Lower altitude = lower dose: how much it really helps

The atmosphere is the shield. At sea level, 1,033 g/cm² of air column overhead attenuates the cosmic-ray cascade down to a small terrestrial contribution. At FL350 the column shrinks to roughly 243 g/cm²; at FL410, to 184 g/cm². Less atmosphere, more particles, higher dose rate. What follows is the CARI-7-derived curve across the full civil cruise band, with the practical rules of thumb the numbers imply.

Dose rate as a function of altitude

The atmospheric depth (the mass per unit area of air above a point) decreases approximately exponentially with altitude with a scale height of about 7 km. Dose rate at a given latitude scales roughly with the secondary-particle flux, which itself rises faster than linear with decreasing depth across the cruise band. The result is approximately:

Flight level	Altitude (m)	Dose rate at 1.5 GV cutoff (about 50–60° geomagnetic), July 2026 (µSv/hr)
FL250	7,620	1.75
FL290	8,840	2.65
FL310	9,450	3.21
FL330	10,060	3.81
FL350	10,670	4.45
FL370	11,280	5.12
FL390	11,890	5.80
FL410	12,500	6.50
FL430 (rare, biz-jet ceiling)	13,110	7.19

The numbers above are CARI-7 outputs at a 1.5 GV vertical cutoff for July 2026, a heliocentric potential of 733 MV ^[1]. Below FL250 dose rate falls more rapidly with decreasing altitude; above FL410 it begins to plateau because we are approaching the altitude where secondary cascade production peaks (the Pfotzer-Regener maximum sits between 15 and 20 km, depending on latitude; slightly above civil cruise altitudes at low latitudes, within the cruise band at the highest latitudes).

The step-climb difference

Commercial aircraft routinely step-climb during long-haul flights as fuel burn reduces weight and a higher cruise altitude becomes feasible. A typical Boeing 787 transatlantic might enter cruise at FL350 and step-climb to FL390 about two-thirds of the way across. That step-climb increases dose rate by 31%, from 4.45 to 5.80 µSv/hr at our reference cutoff.

The fuel-economy reason for step-climbing is real and not in dispute; we are not suggesting carriers should fly lower for dose reasons. But the dose differential is real and worth knowing about if you are comparing routings.

What the Pfotzer maximum implies

The Pfotzer-Regener maximum is the altitude at which the cosmic-ray secondary cascade reaches peak flux, generally between 15 and 20 km. Civil airliners cruise below it; weather balloons and high-altitude research aircraft fly into and through it. The implication for civil aviation is that climbing higher within the cruise band continues to increase dose rate, but the climb is along the rising side of the cascade curve. At very high altitudes (above FL550, the domain of some military aircraft and the retired Concorde) the rate continues to rise; only above ~ 20 km does it begin to fall again toward the top of the atmosphere.

Concorde, which cruised at up to FL600 (about 18 km), had a dose rate roughly 2–3 times that of subsonic aircraft on similar routes. British Airways equipped Concorde with onboard radiation monitors and operating procedures included descent in response to high readings, an unusual concession to in-flight dosimetry that subsonic airline operations do not require ^[2].

The rule of thumb you can carry

Within the civil-cruise band:

• A 4,000 ft (1,200 m) altitude increase from FL350 to FL390 is a 31% increase in dose rate.
• Cruising at FL290 rather than FL390 is about 55% less dose rate for the same latitude.
• The free FAA CARI-7 software will give you the exact number for any specific route, altitude, and date.

Altitude and latitude are comparable levers

Geomagnetic latitude changes in-flight dose rate by a factor of 2.9 between the equator and the poles at FL370 (see our polar guide). Altitude within the civil cruise band changes it by a factor of 3.7 between FL250 and FL410, and 4.1 between FL250 and FL430. So the two levers are comparable, with altitude slightly the wider of the two. They also compound: a polar-cruising long-haul at FL410 is at the high end of both axes at 6.59 µSv/hr; an equatorial cruise at FL290 is at the low end of both at 1.02. That is a factor of about 6.5 between the extremes.

Both are dictated by operational factors: routing by the great circle, winds and airspace, altitude by weight, traffic and fuel economics. Neither is something an individual passenger can adjust.

Why CARI captures altitude well

The atmospheric-depth integration is the cleanest piece of the CARI calculation. The atmospheric column above any latitude/altitude pair is well-known; the cascade transport is well-validated against weather-balloon and high-altitude-aircraft measurements; and the shape of the altitude dependence is better constrained than the absolute level ^[3]. If you trust any single part of a CARI output, trust the relative change with altitude; the overall band we publish on a report is ±25% or ±30%, and altitude is not where most of that comes from.

Operational notes for fliers

Cruise altitude is selected by air traffic control after coordination with the flight crew, subject to wind, traffic, and aircraft performance constraints. The flight crew rarely has more than a few flight-level options at any given moment. The cruise altitude you experience is out of your control as a passenger. The reason to understand the altitude effect is that it helps interpret the per-segment dose number on your report: a transatlantic crossing at FL400 carries about 20% more dose than the same crossing at FL370. Understanding the effect will not change your behavior. Frequent fliers with consistent route patterns can model the effect; everyone else can treat altitude as part of the inherent uncertainty in any single-flight dose figure.

Aircraft type and cruise-altitude correlation

Different aircraft types have different optimal cruise altitudes, which is the practical reason aircraft type matters for dose even though airframe shielding does not. A Boeing 787 typically cruises at FL400-410 on long-haul; a Boeing 737-800 typically cruises FL360-380; an Airbus A380 often steps from FL340 to FL360. The dose-rate differential between FL370 and FL410 is 27%. Over a ten-hour leg at the 1.5 GV cutoff that is about 14 µSv per segment.

A flight planner does not optimize for dose. Cruise altitude is optimized for fuel burn given the wind field, traffic, and the aircraft's performance envelope. For a frequent flier comparing airlines on a route where multiple carriers compete, the carrier flying newer high-altitude long-range types will deliver slightly more dose per segment than the carrier flying older lower-altitude types. The difference is real but modest.

Cabin altitude vs cruise altitude

One source of confusion in popular discussions of flight dose is the distinction between aircraft cruise altitude (where the airframe is) and cabin pressure altitude (the effective altitude the cabin air pressure simulates). Modern airliners typically maintain cabin altitudes between 6,000 and 8,000 ft regardless of cruise altitude; the Boeing 787 and Airbus A350 maintain lower cabin altitudes than older designs. Cabin altitude has no effect on cosmic-radiation dose; the cabin walls do not provide meaningful shielding against penetrating cosmic-ray secondaries. The relevant altitude for dose is the aircraft's actual altitude.

What this means for fuel-versus-dose tradeoffs

Operators considering whether to fly at FL340 vs FL400 on a given route face a fuel-versus-time tradeoff (higher generally means more fuel-efficient cruise; lower means less time at altitude but more drag and worse fuel consumption). The dose differential is not a consideration in operational economics, but it is real. If we imagine a hypothetical airline that wanted to optimize for minimum crew dose, holding fuel and time constraints fixed, they would prefer lower cruise altitudes. But no carrier optimizes this way, because the cost differential is modest and the dose differential is comfortably within everyone's regulatory headroom.

The dose-versus-altitude curve in plain language

If you collapse all of the above into one mental rule: dose rate at typical cruise altitudes scales with the inverse of atmospheric depth, and atmospheric depth decreases roughly exponentially with altitude. In the cruise band, the per-1,000-ft dose-rate change is about 6 to 7%. A 4,000-ft step climb is therefore about 31%. A 10,000-ft difference (FL310 to FL410) is a factor of about 2 in dose rate.

This is one of the more reliable pieces of mental arithmetic in flight dosimetry. The latitude effect is comparable in magnitude but harder to compute mentally because it depends on geomagnetic rather than geographic latitude. Altitude is easier: more altitude, more dose, by roughly the percentages above.

How CARI-7 handles climb and descent

A flight is not all cruise. Climb and descent contribute meaningful dose for short-haul (where they may be 30–50% of total time) and a much smaller share for long-haul (where they are 10–15% of total time). CARI-7 models the climb and descent profile using standard civil-airliner performance assumptions; the integrated dose accounts for the lower per-hour rate at lower altitudes during these phases.

For back-of-envelope estimation, treating climb and descent as roughly half the cruise-altitude dose rate gives a reasonable approximation. For short-haul flights this is the dominant source of uncertainty in single-flight dose estimates; for long-haul it is a small correction.

Reviewed 4 September 2026 · See our methodology and sources.