Inflight Occupational Exposures to Cosmic
Radiation and Magnetic Fields
Reprinted
with permission from Air Line Pilot, January 2000, page 30
by
Capt. Gary C. Butler (Canadian), Ph.D.
Director, ALPA Aeromedical In-Flight Occupational Exposure Research
Scientists have known since 1907 that
radiation originating outside the earth’s atmosphere causes increasing
effects as altitude above the ground increases. Modern scientists generally
believe that stellar flares, stellar coronal mass ejections, supernova
explosions, pulsar acceleration, and galactic nuclei explosions produce and
accelerate this radiation, called cosmic radiation.
Radiation is transmitted through space in
the form of photons or subatomic particles. Cosmic radiation is a mixture of
various types of “ionizing radiation”—a term used if the photons or particles
produce, in the material irradiated, one or more ions, or electrically
charged atoms or group of atoms. Ionization is the primary process by which
radiation exposure affects biological organisms.
Flight crews, while flying, are exposed to
amounts of cosmic radiation that are greater than persons on the ground
receive from both cosmic and terrestrial radiation. Sources of terrestrial
radiation that contribute to whole-body exposure include potassium-40 and the
radioactive decay products of uranium-238 and thorium-232. The three are
found in soil and rock near Earth’s surface.
Galactic radiation
At airliner flight altitudes, high-energy subatomic particles—primarily
protons (95 percent), alpha particles (3.5 percent), and heavier
nuclei—mostly from outside the solar system are colliding with and disrupting
atoms of nitrogen, oxygen, and other atmospheric gases. These collisions
produce photons and additional subatomic particles, which are referred to as
galactic cosmic radiation. The number of galactic radiation particles
entering the atmosphere and, as a consequence, the amount of radiation at
airliner flight altitudes varies inversely with an approximate 11-year cycle
of rise and fall of solar activity. Magnetic fields associated with
low-energy subatomic particles (solar wind) that the sun continuously emits
deflect lower-energy galactic particles that would otherwise enter the
atmosphere; this causes the variation in the amount of galactic radiation.
The solar wind particles are in themselves too low in energy to affect the
amount of radiation at flight altitudes.
Variation with latitude and altitude
At any given cruise altitude, the
galactic radiation dose-rate increases with distance north or south of the
equator until it plateaus at high latitudes. In the northern hemisphere, at a
constant altitude, the galactic radiation level shows little or no increase
above 50 degrees latitude in North America and 60 degrees in Europe and Asia.
Amounts of radiation above the Poles at airliner cruise altitudes are twice
those above the equator at the same altitudes. Earth’s atmosphere provides
significant shielding from cosmic radiation. The lower the altitude, the
thicker the atmospheric layer and, therefore, the greater the protection.
Solar flares
A solar flare is an intense magnetic disturbance on the sun,
resulting in an explosive emission of various kinds of radiation. During some
solar flares, the number and energies of particles emitted from the sun may
temporarily become high enough to significantly increase the amount of
ionizing radiation at flight altitudes, particularly over the polar regions.
Until very recently, solar flares that
would emit particles could not be predicted reliably nor could one predict
how far the radiation would reach, even after the event had occurred.
However, NASA, using the Japanese Yohkoh spacecraft, discovered that S-shaped
patterns often appear on the surface of the sun before these violent solar
eruptions. NASA’s data indicate a strong correlation between an S-shaped
pattern on the sun and the likelihood that an eruption will occur in that
region within days.
At altitude, the radiation resulting from
solar flares is produced in the same way as galactic cosmic radiation is
produced. The particles from the sun, as well as the photons and particles
they produce in the atmosphere, are referred to collectively as solar cosmic
radiation. Between 1956 and 1991, inclusive, approximately 6 solar particle
events occurred during which the radiation at 41,000 feet above the polar
regions probably rose to more than 100 microsieverts per hour. The normal
radiation level at these locations is approximately 12 microsieverts per
hour.
When considering health effects of
ionizing radiation, the amount of radiation that an individual receives is
expressed in terms of sieverts. The sievert is a measure of the biological
harm that ionizing radiation may cause and is the current international unit
for this measurement. The sievert replaces the rem.
1 sievert=100 rem
1 sievert=1,000 millisieverts (mSv)
1 millisievert=1,000 microsieverts (mSv)
Effective dose: When irradiation of the
body is not uniform, the effective dose is an approximately uniform
whole-body dose that would result in the same risk of cancer, hereditary
effects, and length of life lost as the nonuniform exposure. When irradiation
of the body is approximately uniform, the dose and effective dose are the
same.
Radiation exposure and recommended
limits
The FAA (Dr. Wallace Friedberg,
Civil Aeromedical Institute) has estimated the amount of galactic radiation
that aircrew members receive on a wide variety of routes to, from, and within
the contiguous United States. Cumulative dosage, of course, will vary
depending on altitude, latitude, and duration of flight on the route flown.
At the present stage of the solar cycle,
the galactic dose ranges from 0.023 to 0.80 millisievert per 100 block hours.
For example, based on 0.60 mSv per 100 block hours (the mean for a flight
between New York City and Athens, Greece), a pilot flying 700 block hours per
year would receive an annual occupational exposure of 4.2 mSv. In contrast, a
pilot flying 700 block hours on a Chicago-to-San Francisco route (0.41
mSv/100 block hours) would receive an annual dose of approximately 2.8 mSv.
Typically, cosmic radiation exposure for
airline pilots in North America ranges from 3 to 5 millisieverts annually.
These values are considerably lower than the occupational limit of 20
millisieverts per year (5-year average) that the International Commission on
Radiological Protection (ICRP) recommends for a nonpregnant adult.
Considerations during pregnancy
Some recommendations concerning
exposure apply only to pregnant women. The ICRP recommends that once a woman
learns she is pregnant, her occupational exposure to ionizing radiation
should not exceed 1 millisievert for the remainder of the pregnancy. Further,
the exposure of the unborn child should not exceed 0.5 millisievert in any
month (excluding medical exposures), once a pregnancy becomes known.
For radiation protection purposes, the unborn child is assumed
to receive the same dose of cosmic radiation as the mother; but on some
flights, the galactic radiation that an unborn child receives may exceed the
recommended limits. For example, consider two flights: a long (9.5 hours) New
York-to-Athens flight and a short (0.5 hour), low-altitude flight from
Houston to Austin, Tex. The effective dose from the long-haul flight is
approximately 64 microsieverts (mSv), whereas the effective dose from Houston
to Austin is only about 0.14 microsieverts.
The long-haul pregnant pilot will clearly
exceed the 1 mSv limit (64 mSv equals 0.60 mSv/100 block hours/month).
Working on short, low-altitude flights, or alternately, being assigned to a
ground position for the remainder of the pregnancy, could reduce a pregnant
pilot’s exposure to radiation.
All flightcrew members can calculate their
cosmic radiation dose using the FAA computer program CARI-5E. A free download
is available through the Internet at
http://www.cami.jccbi.gov/research/610/600radio.html
Health concerns
The health concerns about aircrew members’ exposure to cosmic radiation
are increased risk of cancer, genetic defects that can be passed on to future
generations, and harm to an unborn child. Death from cancer is the primary
health risk associated with occupational exposure to ionizing radiation; that
is, damage to genetic material (DNA) in the cell is thought to be the
mechanism that underlies the potential risk of increased cancer.
The risk to aircrew members of eventually
dying of cancer as a result of exposure to galactic radiation during a career
of flying, compared to the risk to the general population, is slightly higher,
as are the risks of causing genetic defects and of harming an unborn child.
Research projects investigating health
effects in flight crews caused by cosmic radiation exposure are relatively
few. Generally, while these studies have reported some additional cancers,
their results are far from definitive.
At present, although one cannot exclude
the possibility of harm from exposure to cosmic radiation at the doses likely
to be received during a career of flying, establishing that an abnormality or
disease in a particular individual resulted solely from radiation exposure
would be impossible. Cosmic radiation is but one of the environmental
stressors that affect pilot health.
Among the physiological challenges to the
long-term health of an airline pilot are circadian dysrhythmia, reduced
atmospheric pressure, mild hypoxia, low humidity, noise, vibration, cosmic
radiation, and magnetic fields. In particular, exposure to cosmic radiation
and its potential for causing cancer have recently received considerable
attention.
Given the complexity of an airline pilot’s
environment and the possibility that two or more of the stressors might work
together to cause more harm than each would separately, comprehensive
research into both cosmic radiation and magnetic field exposures in airline
pilots is needed now. However, studies of how radiation affects airline
flight crews are confronted with several challenges.
First, one must deal with the probability
that both ionizing and nonionizing radiation (magnetic fields) enhance the
harm inherent in several other occupational exposures. Second, as health
effects among flightcrew members are small, detecting them may be difficult.
Third, access to a large population of airline pilots has been traditionally
difficult to secure. Fourth, actual measurement of cosmic radiation doses is
not yet on solid ground, which may result in an underestimation of doses. And
fifth, any discussion of potential ionizing radiation risk and/or cancer
incidence assessment must include biological markers of past cosmic radiation
exposure.
Within this context, ALPA (Project
Director: Dr. Gary C. Butler), in conjunction with the Medical University of
South Carolina (Department of Biometry and Epidemiology, Dr. J. Nicholas and
Dr. Dan Lackland), has begun an extensive research program to study these
occupational exposures. Dr. Don Hudson (ALPA’s aeromedical advisor) and Capt.
Allan Campbell (Delta pilots’ Master Executive Council Aeromedical Committee
chairman) are also involved in this project. The FAA’s Civil Aeromedical
Institute (Dr. Friedberg) is also involved in conducting this research.
The research program is now making
ground-based calculations, quantifying magnetic field exposure on the flight
deck, and undertaking an epidemiological survey and exposure assessment. The
epidemiological study has three primary objectives:
·
Assess cancer
incidence in a large group of U.S. and Canadian
airline pilots, through a health survey sent to approximately 9,000
active and 1,000 retired Delta Air Lines pilots and to 1,300 active
and 350 retired Canadian Airlines International pilots.
·
Construct an
extensive database on flight dosage rates, based on
a flight history survey.
·
Analyze the
relationship between exposure to cosmic radiation
and the presence of specific medical conditions.
The results of this study (available in
early winter 2000) should provide initial comprehensive data on cancer risk
assessment and ionizing radiation.
The next phase of this research is to
investigate whether chronic low-dose radiation exposure can be detected in
biological markers; that is, has ionizing radiation induced any chromosomal
changes in circulating white blood cells? The primary objective is to develop
a better understanding of the potential biological hazards in airline pilots
chronically exposed to low radiation doses.
This is the first North American study to
investigate biological markers that may result from radiation exposure in
this professional group. The results of this project, begun in June 1999,
will provide information on the degree to which cosmic radiation exposure
affects biological markers in pilots. In this phase of the research, in
addition to the Medical University of South Carolina, the Fred Hutchinson
Cancer Research Center (Dr. Scott Davis), University of Washington, Seattle,
will be involved.
Magnetic fields
Magnetic fields, one of the
elementary fields of nature, are caused by electric charges and their motion.
Either a permanent magnet or a steady flow of electric current can produce a
static electric field. A steady flow of electric current produces a
direct-current (dc) magnetic field, and electric currents alternating in time
produce alternating-current (ac) magnetic fields. Electric power that
operates devices in the workplace or at home produces magnetic fields.
Electric charges moving to produce a current create magnetic fields.
Magnetic fields are characterized by their
frequency (expressed in hertz) and strength (see “magnetic field strength” in
the glossary). They are generally described in units of microtesla
(1mT=0.000001T) or milligauss (1mG=0.1mT). For example, the earth’s magnetic
field is a static field of approximately 50mT (0.5G). Magnetic fields, unlike
electric fields, can pass through the human body and induce electric
currents.
The presence of humans and animals does
not affect the extremely low frequency magnetic field (ELF). Therefore, the
measured field represents the actual exposure field.
Magnetic fields are known to interact with
biological systems. Particularly at low field strengths, magnetic fields’
biological effects on cell metabolism and growth, gene expression, hormones,
and promotion of tumors have been reported. Some magnetic field effects, such
as the ability to stimulate tissue and bone growth, have been found to be
beneficial; other effects might be harmful.
Biological effects
Researchers have suggested that
magnetic fields suppress pineal body function and production of the pineal
body’s principal hormone, melatonin, thereby increasing the risk of
developing certain cancers, particularly breast cancer. Indeed, female
airline flight attendants in both Finland and Denmark have been documented to
have an increased incidence of breast cancer, which researchers have
suggested is due to their simultaneous exposure to both magnetic fields and
cosmic radiation.
Melatonin generally suppresses other
endocrine glands. Reduced concentrations of melatonin may result in the
pituitary releasing increased prolactin and the gonads releasing increased
estrogen and testosterone. An increase in the amount of circulating estrogen
might, in turn, stimulate proliferation of breast tissue. In addition,
melatonin has been demonstrated to directly inhibit tumor growth in
estrogen-responsive MCF-7 human breast cancer cells in culture.
The magnetic field strength associated
with a reduction in melatonin has been reported to be as low as 0.3 to 1
microtesla (mT) at 50 hertz (Hz) in rats, and the tumor-growth–inhibiting
action of melatonin on human breast cancer cells in culture has been reported
to be blocked at a threshold magnetic field level of 0.2 to 1.2mT. Recent
studies have documented the strength of magnetic fields in aircraft cabins
during flight—the magnetic field may vary from 1 to 10mT.
A more recent paper concluded that the
magnetic field exposure appears to be characterized by frequencies between
100 and 800 hertz and varies in strength depending on the location within the
aircraft, and aircraft type. That is, based on limited measurements, maximum
field strength varied from 0.2 to 0.6mT in economy class, to 1.2mT in first
class, to 1.7mT in the first class galley. Based on these results,
researchers have suggested that magnetic field exposure on the flight deck
may be significantly higher.
These observations provide the incentive
to further explore magnetic field exposure on the flight deck. Other studies
on occupational magnetic field exposure have been inconsistent, but one study
in office workers suggests that melatonin may be reduced during work with
video display units. At 30 centimeters from the screen of a video display
unit, the reported magnetic field level is less than or equal to 0.25 to
0.7mT (compared to approximately 0.1mT in the typical home environment).
Suppression of pineal function has been
implicated in causing prostate cancer and melanoma, both of which have been
found among airline flight crews. Significantly increased incidence of
prostate cancer has been found among Air Canada and British Airways pilots.
When compared with a group of nonflying U.S. Air Force (USAF) officers, male
USAF pilots were found to have more genital cancer and testicular cancer.
Unlike breast tissue, the prostate appears to have a low sensitivity to
ionizing radiation and resultant cancer.
In malignant melanoma, or skin cancer, the
combined effect of solar radiation and reduced melatonin may be important. In
a proportional mortality ratio study, British Airways pilots were found to
have a 6 times greater risk of developing melanoma.
An increased risk of developing malignant melanoma was also found in an
incidence study among USAF pilots.
Because flight crews may work at night,
the effect of light, particularly light at night, on melatonin production
must be considered. Light as perceived by the retina suppresses melatonin
production, and therefore, circulating concentrations of melatonin are
typically low in the day and high at night. However, the greater amount of
melatonin in the body at night does not depend on being asleep or awake; if
the light level is dim, a person will maintain a normal rhythm even if he or
she is awake all night. Because aircraft lights are typically dimmed during
night flight, any reduced nighttime melatonin production among flight crews
may be related primarily to magnetic fields.
Electromagnetic fields (EMF) may work
together with other factors in the cancer process. Researchers have proposed
that an EMF-induced increase in the body of chemicals called “free radicals”
may inhibit a cell’s ability to protect itself against an attack by such
antioxidants as a toxic chemical or ionizing radiation. The free-radical
increase may result from EMF disrupting calcium balance or reducing
melatonin. Recent studies have shown melatonin to be a potent hydroxyl
radical scavenger, preventing cancer-causing damage to nuclear DNA. Thus,
reduction in melatonin might increase the likelihood of developing cancer and
suffering DNA damage. In addition, magnetic field exposure might induce or
prolong the half-life of free radicals, which melatonin is known to scavenge.
Several studies support the possibility
that magnetic fields may inhibit a cell’s ability to protect itself from
ionizing radiation. Human lymphocytes exposed to both ionizing radiation and
60 Hz magnetic fields have demonstrated a greater number of chromosomal
aberrations than have been observed after exposure to ionizing radiation
alone. T-lymphocytes previously exposed to strong magnetic fields have been
found to be more radiosensitive than control cells, and exposure to magnetic
fields has been found to induce greater ionizing radiation effects in the
cultured mammalian cell line FM3A.
Research
The primary purpose of the ALPA
co-sponsored research is to help pilots develop a better understanding of
their exposure to magnetic fields on the flight deck.
The initial phase of this research, which
was recently completed, involved quantifying magnetic field exposure of
airline pilots during domestic and international flying on four different
aircraft types (B-737-200, A320, B-767-300ER, and B-747/400) using an EMDEX
II meter. Results will be published in an upcoming issue of Air Line Pilot.
The next phase of this research will involve more comprehensive measurements
including using a multi-wave meter to capture transients of the magnetic
field.
Glossary
• Cancer is a malignant tumor of potentially unlimited growth, capable of
invading surrounding tissue or spreading to other parts of the body by
metastasis.
• Carcinogen is any cancer-producing substance.
• Chromosome—DNA of each cell is packed into chromosomes within the cell
nucleus. Each human somatic cell contains 23 pairs of chromosomes, or 46
total chromosomes.
• Endocrine glands secrete hormones into the circulatory system and influence
tissues that are separated by some distance from the endocrine glands.
Hormones act on specific tissues and influence the tissues’ activity in a
specific fashion.
• ELF—extremely low frequency—is usually associated with frequencies in the
order of 3 Hz to 3 kHz.
• Epidemiology is the study of the prevalence and spread of disease in the
community. The two main types of epidemiological studies of chronic disease
are cohort (follow-up) or case control (retrospective).
• Gauss is the centimeter-gram-second (cgs) unit of magnetic field density; 1
G=10–4 tesla (T) (tesla is the SI unit of magnetic field density); 1mG=0.1mT.
• Free radicals—In chemistry, a radical is group of elements or atoms usually
passing intact from one compound to another, but usually incapable of
prolonged existence in a free state. Free radicals are radicals in a
transient uncombined state, an atom or group of atoms carrying an unpaired
electron and no charge: e.g., hydroxyl and methyl.
• Hertz (Hz) is the SI unit of frequency. 1 Hz = sec–1
• Ionizing radiation is radiation sufficiently energetic to eject electrons
from an atom. Ionizing radiation includes x and gamma radiation, electron
(beta radiation), alpha particles (helium nuclei), and heavier charged atomic
nuclei.
• Melatonin, a hormone that the pineal body secretes in the brain’s
epithalamus, exerts a generally suppressive action on other endocrine glands.
Changes in the amount of daylight during each day regulate pineal secretion;
that is, increased daylight causes, in the retina of the eye, impulses that
are propagated to the brain, and decreased pineal secretions result. In the
dark, pineal secretion increases.
• Metastasis, in cancer, is the appearance of new-growth tumors (neoplasms)
in parts of the body remote from the primary tumor; metastasis results from
dissemination of tumor cells by the lymphatics or blood vessels, or through
serous cavities or subarachnoid or other spaces.
• Oncogenes are genes that carry the potential for cancer.
• Pineal body is located in the epithalamus of the brain and functions
as an endocrine gland that secretes hormones that act on the hypothalamus or
the gonads to inhibit reproductive functions. The pineal body has also been
shown to affect the activity of the adrenal glands, the thyroid, and the
pancreas, and may be involved in the sleep-wake cycle.
• SI, Systeme International, is an internationally adopted system of units,
for example: meter, kilogram, and coulomb.
• Tesla (T)—Magnetic fields are expressed in tesla (T) or gauss (G), where
1T=104G. Because the range of magnetic fields encountered are quite small, the
fields are generally described in units of microtesla (1mT=0.000001T) or
milligauss (1mG= 0.001G). For example, Earth’s magnetic field is a static
field of about 50mT (0.5G), and a current of 50 amperes in a straight
wire produces a magnetic field of 100mT at a distance of 10 centimeters.
Tesla is the SI unit of magnetic field density.
• White blood cells (lymphocyte), formed in lymphoid tissue throughout the
body, produce antibodies and other chemicals that are responsible for
destroying microorganisms; white blood cells are also involved in allergic
reactions, graft rejection, tumor control, and immune system regulation.
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