How a flight's radiation dose is calculated

CARI-7: the FAA's model

CARI-7 is software from the FAA's Civil Aerospace Medical Institute (CAMI) in Oklahoma City that calculates the dose aircraft occupants receive from cosmic radiation. CAMI has developed the CARI programs since the late 1980s; version 7 models the particle spectra of galactic and solar cosmic radiation, how the solar wind modulates them, how Earth's magnetic field deflects them, and how they cascade through the atmosphere (FAA: CARI-7 and CARI-7A; Copeland, CARI-7 Documentation: Particle Spectra, DOT/FAA/AM-21/04, 2021).

Given two airports, a month, and a flight profile (minutes climbing, minutes at each cruise altitude, minutes descending), CARI-7 follows the great-circle route and adds up the dose minute by minute. The FlightDose app runs it on its server for every flight a user logs. The route pages on this site use the same CARI-7 installation and settings, run offline for one standard flight per direction.

Why latitude matters

Cosmic rays are charged particles, and Earth's magnetic field turns them aside. Over the tropics the field runs parallel to the ground and only the most energetic particles get through; towards the magnetic poles it dips into the ground and shields almost nothing. The FAA puts galactic cosmic radiation over the polar regions at about twice the level over the geomagnetic equator at the same altitude (AC 120-61B, 6a). In CARI-7's grid for August 2026 at 35,000 ft, the rate runs from 1.5 µSv/h over the equator to 4.7 µSv/h near the poles (see the map).

That is why routes over the North Atlantic, Canada, Alaska and the Arctic carry more dose per hour than routes across the tropics, and why a great-circle route that bends towards the pole, like New York to Hong Kong, collects more than its distance alone suggests.

Why altitude matters

The atmosphere is the shield, and every thousand feet of climb leaves less of it overhead. On the New York to London route the same 6 h 18 min flight receives 19.2 µSv at 31,000 ft, 22.7 µSv at 33,000 ft, 26.5 µSv at 35,000 ft, 30.5 µSv at 37,000 ft, 34.6 µSv at 39,000 ft, 38.8 µSv at 41,000 ft. The cruise altitude is often the biggest difference between two flights on the same route.

The solar cycle

The sun's magnetic activity rises and falls over about 11 years. When the sun is active, its wind carries a stronger magnetic field out through the solar system and turns away more of the galactic cosmic rays before they reach Earth; when it is quiet, more get in. So flight doses are highest around solar minimum. The last minimum was around the end of 2019; NASA and NOAA announced in October 2024 that the sun had reached the maximum period of solar cycle 25 (NOAA SWPC).

CARI-7 takes the sun's state from a monthly value the FAA publishes, the heliocentric potential. FlightDose updates it weekly from the FAA; a flight in a month the FAA hasn't published yet uses the newest month available. On the New York to London route the swing between quiet and active years is about 66% (33.8 µSv in April 2020, 20.4 µSv in January 2026).

Solar storms are different: occasional bursts of energetic particles from the sun itself. They are rare, short and hard to predict, and CARI-7's monthly figures don't include them. More on solar storms.

What "effective dose" means

Every number on this site is an effective dose, in millisieverts (mSv) or microsieverts (µSv; 1 mSv = 1,000 µSv), using the weighting factors of ICRP Publication 103 (2007). Effective dose weights each kind of radiation by how damaging it is and each organ by how sensitive it is, then adds them up, so that very different exposures (a flight, a CT scan, a year of background radiation) can be compared on one scale. It is a tool for comparing risk across a population. It is not a measurement of what happened inside any one person.

Time in the air, in each direction

CARI-7 doesn't work out how long a flight takes; it is told. The app uses the time a traveller enters, or the airline's scheduled block time less taxiing, and otherwise estimates it. The standard flights on this site use the estimate:

20 minutes for the slow climb and descent, plus the great-circle distance at 870 km/h, adjusted for a prevailing westerly wind of up to 75 km/h: strongest at 45° latitude, nothing at the equator or the poles, and projected on the route's bearing, so it helps eastbound flights and slows westbound ones.

That is why the two directions of a route have different doses. JFK → LHR takes about 6 h 18 min and LHR → JFK about 7 h 16 min, so the westbound flight receives 31 µSv against 26.5 µSv. The estimate can't know the day's jet stream, which moves a long crossing by 30 to 60 minutes.

Limits and recommendations

• Aircrew. The FAA treats cosmic radiation as occupational exposure for crew and recommends the ICRP's limit: a five-year average of 20 mSv a year, with no more than 50 mSv in any one year. It is a recommendation, not a regulation, and the FAA notes these are "upper limits of acceptability", not thresholds of harm (AC 120-61B, 7b). More for aircrew.
• Pregnancy. For a pregnant crewmember the FAA recommends limiting the dose to the conceptus to no more than 0.5 mSv a month (AC 120-61B, 7c). More on flying while pregnant.
• The public. ICRP Publication 103 sets 1 mSv a year as the limit for the public from planned sources of radiation; natural background and medical exposures are outside it. FlightDose shows it as a reference point.
• Background. The average person in the United States receives about 3.1 mSv a year from natural sources, 0.33 mSv of it from cosmic radiation at ground level (NCRP Report No. 160, 2009, as tabulated in AC 120-61B).

What the numbers can't know

• Your actual flight. The route pages describe one standard flight. Real flights differ in altitude, step climbs, track and time in the air.
• The actual track. CARI-7 follows the great circle; airlines follow tracks that bend with the jet stream and air-traffic routes.
• Solar storms. A solar particle event can add dose at high latitudes for hours. The FAA says the dose from one cannot be estimated in advance (AC 120-61B, 9a).
• The aircraft and you. The model doesn't include the aircraft's own structure, and effective dose is a population average, not a personal measurement.

So every number here is an estimate, not a measurement, and none of it is medical advice.

Sources

1. Federal Aviation Administration. Advisory Circular AC 120-61B, In-Flight Radiation Exposure. 21 November 2014.
2. Federal Aviation Administration, Civil Aerospace Medical Institute. CARI-7 and CARI-7A.
3. Copeland K. CARI-7 Documentation: Particle Spectra. DOT/FAA/AM-21/04. FAA Office of Aerospace Medicine, March 2021.
4. International Commission on Radiological Protection. The 2007 Recommendations of the ICRP. ICRP Publication 103. Ann. ICRP 37 (2–4), 2007.
5. National Council on Radiation Protection and Measurements. Ionizing Radiation Exposure of the Population of the United States. NCRP Report No. 160, 2009.
6. NOAA Space Weather Prediction Center. Joint solar maximum announcement, NASA and NOAA. 15 October 2024.
7. FAA. Heliocentric potential values, the monthly solar input to CARI-7.

The same method, as data: /data/v1/methodology.json. Who publishes it: about FlightDose.