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total radiation exposure.
Latitude
The earth’s magnetic field acts to deflect many charged
particles of solar and galactic origin that would otherwise
enter the atmosphere. This shielding is most effective at
the equator, but it decreases, moving towards the poles,
and its lowest over the magnetic poles of the earth (North
Pole and South Pole). At aircraft cruise altitudes, galactic
radiation doses over the magnetic poles are about twice
those over the equator. Since commercial aircraft usually
fly high latitude (polar) routes between the United States
and Europe or Asia, exposure is greater in international
operations.
Solar Cycle
Finally, the amount of galactic and solar cosmic radiation
particles entering the earth’s atmosphere varies with the
The photo on the
left is a solar flare
with an eruptive
prominence… The
photo on the right
depicts a fairly
large solar flare
FSI - HUMAN FACTORS 11
approximate 11-year cycles of rise and decline in solar
activity. Although we can influence the first two variables
through flight bids - altitude/duration of flight and latitude,
we have no control over solar cycles.
How much exposure is too much?
There are a number of national and international organizations
that provide guidelines for ionizing radiation exposure
limits; unfortunately, these guidelines are not uniform.
It is important to note that to the best of our current medical
knowledge, it appears that no adverse impacts have
been noted for any exposure less than 100 millisieverts.
The guidelines cited below are all in millisieverts (once
again, a millisievert is one thousand microsieverts).
National Council on Radiation Protection and Measurements
(NCRP)
50 mSv per year for occupationally exposed workers
5 mSv for pregnant crewmembers, distributed over
nine months with no single month greater than 0.5
mSv
1 mSv per year for the general public, with a lifetime
limit f 10 mSv x age
5 mSv per year for employees involved in nonexposed
radiation occupation
European Council
6 mSv per year
Pregnancy: the dose to the unborn child will be as low
as reasonably achievable and should not exceed 1 mSv
during (or remainder) of the pregnancy
Nursing women will not be employed in work involving
a significant risk of bodily radioactive contamination
( > 1 mSv / year)
International Commission on Radiological Protection
(ICRP)
20 mSv per year (the average over five years) with no
more than 50 mSv in any one year. This value includes
background, on-the ground exposure (which is around
3-5 mSv per year)
5 mSv per year for employees involved in nonexposed
radiation occupation
1 mSv total limit during pregnancy
Table 1 on page 9 illustrates the estimated dose to air carrier
crewmembers from cosmic radiation received for each
of 32 nonstop flights on a variety of routes. Depending on
the particular flights flown during a work year, the annual
dose, based on flying 850 block hours per year, can range
from 1.19 millisieverts on short range domestic flights to
5.1 millisieverts on long range international routes (i.e.
Athens to New York). This amounts to 5.9% to 25.5% of
the recommended annual ICRP limit of 20 mSv per year
for adult occupational exposure.
It must be pointed out that potentially large doses of ionizing
radiation can be emitted from the sun over short periods
of time (coronal mass ejections and solar flares). Fortunately,
these so-called solar particle events are rare.
Since 1956, there have not been any events during which
the hourly average dose rate exceeded 1 mSv / hour at
40,000 feet - however, they are unpredictable.
Obtaining estimates of cosmic radiation
There are ways of estimating the amount of cosmic radiation
received by crewmembers. The FAA has developed a
computer software program entitled “CARI-6” which provides
an estimated dose for a particular flight based on certain
parameters of the flight. The program require s the
point of origin, point of arrival, time in route and altitude
parameters, and then will calculate an estimated dose received
during the flight. CARI-6 also takes into account
the phase of the solar cycle. This number is updated approximately
monthly and can be obtained from the FAA’s
Website (http://www.cami.jccbi.gov/AAM-
600/610/600Radio.html). A copy can be downloaded from
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