Radon Glossary
Clear definitions of the terms you will find in radon reports, maps and regulations — from Bq/m³ to sub-slab depressurization.
Radon basics
Radon (Rn-222)A naturally occurring radioactive noble gas produced by the decay of radium-226 in the uranium-238 chain. It is colorless, odorless and tasteless, seeps out of soils and rocks, and can accumulate indoors. The WHO classifies it as the second leading cause of lung cancer after smoking.Radon entry routesThe openings through which soil gas enters a building: cracks in slabs and walls, construction joints, gaps around service pipes and cables, sumps, floor drains and crawl spaces. Identifying entry routes is a key step in designing an effective mitigation system.Soil gasThe air contained in the pores of the ground beneath and around a building. Soil gas typically carries much higher radon concentrations than outdoor air and is the main source of indoor radon when it is drawn into the building through the foundation.Stack effectThe pressure difference created when warm indoor air rises and escapes through upper parts of a building, drawing replacement air — including radon-laden soil gas — in through the lowest levels. It explains why indoor radon is usually highest in winter and at night.Thoron (Rn-220)A radioactive isotope of radon produced in the thorium-232 decay chain. Its half-life is only about 56 seconds, so it rarely travels far from its source; indoor exposure usually matters only where building materials contain significant thorium.Uranium decay chainThe sequence of radioactive decays that starts with uranium-238, present in most soils and rocks, and produces radium-226 and then radon-222. Because uranium is ubiquitous in the Earth’s crust, radon is generated everywhere — what varies is how much reaches indoor air.
Units
Becquerel (Bq)The SI unit of radioactivity: one becquerel equals one radioactive disintegration per second. Radon in air is expressed as an activity concentration, in becquerels per cubic metre (Bq/m³).Becquerels per cubic metre (Bq/m³)The standard unit for radon concentration in air in Europe and in WHO guidance: the number of radon disintegrations per second in each cubic metre of air. Reference levels are typically 100–300 Bq/m³. 1 pCi/L = 37 Bq/m³.Picocuries per litre (pCi/L)The traditional unit for radon concentration used in the United States. One picocurie per litre equals 37 becquerels per cubic metre; the US EPA action level of 4 pCi/L therefore corresponds to about 148 Bq/m³.Sievert (Sv)The SI unit of effective radiation dose, which weights the absorbed energy by radiation type and tissue sensitivity. Radon exposure is often converted to millisieverts per year (mSv/y) to compare it with other radiation sources; in many countries radon is the largest natural contributor to the population dose.Working Level (WL)A historical unit for the concentration of radon decay products in air, originally defined for uranium mines. Cumulative occupational exposure is measured in Working Level Months (WLM), a quantity still used in epidemiological studies of miners.
Measurement
Activated charcoal testA short-term test in which a canister of activated charcoal adsorbs radon for two to seven days and is then analysed in a laboratory. It provides a quick, low-cost screening value, but its result should never be the sole basis for a mitigation decision.Alpha-track detectorA passive detector containing a small piece of plastic that alpha particles from radon decay mark with microscopic tracks. After an exposure of one to twelve months, a laboratory counts the tracks to calculate the average radon concentration.Annual average concentrationThe radon concentration averaged over a full year, the quantity to which most reference levels formally apply. It is estimated from long-term measurements, sometimes adjusted with seasonal correction factors.Continuous radon monitorAn electronic device that measures and records radon concentration over time, typically displaying hourly or daily values. Useful for observing variations, screening and checking mitigation systems, though laboratory-analysed long-term tests remain the standard for regulatory decisions.Electret ion chamberA passive detector in which an electrostatically charged disc (electret) loses charge as radiation from radon ionizes the air inside a chamber. The measured voltage drop over the exposure period gives the average radon concentration.Long-term testA radon measurement of at least 90 days, ideally covering the heating season. It averages out daily and seasonal variations and is the basis for comparing against official reference levels and deciding whether to mitigate.Passive detectorA radon detector with no power supply or moving parts, such as alpha-track or electret detectors. It is left in place for weeks or months and then analysed in a laboratory, making it the usual choice for the long-term measurements regulations rely on.Radon mapA map published by a national authority showing the estimated radon potential of each area, based on geology and measurement campaigns. Maps identify priority areas for policy, but they never replace measuring a specific building.Radon potentialA classification — typically low, medium or high — of how likely buildings in an area are to have elevated indoor radon, derived from geology, soil permeability and measured data. High-potential areas are often designated as radon priority areas with additional legal obligations.Radon testingThe process of measuring the radon concentration in indoor air with a detector. Because levels vary greatly between buildings — even neighbouring ones — testing is the only way to know the level of a specific home or workplace.Short-term testA radon measurement lasting from two days to about three months. Because radon varies daily and seasonally, short-term results are only indicative; they are useful for screening or real-estate deadlines but should be confirmed with a long-term test.
Mitigation
Post-mitigation verificationA new radon measurement carried out after a mitigation system is installed to confirm that it works. A long-term follow-up test is recommended, plus periodic re-testing over the years, since fans age and buildings change.Radon barrierA continuous airtight membrane installed in the floor or foundation to block soil gas from entering a building. Standard in new construction in radon priority areas; its effectiveness depends critically on sealing all joints and penetrations.Radon fanThe in-line fan that powers an active soil depressurization system, running continuously to keep the ground under the building at lower pressure than the interior. Fans typically last 5–15 years and should be checked periodically, often with a pressure indicator.Radon mitigationThe set of building techniques used to reduce indoor radon concentration, from sealing entry routes and improving ventilation to active soil depressurization. Well-designed systems routinely reduce levels by 50–90% or more.Sealing entry routesClosing cracks, joints and service penetrations with appropriate sealants to reduce the pathways available to soil gas. Sealing alone rarely brings high levels below the reference level, but it improves the performance and efficiency of active systems.Sub-membrane depressurizationThe equivalent of sub-slab depressurization for crawl spaces: the earth floor is covered with an airtight membrane and a fan extracts the soil gas from beneath it, preventing radon from mixing with the air under the building.Sub-slab depressurization (SSD)The most effective and widely used mitigation technique: a suction point is created under the building’s slab and a fan extracts the radon-laden soil gas through a pipe, releasing it above the roof before it can enter the building.Sump depressurizationA variant of active soil depressurization that uses an existing sump or a purpose-built radon sump under the floor as the suction point. Common in buildings with drainage sumps or where a sump can be excavated from inside.VentilationIncreasing the supply of outdoor air to dilute indoor radon, either in the occupied space or in the underfloor void or crawl space. Mechanical ventilation with heat recovery can reduce moderate radon levels while limiting energy losses.
Regulation & authorities
Action levelThe term used by the US EPA for the concentration at which it recommends fixing a building: 4 pCi/L (about 148 Bq/m³). The EPA also suggests considering mitigation between 2 and 4 pCi/L.CSN (Consejo de Seguridad Nuclear)Spain’s nuclear safety and radiation protection regulator. Regarding radon, it publishes the national radon potential cartography, participates in the national radon action plan and issues guidance for measurement and mitigation.CTE DB-HS6 (Spain)The section of the Spanish Building Code (Código Técnico de la Edificación, Documento Básico HS6) that regulates protection against radon in buildings. It lists the municipalities where protective measures are required and sets a reference level of 300 Bq/m³.Directive 2013/59/EuratomThe EU Basic Safety Standards Directive that obliges member states to establish a national radon action plan, set reference levels not exceeding 300 Bq/m³ for dwellings and workplaces, identify radon priority areas and regulate radon in workplaces and new buildings.Radon priority areaAn area officially designated by a national authority where a significant share of buildings is expected to exceed the reference level. Designation typically triggers obligations such as workplace measurements and radon protection in new construction.Reference levelThe radon concentration above which action to reduce exposure is considered warranted. The EU sets a maximum reference level of 300 Bq/m³ for homes and workplaces (Directive 2013/59/Euratom); the WHO recommends 100 Bq/m³ where feasible. It is not a strict safe/unsafe boundary — risk exists at any level.UKHSA (UK Health Security Agency)The UK public health agency responsible for radon, operator of the UKradon service. It publishes the UK radon maps, defines Radon Affected Areas and sets the UK action level of 200 Bq/m³ for homes, with a target level of 100 Bq/m³.US EPA (Environmental Protection Agency)The United States federal agency that leads national radon policy. It recommends fixing homes at or above 4 pCi/L (about 148 Bq/m³), publishes the US radon zone map and testing/mitigation guidance, and coordinates radon programs with the states.
Health
Alpha radiationIonizing radiation consisting of helium nuclei emitted by radon and its progeny. Alpha particles travel only micrometres in tissue but deposit their energy densely, which makes inhaled alpha emitters particularly damaging to the cells lining the lungs.Group 1 carcinogen (IARC)The highest classification of the International Agency for Research on Cancer, reserved for agents with sufficient evidence of carcinogenicity in humans. Radon has been classified as a Group 1 carcinogen since 1988, based on studies of miners and residential epidemiology.Radon and smoking (combined risk)Tobacco smoke and radon multiply rather than merely add their effects: at the same radon level, the absolute lung cancer risk of a smoker is many times that of a never-smoker. Quitting smoking and reducing radon together provide the largest risk reduction.Radon lung cancer riskThe probability of developing lung cancer from radon exposure, which grows roughly linearly with concentration and duration — pooled European studies estimate about 16% higher risk per 100 Bq/m³ of long-term exposure. The WHO attributes 3–14% of lung cancers to radon depending on the country.Radon progeny (decay products)The short-lived radioactive isotopes (polonium, lead, bismuth) produced when radon decays. Attached to aerosols, they are inhaled and lodge in the airways, where their alpha emissions irradiate lung tissue — they, rather than radon gas itself, deliver most of the dose.