Flying exposes passengers and crew to increased levels of natural radiation compared to the ground. This exposure is due to cosmic radiation, which originates from outside Earth’s atmosphere. The increase in radiation is a direct consequence of operating at high altitudes where the natural atmospheric shielding is significantly reduced. This article will explain the source of this radiation, the factors that cause the dose to change, how the exposure is measured, and what the total dose means for a traveler’s health.
The Origin of In-Flight Radiation
The radiation encountered during a flight is primarily composed of high-energy particles known as Galactic Cosmic Radiation (GCR). These particles originate from distant astronomical events, such as supernovae, and constantly bombard our solar system. A smaller, less frequent source comes from the Sun in the form of Solar Particle Events (SPEs), which are bursts of energetic particles released during solar flares or coronal mass ejections.
On the ground, Earth’s dense atmosphere and its magnetic field provide substantial protection against these energetic particles. The magnetosphere deflects many charged particles, and the atmosphere acts as a massive physical shield.
When a primary cosmic ray particle penetrates the atmosphere, it collides with air molecules, creating a cascade of secondary particles, which include neutrons, protons, and electrons. Commercial airliners fly at altitudes, typically between 30,000 and 40,000 feet, where the amount of atmospheric shielding overhead is significantly diminished. This lower density of air results in a higher flux of these secondary particles reaching the aircraft cabin.
Variables That Determine Exposure Levels
The actual radiation dose received during a flight is highly variable and depends on three main factors. The first is flight altitude; a higher cruising altitude means less atmosphere is available to attenuate the cosmic rays, leading to a greater radiation dose.
Another element is the flight’s latitude, which relates to the shielding provided by Earth’s magnetic field. The field is strongest at the equator, deflecting more cosmic rays, but is weakest near the North and South Poles. Consequently, flights that follow polar routes experience higher dose rates than equatorial flights of similar duration.
The third factor is the solar cycle, which affects the intensity of GCR reaching Earth. Paradoxically, when the Sun is highly active, its powerful magnetic field shields the inner solar system from GCR, resulting in lower radiation levels at aviation altitudes. Conversely, during periods of low solar activity, known as the solar minimum, the GCR intensity increases, leading to a higher in-flight radiation dose.
Quantifying the Radiation Dose
Radiation exposure is measured in units of Sieverts (Sv), typically expressed as the much smaller millisievert (mSv) or microsievert (\(\mu\)Sv) for aviation doses. At typical cruising altitudes, the average dose rate is approximately 4 \(\mu\)Sv per hour.
To put this into perspective, the average worldwide dose from natural background radiation on the ground, which includes radon gas and cosmic rays at sea level, is about 2.4 mSv per year. A transatlantic round-trip flight, such as from Frankfurt to New York and back, exposes a passenger to an estimated total dose of approximately 100 \(\mu\)Sv (0.1 mSv).
This flight dose is roughly equivalent to a single chest X-ray. The exposure from a long-haul flight is only a small fraction of the annual background dose, adding about 4% to the yearly total for the average person.
For occupational exposure, international bodies like the International Commission on Radiological Protection (ICRP) recommend a limit of 20 mSv per year, averaged over five years, for radiation workers. Air crew are classified as occupationally exposed, and their average annual dose generally falls between 0.2 mSv and 5 mSv depending on their routes and flight hours.
Assessing the Health Risk of Flying
For the vast majority of the public, the radiation dose from occasional air travel is negligible and is not associated with any measurable increase in health risk. The exposure from a few flights per year falls well within the normal variation of natural background radiation that people experience simply by living in different geographic locations.
The primary concern is for air crew, who accumulate a dose over many years. Air crew are classified as occupationally exposed workers, and their cumulative exposure is monitored and managed to ensure it stays below the recommended limits.
Pregnant travelers are often concerned, but the radiation impact on the fetus from casual flying is considered trivial. However, pregnant flight crew members are advised to manage their schedules to keep the total dose to the fetus below the ICRP-recommended limit of 1 mSv for the entire pregnancy.