Perspectives: Ionizing Radiation Safety

Air transport crewmen are occupationally exposed to higher doses of ionizing radiation than are most members of the general population. The Federal Aviation Administration’s (FAA’s) Civil Aerospace Medical Institute (CAMI), therefore, has come up with safety recommendations, as well as a method of estimating the amount of ionizing radiation a crew is exposed to on given routes. First, however, one should understand where ionizing radiation comes from and how it is formed.

The principal ionizing radiation is galactic cosmic radiation, also called galactic radiation. Its main source is believed to be supernovae (exploding stars). The primary galactic radiation consists of fast-moving subatomic particles (mostly hydrogen nuclei, called protons) that, on entering the Earth’s atmosphere, collide with the nitrogen, oxygen and other air molecules, generating additional subatomic particles, X-rays and gamma rays. The particles that enter the atmosphere and the particles, X-rays and gamma rays generated are collectively referred to as galactic radiation.

Infrequently, a disturbance in the sun (solar flare, coronal mass ejection) may lead to a sudden increase in radiation at flight altitudes, similar in qualitative composition to galactic radiation. Interaction of galactic or solar radiation with the molecules that make up the tissues and organs of the human body may lead to health effects.

Aircraft occupants have several natural sources of protection from environmental ionizing radiation. The atmosphere provides partial shielding: the lower the altitude, the thicker the atmospheric shielding, the greater its density, and therefore, the greater the protection. The Earth’s magnetic field, also called the geomagnetic field, deflects many radiation particles that would otherwise enter the atmosphere. In general, the magnetic shielding is greatest over the geomagnetic equator (near the geographic equator) and gradually decreases to zero as one goes north or south.

The magnetic fields carried by the subatomic charged particles (solar wind) continuously emanating from the sun also reduce the intensity of the galactic radiation incident on the atmosphere. These particles are too low in energy to contribute to the radiation level at aircraft flight altitudes. But irregularities in the magnetic fields that they carry scatter some of the galactic radiation particles, thereby reducing the number that would otherwise enter the atmosphere.

When solar activity is high, the solar wind carries more irregularities, resulting in more scattering and a correspondingly greater decrease in the galactic radiation in the atmosphere. There is an approximately 11-year rise and decline in solar activity, with a corresponding decline and rise in galactic radiation in the atmosphere. The figures (left) depict the galactic radiation level in the atmosphere over a 44-year period. They show its relation to altitude, geographic location and stage of the solar activity cycle.

FAA’s CAMI has recommended limits for occupational exposure of air carrier crews to ionizing radiation. It has developed and made available to the public computer programs for estimating the amount of galactic radiation received during air travel. The most popular of the programs, CARI-6, estimates the radiation dose received on a flight between two airports when the aircraft flies approximately a great-circle route. There is a link from www.cami.jccbi.gov/radiation.html to a version of CARI-6 that can be run directly on the Internet. A special version of CARI-6, called CARI-6M, allows the user to specify a detailed flight route. Both CARI-6 and CARI-6M can be downloaded from the Web site.

A document entitled "What Commercial Aircraft Crewmembers Should Know About Their Occupational Exposure to Ionizing Radiation" can be accessed from the Web site. Topics covered in the document include: the nature of ionizing radiation, recommended radiation exposure limits, health concerns, health risk estimates, and ionizing radiation from the sun.

Wallace Friedberg is a research physiologist and team leader of the Radiobiology Research Team. Kyle Copeland is a health physicist and member of the research team. Both are employed by the Federal Aviation Administration’s Civil Aerospace Medical Institute in Oklahoma City, Okla.