The air we breathe is a mixture of gases, primarily nitrogen, oxygen, and argon. Like all elements, these gases exist in various forms known as isotopes. An isotope of an element contains the same number of protons—which determines its identity—but a different number of neutrons, resulting in a slightly different atomic mass. Our atmosphere is constantly circulating these different atomic variants, which raises a fundamental question: are the isotopes making up the air stable or unstable?
Defining Isotope Stability
The stability of an isotope is determined by the composition of its nucleus, specifically the balance between the number of neutrons and protons. A stable isotope possesses a neutron-to-proton ratio that allows the strong nuclear force to effectively counteract the repulsive electrical forces between the positively charged protons. For elements with a low atomic number, the most stable configurations generally have an almost equal number of neutrons and protons.
As the number of protons increases, however, more neutrons are needed to act as a kind of nuclear glue to maintain this structural integrity. If an isotope contains a ratio of neutrons to protons that falls outside this narrow “band of stability,” it is considered unstable. These unstable isotopes are radioactive and spontaneously decay, shedding excess energy or particles to achieve a more stable configuration.
The speed of this decay is measured by its half-life, which is the time required for exactly half of the radioactive atoms in any given sample to transform into a different nuclear species. Isotopes with half-lives ranging from fractions of a second to billions of years are present in nature.
Stability of Nitrogen and Oxygen
The overwhelming majority of the air we inhale is composed of nitrogen and oxygen, and their most common isotopes are non-radioactive. Nitrogen makes up approximately 78% of the atmosphere, with two primary stable isotopes: N-14 and N-15. Nitrogen-14 is by far the most abundant, accounting for about 99.6% of all nitrogen atoms in the air.
This N-14 isotope is unusual because it is one of the few stable nuclides that contains an odd number of both protons and neutrons—seven of each. The much rarer N-15 isotope, which makes up the remaining 0.4%, is also completely stable. These two forms have existed on Earth since its formation and do not undergo any form of radioactive decay.
Oxygen, which makes up about 21% of the atmosphere, also consists almost entirely of stable isotopes. The most common form is O-16, which accounts for approximately 99.76% of atmospheric oxygen. This isotope is highly stable due to its equal complement of eight protons and eight neutrons.
The remaining fraction of atmospheric oxygen is composed of two other stable, heavier isotopes: O-18 and O-17. Oxygen-18 has an atmospheric abundance of about 0.20%, while O-17 is the least abundant at approximately 0.04%. Because N-14 and O-16 constitute over 99% of the air, the vast bulk of the atmosphere is entirely stable.
Natural Radioactivity in Trace Gases
While the primary gases are stable, the air does contain minute traces of naturally occurring radioactive isotopes known as radionuclides. The most significant of these is Carbon-14 (C-14). Carbon-14 is continuously generated in the Earth’s upper atmosphere when cosmic rays collide with nitrogen atoms, transforming them into this unstable carbon variant.
This cosmogenic nuclide is a beta emitter with a half-life of 5,730 years, meaning it slowly decays back into stable nitrogen. Because C-14 is quickly oxidized into carbon dioxide, it circulates throughout the atmosphere and is incorporated into all living things through respiration and photosynthesis. However, its concentration is extremely small, with only about one C-14 atom for every one trillion stable carbon atoms.
Another atmospheric trace gas with a link to radioactivity is Argon, which makes up almost 1% of the air. The most common isotope, Ar-40, is a stable gas. The reason for its high abundance, however, is the very slow decay of the long-lived radioactive isotope Potassium-40 (K-40) found in the Earth’s crust.
As K-40 decays in rocks and minerals, it produces Ar-40 gas, which is then released into the atmosphere over geological time. Therefore, while the argon gas itself is stable, its prevalence in the air is a direct result of a naturally occurring radioactive process within the planet.
Contextualizing Atmospheric Radiation
The presence of radionuclides like C-14 in the air contributes to the natural background radiation exposure that everyone receives. This total exposure varies globally but averages around 2.4 millisieverts (mSv) per year. The dose received from inhaling and ingesting C-14 as part of the carbon cycle is an extremely small fraction of this total.
The average internal dose from C-14 is estimated to be about 12 microsieverts (µSv) annually, or about 0.012 mSv. In comparison, the internal dose from the naturally radioactive Potassium-40 (K-40) that is also incorporated into the body is substantially higher, contributing around 0.17 mSv per year.
The largest single source of natural radiation exposure is typically the inhalation of Radon gas, which can contribute over 1.2 mSv annually. The small dose from C-14 is also minor compared to the dose from cosmic radiation at sea level, which is approximately 0.3 mSv. The air is overwhelmingly stable, and the trace amounts of radioactive isotopes present are simply a constant, harmless component of our natural environment.