[ RadSafe ] Dose from Airtravel

[Joseph Shonka]

Dear Michael

On Tue, May 23, 2017 at 7:31 AM, Krzesniak, Michael F CIV NSWC Crane, Crane
Bldg 3059 Code 6054 <michael.krzesniak at navy.mil> wrote:Having trouble
searching the Radsafe digest and didn't find a good source in on-line
search.

Looking for dose estimates for checked and carry-on baggage for commercial
air travel.  Also, dose estimates for passengers.


There is a lot of information, some of which is incomplete.  Let me start
by noting that a recent New England Journal of Medicine paper (
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4482339/ ) has a meta study of
all epidemiology studies of aircrew and found that aircrew have twice the
level of melanoma as the general population (despite a strong healthy
worker effect).  The authors attributed the effect to be due to the 3% to
5% increase in UVA + UVB per 1000 meters of altitude.  Although average
aircrew lifetime exposures to ionizing radiation can produce a 50%
probability of causation (using IREP, the online code for EEOICPA) for
melanoma, UV is the most likely cause with radiation a contributing
factor.  There are also no regulations on US aircrew concerning UV
exposure.   Most epi studies also show excess breast cancer in female
aircrew, which has been attributed to hormonal changes due to diurnal
changes for aircrew serving on long duration flights.  Again, IREP shows
that radiation exposure would be a contributing factor for breast cancer.
So US aircrew are the only radiation workers whose employers (the carriers)
have no requirements for controlling their radiation exposure, and are the
only group of radiation workers who have been found to have two types of
cancer in excess for which their radiation exposure may be considered a
contributing factor.

I would caution against studies emphasizing averages.  If you average lower
altitude (thus lower dose rate) commuter flights (for which aircrew would
average less than 100 mSv per year) with high altitude flights (which have
higher annual doses), an average value is noted, but is it typical?  US
aircrew flying under 14CFR135 are limited to 1200 hours *flight time*
annually, and unions for the larger carriers often have agreements for
pilots that are lower.   However, the hours are not counted if the (US)
pilots are not in command (e.g. resting aircrew on long duration flights or
flights taken as passenger status for re positioning do not accrue *flight
time*).  Flight attendants have no annual hourly limit.  Thus, the FAA's
estimate (used to justify the lack of regulations) based on 750 hours does
not actually equal the average annual time in flight (as opposed to *flight
time*) of aircrew.  With the quiet sun over the next few years, transpolar
flights will have higher doses from GCR (~0.15 mSv Chicago to Bejing, or
0.75 mSv per 100 block hours.  Lacking regulations and controls, some US
aircrew could exceed 20 mSv, unless they voluntarily reduce income and
choose lower exposure routes.  Should radiation protection rely on workers
turning down more income to reduce their exposure?

I think of it this way.  The average nuclear power plant worker's annual
exposure is now down to around 1 mSv.  So, does anyone think we should
eliminate all controls on worker exposure at power plants and substitute
advisory circulars that tell workers if they don't want radiation, consider
not working in certain areas of the plant and have the utility offer lower
salaries for those positions?  Can we avoid making any external radiation
measurements in the plant and substitute computer estimates?  The FAA's
advisory circular (AC120-61B) does just that for aircrew.

Most analyses of aircrew doses only consider galactic cosmic radiation.
They ignore solar proton, neutron and gamma events as well as terrestrial
gamma flashes (from lighting).  Ed Bramlitt and I summarized these sources
in a note (Health Physics January 2015, Volume 108, Number 1 RADIATION
EXPOSURE OF AVIATION CREWMEMBERS AND CANCER Bramlitt and Shonka DOI:
10.1097/HP.0000000000000166 ).

Concorde was required to have radiation detection on board.  The cockpit
dial showed the pilot green, yellow and red.  Red (0.5 mSv/hr) required
slowing to subsonic, reducing altitude and diverting to the closest airport
due to fuel constraints.  Red would occur during solar flares.  During
operations, Air France had to execute that emergency procedure 4 times.  On
9/29/89, the BA flight from New York to London cleverly diverted south of
the great circle route reducing dose rate a factor of 5 (keeping the dose
rate below 0,5 mSv) while adding 15 minutes to the flight time.  (IEEE
TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 54, NO. 4, AUGUST 2007 1071 Solar
Particle Events in the QinetiQ Atmospheric Radiation Model Dyer, et al.)
However, all aircraft flying at high latitudes on that day had elevated
exposure not accounted for by codes such as the FAA's CARI software that
only consider GCR.  What was the collective dose for all (high latitude)
passengers and aircrew on that date?

NASA was so concerned about terrestrial gamma ray flashes (TGF, radiation
from lightning, seen by satellites measuring gamma ray bursts) that they
asked for a meeting with the FAA in 2008.  There is a published estimate of
100 mSV for TGFs of less than 100 meter diameter, with lower (or higher)
doses for larger (or smaller) diameters. (JOURNAL OF GEOPHYSICAL RESEARCH,
VOL. 115, D09206, doi:10.1029/2009JD012039, 2010 Estimation of the fluence
of high‐energy electron bursts produced by thunderclouds and the resulting
radiation doses received in aircraft  Dwyer et al. ).  As a result of that
FAA-NASA meeting, AC120-61B was amended to advise flight attendants and
pilots to avoid flying over or through thunderstorms.  However, worldwide
aircraft are stuck by lightning about 80 times per day.  I mention this not
to suggest that the radiation is present, but to show that aircraft are in
the vicinity of lightning and may also be subject to radiation from TGFs
with similar frequency.   Aside from a few satellites 550 km up in space,
TGF is not routinely measured.

My answer to your question, baggage (either carry on or checked) and
passengers on a single flight from GCR alone would be less than 0.15 mSv.
However, doses of more than 100 mSv are possible, either from TGFs to a
single aircraft or for all aircraft at higher latitudes from solar flares,
particularly for trans-polar flights.

-- 
Joseph J. Shonka, Ph.D.
119 Ridgemore Circle
Toccoa, GA 30577
770-509-7606