What is Radon

Radon sits at the bottom of Group 18 — the noble gases — in the seventh period of the periodic table. Its electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁶, with a completely filled outer p-shell (six valence electrons), which accounts for its chemical inertness.

Under standard conditions radon is a colorless, odorless, tasteless, monatomic gas with a density of 9.73 kg/m³ at 0°C and 1 atm — making it the densest gas at room temperature, roughly seven and a half times heavier than air. It condenses to a colorless liquid at −61.7°C and freezes to a glowing yellow-then-orange solid at −71°C; the glow is produced by the intense alpha radiation from its own decay.

Radon is the only gaseous product in the uranium and thorium decay series, and this unique property — being both radioactive and a gas — is what makes it a health hazard: unlike its parent radium and its daughter polonium (both solids), radon escapes from rock and soil and accumulates in enclosed air.

Radon has 39 known isotopes, all radioactive, spanning mass numbers 193 to 231. Only three occur naturally as members of the primordial decay series. Each is named after its parent chain, and each has a different half-life and health significance.

Three factors make ²²²Rn the primary indoor radon concern. First, its parent radium-226 has a very long half-life (1600 years) and is widespread in uranium-bearing rocks and soils, providing a persistent, nearly constant supply of ²²²Rn.

Second, the 3.82-day half-life of ²²²Rn itself is long enough for the gas to diffuse through meters of permeable soil and enter buildings before decaying, yet short enough to deliver significant radioactivity once indoors.

By contrast, ²²⁰Rn (thoron) decays in under a minute, so it rarely travels far from its point of origin; ²¹⁹Rn (actinon) decays in about 4 seconds and contributes almost nothing to indoor air. When we speak of "radon" in the context of indoor air quality and health, we mean ²²²Rn.

Radon-222 undergoes alpha decay with a half-life of 3.8235 days (or 3 days, 19 hours, 45 minutes). The decay is essentially 100% by emission of an alpha particle with an energy of 5.4897 MeV, producing polonium-218.

An alpha particle is a helium-4 nucleus (two protons, two neutrons) emitted at high velocity. The 5.49 MeV alpha from ²²²Rn travels about 4 centimeters (roughly 1.5 inches) in air before coming to rest, depositing its energy along a dense ionization track. In tissue or water, the range is only about 40–70 micrometers — shorter than the thickness of the outer dead layer of human skin.

This means radon gas outside the body is essentially harmless: the alpha particle cannot penetrate skin. The hazard arises when radon is inhaled. Most inhaled radon is exhaled before it decays, but its decay products — solid, radioactive, charged atoms — attach to aerosols or airway surfaces and decay in place, irradiating lung tissue from within.

Radon was discovered in 1899 by Ernest Rutherford and Robert B. Owens at McGill University while investigating the "radioactive emanation" from thorium compounds — what we now know as thoron (²²⁰Rn).

In 1900, Friedrich Ernst Dorn in Germany observed that radium compounds similarly released a radioactive gas; this was ²²²Rn. The gas was initially called "radium emanation" and later "niton" (from the Latin nitens, shining) before the name radon was officially adopted in 1923.

Marie and Pierre Curie and André-Louis Debierne isolated the gas and studied its properties in the early 1900s. The first accurate measurement of radon's half-life (3.825 days) came in 1911, and its density was first measured in 1910.

When a radon atom decays, it doesn't become stable — it initiates a cascade of further radioactive decays through a chain of short-lived "progeny" or "daughters." For ²²²Rn, the decay sequence is:

²²²Rn → ²¹⁸Po → ²¹⁴Pb → ²¹⁴Bi → ²¹⁴Po → ²¹⁰Pb (which then continues through ²¹⁰Bi and ²¹⁰Po to stable ²⁰⁶Pb)

Unlike radon, which is a chemically inert gas, these progeny are solid metal atoms (polonium, lead, bismuth) that are chemically reactive, carry electrical charges, and attach to dust particles or directly to the surfaces of lung airways when inhaled.

The first four members — ²¹⁸Po (half-life 3.05 min), ²¹⁴Pb (26.8 min), ²¹⁴Bi (19.9 min), and ²¹⁴Po (164 microseconds) — decay quickly. Two of them, ²¹⁸Po and ²¹⁴Po, emit high-energy alpha particles (6.00 and 7.69 MeV). It is these alpha emissions from deposited progeny, not from radon gas itself, that deliver essentially all the lung dose and drive the cancer risk.

Alpha particles deposit energy in dense, linear tracks — a property quantified as high linear energy transfer (LET), typically around 100–200 keV/μm in tissue. This concentrated energy deposition produces clustered ionizations and complex, difficult-to-repair DNA damage in cells along the track.

The biological effectiveness of alpha radiation is about 20 times higher than that of X-rays or gamma rays per unit absorbed dose, as reflected in the radiation weighting factor of 20 assigned to alpha particles by international radiation protection standards (ICRP).

Because alpha particles have a very short range (tens of micrometers in tissue), they are only hazardous when the radioactive source is inside or in direct contact with the body. Radon progeny deposited on bronchial airway surfaces irradiate the sensitive basal and secretory cells of the epithelium at point-blank range, initiating the DNA mutations that can lead to lung cancer.

Radon is detected and measured by exploiting its radioactive decay. The most common methods detect either the alpha particles emitted by radon and its progeny, or the gamma rays emitted by progeny nuclides.

Alpha detection methods include continuous radon monitors (which use ion chambers, scintillation cells, or silicon detectors to count alphas in real time) and passive devices such as alpha-track detectors (which record damage tracks left by alphas in a plastic film over weeks or months) and electret ion chambers (which measure ionization-induced voltage drop).

Gamma detection methods, such as gamma spectroscopy using high-purity germanium or sodium iodide detectors, identify radon indirectly by detecting the characteristic gamma rays from ²¹⁴Pb (352 keV) and ²¹⁴Bi (609 keV). Scintillation counting with zinc sulfide or Lucas cells detects alpha scintillations.

Liquid scintillation counting is used for radon in water samples. Activated charcoal canisters passively adsorb radon from air and are later analyzed by gamma counting.

Radon activity concentration — the number of radioactive decays per unit time per unit volume — is measured in becquerels per cubic meter (Bq/m³) in SI units or picocuries per liter (pCi/L) in the traditional unit still widely used in the United States.

The conversion is: 1 pCi/L = 37 Bq/m³. One becquerel equals one radioactive decay per second; one picocurie equals 0.037 Bq, or about 2.22 decays per minute. A typical indoor radon level of 50 Bq/m³ (~1.35 pCi/L) corresponds to roughly 110 radon atoms decaying per second in each cubic meter of air.

A single short-term radon measurement can vary widely due to weather, ventilation, and seasonal effects. Long-term measurements (90 days to one year) are needed to estimate annual average exposure, which is what correlates with health risk.

International and national authorities use reference levels or action levels as decision thresholds. The World Health Organization recommends a reference level of 100 Bq/m³ (2.7 pCi/L), with 300 Bq/m³ as an upper limit if 100 cannot be achieved. The US EPA action level is 148 Bq/m³ (4 pCi/L). The European Union directive sets 300 Bq/m³ for existing buildings and 200 Bq/m³ for new construction.

These levels represent points above which remedial action is recommended to reduce exposure. There is no safe threshold — risk increases linearly with exposure even at low levels — but remediation becomes increasingly cost-effective and urgent as concentrations rise above reference levels.

Radon levels above 1000 Bq/m³ (~27 pCi/L) are uncommon but occur in high-radon areas, especially in poorly ventilated basements over uranium-rich or fractured bedrock, and warrant immediate mitigation.