Notes on the Natural Radiation Hazard at Aircraft Altitudes

In flight there are two principal sources of natural radiation to consider:

  1. Galactic Cosmic Rays (GCR) which are always present, and
  2. Solar Energetic Particle (SEP) events, sometimes called Solar Cosmic Ray (SCR) events, which occur sporadically.

GCR provide an inescapable radiation background which varies over the solar cycle of about 11 years. GCR are maximum when solar activity is minimum and vice-versa. The resulting dose equivalent at aircraft also varies quite strongly with altitude and latitude The following table provides estimates of the radiation dose equivalent at the times of a recent solar minimum (10/86) and solar maximum (7/89) for representative high and low latitude locations at 90 degrees west longitude as a function of altitude. These values were obtained through the use of the CARI-6 program developed by the Civil Aeromedical Institute of the Federal Aviation Administration.

Dose Equivalent Rate (micro-sieverts per hour (uSv/h)
Solar Minimum (10/86)
Solar Maximum (7/89)
Altitude
(x1000 ft)
35 degrees
North Latitude
70 degrees
North Latitude
35 degrees
North Latitude
70 degrees
North Latitude
0
10
20
30
40
50
60
70
80
0.0401
0.190
0.985
3.25
6.78
9.71
11.1
11.4
11.2
0.0412
0.207
1.14
4.06
9.02
13.8
17.1
19.2
20.6
0.0374
0.173
0.875
2.85
5.88
8.36
9.49
9.68
9.44
0.0380
0.181
0.953
3.24
6.99
10.3
12.3
13.3
13.8

We are continually exposed to dose equivalent rates of about 6 to 12 uSv per day at sea-level due to GCR and radiation from the ground which is dependent on our location, as well as radiation from within our own bodies. Thus a two-hour flight at conventional altitudes (20,000 - 40,000 ft) may approximately double one's radiation dose for the day.

Solar particle events of sufficient size to provide significant exposure increases are rare1. In 11 years one could expect up to three events, each of which might produce an equivalent dose rate up to 200 uSv/hr for a few hours at aircraft altitudes. Assuming 3 unusually long duration, maximum events of 1 day each yields a probability of being at altitude on a 2-hour flight during that time of about 1 in 1500.

Risk Factors:

The NCRP2 (National Council on Radiation Protection and Measurement) estimates the cancer risk probability for a population of all ages at 0.05 per Sv. For example then, the estimated cancer risk for the population to a dose of , say, 200 uSv is equal to: 0.05 x 0.000200 = 1 additional cancer in a population of 100,000. The risk for infants is about twice that figure, while the rate of radiation induced mental retardation is a special risk for the fetus (an effect not observed in adults) and in the most sensitive period, 8 - 15 weeks of gestation, is about 0.4 per Sv, or a risk factor of 8 per 100,000.

Further information:

It is the province of the NOAA Space Environment Center to provide the best space environment information available. The Center does not provide judgments regarding health issues.

If you are concerned and would like further information, you might consult commercial services such as In-Flight Radiation Protection Services or High Altitude Radiation Monitoring Service which purport to advise travelers on flight radiation issues.

References:
1 NOAA Space Weather Scales (/NOAAscales/)
2 Radiation Exposure and High-Altitude Flight, NCRP Commentary No.12, 1995

The information presented here was prepared by H. H. Sauer CIRES/NOAA
March 2000