheral vision to detect motion. Scan at, above and below the horizon.
Be sure to scan laterally also. FAA mishap figures show that you are five
times more likely to experience a midair collision with an aircraft moving
in the same direction as your aircraft as you are to experience a head-on
Visual Scanning - Night Time
Night time visual scanning is completely different from day time scanning.
It is not intuitive and therefore, must be learned and practiced. Recall
that there is no ability to focus directly on objects in low light
conditions (night blind spot). You must rely on images arriving on the
rods in the periphery of the retina. A pilot searching for objects at
night should look off-centre about 10-15 degrees from where it is
expected. Once it is detected, looking directly at it will cause it to
disappear. Instead, the Air Force teaches a "diamond scan" technique. This
involves looking above, below and to each side of an object in a diamond
shaped scanning pattern to keep it from bleaching out and disappearing.
See the article from Flight Safety Foundation on "Darkness Increases Risks
Night vision is affected by many factors. Light is the most significant.
Dark adaptation takes about 20-30 minutes. A single exposure to a bright
light will cause night vision to deteriorate until the adaptation can
occur again. If outside during the day or flying, sunglasses should be
worn to allow full night adaptation to occur as lighting diminishes.
Cockpit lighting should be dimmed to the minimum necessary to see
instruments while allowing maximum night vision adaptation.
Red filters on lights will also prevent washout of night vision, but red
writing and lines on maps, charts and operating handbooks may not be
visible if viewed with red filtered light. Nutrition is critical as
mentioned above. Vitamin A and beta-carotene are critical nutrients for
optimum night vision. Smoking degrades night vision by about 20%. Vision
is also very sensitive to oxygen levels. The retina uses large amounts of
oxygen, particularly at night. Oxygen supplementation is recommended at
cabin pressures above 5,000 feet MSL at night.