In brief. A shelter is an underground room, in contact with the ground and built to be very tightly sealed: these are the conditions in which radon and damp accumulate most easily. In Italy Legislative Decree 101/2020 (D.Lgs. 101/2020) sets a reference level of 300 Bq/m³ for existing dwellings, falling to 200 for those built after 31 December 2024. The risk lies mainly in everyday use, when the shelter serves as a cellar, gym or archive, rather than in the days of an emergency.
What radon is and why it builds up underground
Radon is a natural radioactive gas, odourless and colourless. The Istituto Superiore di Sanità (ISS, Italy's National Institute of Health), on a page updated in May 2025, explains that it comes from the decay of radium and originates mainly from the soil, and to a lesser extent from some building materials such as tuff. According to ISS estimates, exposure to radon is responsible for over 3,000 cases of lung cancer in Italy every year.
The International Atomic Energy Agency (IAEA) identifies it as the second cause of lung cancer after smoking and notes that it accumulates in enclosed spaces. The World Health Organization (WHO) fact sheet of January 2023 adds the details that matter to designers: concentrations are usually higher in cellars, basements and rooms in contact with the ground, and the gas enters through cracks in floors, joints between floors and walls, gaps around pipes and cables, sumps and drains. According to the WHO, the risk of lung cancer increases by about 16% for every additional 100 Bq/m³ of long-term average concentration, and for smokers the risk is about 25 times higher.
A shelter combines all these factors. It is underground, has few air changes when the ventilation is off and, in peacetime, often stays closed for weeks.
Thresholds in Italy and elsewhere
Legislative Decree 101/2020, in article 12, sets the reference levels as an annual average: 300 Bq/m³ for workplaces and existing dwellings, 200 Bq/m³ for dwellings built after 31 December 2024. The values are reported, among others, by ARPA Puglia in its 2022 environmental report. The National Radon Action Plan 2023-2032 was adopted by a decree of the President of the Council of Ministers of 11 January 2024. Which threshold applies to a shelter depends on how the room is used, and must be assessed with the engineer together with the other regulations and permits in Italy and Europe.
The WHO recommends a national reference level of 100 Bq/m³ and, where this cannot be achieved, no higher than 300. Switzerland shows how hard it is to move from rules to practice: Swissinfo reported in January 2020 that, according to a federal report, radon had been measured in only 6% of buildings, while the Federal Office of Public Health estimates between 200 and 300 deaths a year from radon-related lung cancer.
How it is measured
Maps are not enough. The ISS regards estimates based on maps or soil measurements as unreliable and identifies direct measurement as the most reliable way to know the level in a room. Small passive detectors are used, left in place for twelve months, because the concentration varies greatly with the seasons and the weather. According to the ISS, a single measurement generally costs no more than a few tens of euros. To estimate the annual average, the WHO indicates a measurement of at least three months.
For a shelter the practical criterion is simple: measure in the shelter itself, in the conditions in which it will actually be used, and repeat the measurement after work affecting floors, walls or systems. Good design reduces the risk, but only measurement tells you the real level.
Prevention costs less than remediation
The IAEA observes that preventing accumulation in new homes normally costs less than intervening afterwards. The techniques cited by the WHO and the ISS are well known: ventilating beneath the floor, creating a radon sump with a fan under the slab, sealing floors and walls, improving room ventilation. According to the WHO, passive systems can reduce concentrations by over 50%, and fan-assisted ones by even more. The ISS also points out that remediation work, especially in workplaces, requires professionals who have completed a qualification course of at least 60 hours.
In a shelter part of the work coincides with what is needed anyway. The 2024 brochure on large shelters from the Swiss Federal Office for Civil Protection requires gaps to be hermetically sealed, starting with duct penetrations: the same entry route the WHO identifies for radon. A suction system under a slab designed to withstand pressure, on the other hand, must be agreed with the structural engineer from the outset. Air and ventilation are covered on the page on shelter air, ventilation and filtration.
Damp and condensation
Damp is the second hidden risk. The WHO indoor air quality guidelines on dampness and mould, from 2009, associate damp environments with more respiratory symptoms, allergies and asthma, and identify avoiding persistent damp on internal surfaces and in the building fabric as the main measure.
In an underground room the walls often stay cooler than the air, and when the air is humid condensation forms. The Swiss system allows for this: the 2023 FOCP brochure "Il rifugio" (The shelter) requires the ventilation's condensate container to be emptied before use. The 2023 maintenance leaflet from the civil protection service of the Canton of Lucerne wants floor drains to be clean and filled with water, because empty ones smell. The WHO, for its part, lists drains and sumps precisely among radon's entry routes.
Damp also affects the system: NBC filters work with activated carbon, which loses effectiveness when damp, and a humid room is not the ideal place to store them.
The typical mistake: using the shelter without measuring it
The finished shelter is often immediately turned into a wine cellar, gym or document archive. The Swiss brochure allows uses such as cellar, storeroom, hobby room or archive, provided the envelope, doors and ventilation are not altered. But a paper archive deteriorates in a damp room, and a gym used every day in a room with high radon exposes people in exactly the way that matters for the risk: many hours, for years.
NetCom's view
NetCom starts from the client's real need, and the real need includes the everyday use of the room, not just the emergency scenario. That is why radon and damp are part of the initial assessment, together with the risks the shelter must protect against.
NetCom coordinates the security project and brings together the qualified professionals who deal with structures, systems and remediation; installation is entrusted to selected partner companies working to NetCom protocols. Measurements and documents remain confidential.
Frequently asked questions
What is the radon reference level in Italy?
Legislative Decree 101/2020 sets an annual average of 300 Bq/m³ for workplaces and existing dwellings and 200 Bq/m³ for dwellings built after 31 December 2024. Which value applies to a shelter depends on its use.
How is radon measured in an underground room?
With passive detectors left in the room for twelve months, as indicated by the Istituto Superiore di Sanità, which regards estimates based on maps as unreliable. For a shelter it is best to measure under real conditions of use.
Is the shelter's ventilation enough against radon?
It helps when it is running, but in peacetime it is often switched off. Effective measures, such as sealing and sub-slab suction, must be planned in the design and coordinated with the structure.
If you are considering a shelter in an underground room and want to plan it with radon and damp in mind too, NetCom's page on private bunkers and shelters explains how to request an initial confidential conversation.
To assess a shelter on your property: Private bunkers and shelters · Preliminary shelter check · Who builds private bunkers in Italy · Bunker cost
Sources
- Protezione dal radon (Radon protection) — Istituto Superiore di Sanità, published on 13 November 2019, updated on 7 May 2025.
- Radon — World Health Organization, 25 January 2023.
- Radon — International Atomic Energy Agency (IAEA).
- Indoor radon activity concentration (State of the Environment, Physical agents 2021) — ARPA Puglia, February 2022.
- Radon Action Plan 2023-2032: the DPCM published in the Official Gazette — LavoriPubblici.it, 23 February 2024.
- WHO guidelines for indoor air quality: dampness and mould — World Health Organization, 2009.
- Il rifugio (The shelter). Information on the purpose, structure and use of public shelters — Federal Office for Civil Protection (FOCP), June 2023.
- Grandi rifugi (Large shelters). Information on shelters for 200 to 800 people — FOCP, October 2024.
- Merkblatt für den Unterhalt von Schutzräumen — Canton of Lucerne, version of 5 July 2023.
- Anti-radon measures found 'to be lacking' — Swissinfo, 31 January 2020.
