Radon Decay Products

Radon-222 alpha-decays to produce a cascade of short-lived solid progeny:

The first five members (Rn-222 through Po-214) all have half-lives under 30 minutes, so they decay in situ in the airways before clearance mechanisms remove them, delivering alpha energy directly to bronchial tissue.

Lead-210's 22.3-year half-life is enormously longer than lung residence time (hours), so Pb-210 atoms formed in the airway are cleared before they decay. Its contribution to acute bronchial alpha dose is negligible, and the chain is treated as "ending" at Pb-210 for radon health purposes.

Only three chain members emit alpha particles: Rn-222 (5.49 MeV), Po-218 (6.00 MeV), and Po-214 (7.69 MeV). Alpha particles are high-LET (linear energy transfer) radiation that deposit large energy over a very short range — a 5.5 MeV alpha travels only ~4 cm in air and tens of micrometers in tissue.

Deposited Po-218 decays by alpha. Even Pb-214 and Bi-214 that deposit ultimately decay through to Po-214, which delivers its 7.69 MeV alpha. Health-physics dosimetry commonly neglects beta contribution because the biological effect of beta particles is far less significant than the two alpha emitters.

Calculated dose-conversion factors attribute only a small fraction to beta radiation (~0.21 mSv/WLM for betas vs ~10 mSv/WLM total). Po-218 and Po-214 are responsible for the highest dose deposition in bronchial epithelium.

When a progeny atom is born by alpha recoil, it is typically a small, charged ion. Within a fraction of a second it clusters with trace gases and water vapor to form an "unattached" cluster of ~0.5–5 nm diameter with very high diffusion coefficient.

These clusters attach to ambient aerosol particles within ~1–100 seconds, becoming the "attached" fraction on particles of ~100–1000 nm (accumulation mode dominant, AMAD typically 200–450 nm).

What determines the split: ambient aerosol concentration. More particles → faster attachment → smaller unattached fraction. Clean, poorly ventilated air yields higher unattached fractions; cigarette smoke, cooking, candles sharply lower it.

If radon were sealed in stagnant air for several hours (about 3+ hours), each progeny would build up to the same activity as the radon (secular equilibrium, F = 1). In real indoor air, ventilation, plate-out, and filtration continuously remove progeny before equilibrium, so progeny activities are lower (disequilibrium, F < 1).

The equilibrium factor F is the ratio of equilibrium-equivalent concentration (EEC) to actual radon gas concentration. Formally:

F = (0.106·C₍Po-218₎ + 0.514·C₍Pb-214₎ + 0.380·C₍Bi-214₎) / C₍Rn-222₎

Typical values: UNSCEAR and ICRP adopt F = 0.4 for indoor air and 0.6 for outdoor air. BEIR VI assumes F = 0.4 in homes. Measured values range ~0.1–0.9, mostly within ±30% of 0.4.

What influences F: ventilation rate (dominant factor — more ventilation lowers F), aerosol concentration, surface-to-volume ratio, plate-out rate, and filtration (which can drop F sharply).

The potential alpha energy of an atom is the total alpha energy it will emit as it decays down to Pb-210. The PAEC of a mixture is the sum of potential alpha energies of all progeny per unit volume.

Per-atom energies: Po-218 = 13.69 MeV (its own 6.00 MeV + downstream 7.69 MeV from Po-214); Pb-214 = Bi-214 = Po-214 = 7.69 MeV (only downstream/own Po-214 alpha).

Units: Working Level (WL) — historical unit = any combination of progeny in 1 L of air that will emit 1.3 × 10⁵ MeV of potential alpha energy. 1 WL = 2.08 × 10⁻⁵ J/m³. SI unit: joule per cubic meter (J/m³).

Cumulative exposure: Working Level Month (WLM) = exposure to 1 WL for 170 hours. 1 WLM = 3.54 mJ·h/m³.

How PAEC is calculated: From measured progeny concentrations, PAEC = Σ (activity × potential-alpha-energy-per-unit-activity). Equivalently, PAEC (J/m³) = 5.56 × 10⁻⁹ × EEC (Bq/m³), or from radon gas: PAEC = 5.56 × 10⁻⁹ × F × C₍Rn₎.

Most inhaled radon gas is exhaled before it decays (3.8-day half-life is long vs lung residence time), so radon itself delivers little dose. The dose comes from progeny that deposit and decay in airways.

Because the progeny-to-gas ratio (F) varies widely (0.1–0.9) with ventilation, aerosols, and plate-out, two rooms with identical radon readings can present substantially different true exposures.

The health-relevant quantity is PAEC (or exposure in J·h/m³ or WLM), which equals radon × F × constant. Regulators measure gas (simple, cheap) and multiply by assumed F (~0.4) — adequate for homes but a real uncertainty source in mines, caves, or air-filtered spaces where F departs from 0.4.

The unattached fraction adds further variation because unattached progeny deposit far more efficiently in airways.

Deposition depends on particle size (attachment state), breathing pattern, and airway geometry.

Unattached clusters (~1 nm): deposit by Brownian diffusion with very high efficiency in extrathoracic and upper bronchial airways. One model computed ~96% extrathoracic deposition for healthy adults, with highest bronchial deposition around airway generations 13–14.

Attached progeny (~100–450 nm): deposit far less efficiently in extrathoracic region (~6–8%) and penetrate variably into tracheobronchial and alveolar regions via impaction and sedimentation.

Hot spots: computational fluid dynamics shows highly non-uniform deposition, with concentration at airway bifurcation "carinal ridges." Local cell-nucleus doses at hot spots can reach ~22 Gy vs average ~0.1 Gy, driving multiple-hit cellular damage.

The bronchial epithelium (basal and secretory target cells) is the principal tissue at risk; highest deposition-density occurs in first/second bronchial generations.

ICRP Human Respiratory Tract Model (HRTM), Publication 66 (1994): regional lung deposition/clearance model used to compute regional deposition and dose.

ICRP Publication 65 (1993): epidemiologically based dose conversion convention (lifetime excess risk 2.8×10⁻⁴/WLM; rounded 5 mSv/WLM workers, 4 mSv/WLM public).

ICRP Publication 137 (2017): current dosimetric coefficients — rounded recommendation 3 mSv per mJ·h/m³ (~10 mSv/WLM) for most circumstances (equivalent to 6.7 nSv per Bq·h/m³ at F = 0.4).

Detailed values: 3.3 mSv per mJ·h/m³ (12 mSv/WLM) for mines, 5.7 mSv per mJ·h/m³ (20 mSv/WLM) for indoor workplaces, 6.7 mSv per mJ·h/m³ (24 mSv/WLM) for tourist caves, sedentary work/homes ~4 mSv per mJ·h/m³ (14 mSv/WLM).

Jacobi (1972) room model / Jacobi–Porstendörfer model: standard indoor air model describing dynamic balance of free and attached progeny via rate parameters (ventilation, attachment, deposition), from which F and fₚ are derived.

Radon is the second leading cause of lung cancer after smoking. WHO estimates radon causes 3–14% of all lung cancers depending on national radon levels and smoking prevalence.

Risk increases by about 16% per 100 Bq/m³ increase in long-term average radon concentration (Darby et al., 2005, 13-study European pooling, BMJ).

US EPA attributes about 21,000 US lung-cancer deaths per year to radon, of which about 2,900 occur among people who have never smoked.

Alpha particles emitted inside the lung are intensely damaging (high LET), shredding DNA in bronchial epithelial cells and initiating mutations that can lead to lung cancer. Alpha particles can't penetrate skin from outside, but emitted inside the airway they irradiate target cells at point-blank range.

IARC classifies radon as a Group 1 human carcinogen. WHO, EPA, and all major health authorities recommend testing and mitigation where levels exceed reference thresholds.