Is radon a risk in an underground shelter or basement?

Short answer. Yes. Radon, a natural radioactive gas from the soil, is usually most concentrated in basements, cellars and rooms in contact with the ground ². WHO links it to 3% to 14% of lung cancers and recommends a reference level of 100 Bq/m³, never above 300 Bq/m³ ². The only way to know a shelter's level is a long-term test of at least three months ¹.

Why is radon a concern below ground?

Radon enters buildings from the soil, and the main way it moves indoors is pressure-driven airflow from the soil into the occupied space ¹. WHO notes that levels are usually highest in basements and cellars and in living spaces in contact with the ground, although considerable levels can also occur above the ground floor ². Health Canada says the same: radon levels are generally highest in basements and crawl spaces ⁸.

An underground shelter or a basement safe room sits exactly where radon tends to collect. That does not mean every shelter has a problem. WHO stresses that indoor concentrations vary with the construction of the building and ventilation habits, and vary by season, day and even hour ¹. That variability is why measurement, not guesswork, decides.

How serious is the health risk?

WHO describes radon as the second most important cause of lung cancer after smoking in many countries, and the primary cause among people who have never smoked ¹. Lung cancer risk rises by about 16% for each 100 Bq/m³ increase in long-term average radon concentration ². There is no known threshold below which radon presents no risk ¹.

The risk depends on long exposure, not a single night. Reference levels are set on the annual average concentration ¹⁵⁶. EPA's lifetime risk estimates show how strongly smoking multiplies the effect:

Source: EPA ³.

EPA estimates that radon causes about 21,000 lung cancer deaths a year in the United States, about 2,900 of them among people who have never smoked ³.

Does a shelter used only in emergencies need testing?

It depends on how the room is used day to day. Health Canada's guideline applies to the "normal occupancy area", meaning any part of a building where a person is likely to spend more than 4 hours per day; it excludes crawl spaces, unfinished basements and rarely used areas such as storage rooms ⁶. A shelter that stays closed most of the year is closer to a storage room under that definition.

Many basement shelters do double duty as an office, a guest room, a gym or a teenager's den. Once someone spends hours there every day, the room becomes the "lowest lived-in level" that Health Canada asks people to test ⁷. EPA likewise says that if you begin occupying a lower level of your home, such as a basement, you should retest on that level ⁴.

There is a practical reason to test even a rarely used shelter: a long emergency stay could keep people in the room around the clock for days or weeks. Knowing its radon level in advance is part of knowing the space.

How do you test a shelter or basement for radon?

The key points from the agencies:

• Duration. WHO says estimating the annual average requires reliable measurements for at least three months and preferably longer; short-term measurements give only a crude indication ¹. WHO defines a long-term measurement as 3 months up to 1 year ¹.
• Season. Health Canada advises a 3-month test ideally during the fall or winter ⁸, and its technical guide asks that at least 91 days of the test fall within the heating season ⁷.
• Where. Test in the normal occupancy area of the lowest lived-in level ⁷. For a shelter, that means inside the room itself, not the floor above.
• Device quality. Health Canada points to measurement professionals certified under the Canadian National Radon Proficiency Program (C-NRPP) ⁸; WHO recommends quality assurance for measuring devices ¹.
• Retest after changes. WHO notes that renovation which alters structure, heating, cooling or mechanical systems can open new entry routes for radon or change air pressure patterns ¹.

What official agencies recommend

Agencies agree on the method (long-term testing, then action above a reference level) but not on the number.

A note on units: the United States measures radon in picocuries per litre (pCi/L), most other countries in becquerels per cubic metre (Bq/m³). EPA expresses its 4 pCi/L action level as 150 Bq/m³ ³; the exact unit conversion (1 pCi/L = 37 Bq/m³) gives 148 Bq/m³. In practice, the U.S. and Canadian thresholds are both stricter than the EU ceiling, and WHO's preferred level is the strictest of all.

The EU directive also requires member states to promote action to identify dwellings above the reference level and to encourage radon-reducing measures in them ⁵. Each EU country sets its own figure, which can be lower than 300 Bq/m³; check the national radon authority.

What if the result is high?

Do not try to engineer a fix from a guide. Radon reduction is specialised work, and how it is done depends on the foundation, the soil and the ventilation of the specific building. Health Canada recommends hiring a certified radon mitigation professional to find the best way to reduce the level ⁸, and says levels can be reduced by more than 80% for about the same cost as common repairs such as replacing a furnace ⁸.

WHO's comparison of control options shows why a professional assessment matters: sealing alone has limited potential for radon reduction, especially over time, because it does not address pressure-driven airflow ¹. In WHO's table of radon control options for new construction, every option, from sealing and ventilation to soil depressurization, calls for radon or pressure testing to check that it works ¹.

For a shelter, any change to ventilation or to the envelope also affects other functions, such as filtration, overpressure or blast protection. Those trade-offs belong to a qualified engineer working with a certified radon professional.

Frequently asked questions

Is a short emergency stay in a high-radon shelter dangerous?

Radon's health effect is tied to long-term average exposure, which is why reference levels are set on annual averages ¹². A short stay matters far less than years of daily use, but WHO notes there is no known threshold below which radon presents no risk ¹. Testing in advance removes the guesswork.

Can I test with a short-term kit?

Short-term kits give a quick indication, but WHO calls them only a crude indication of the actual concentration ¹. WHO counts only measurements of 3 months up to 1 year as long-term ¹, and Health Canada calls for a long-term test of 3 months ⁸, with at least 91 days in the heating season ⁷. Use a long-term test for decisions about a room people will occupy.

Does a buried container or concrete bunker block radon?

Not reliably. Radon moves through soil and into structures by pressure-driven airflow, and sealing surfaces alone has limited and often declining effect ¹. Only a measurement inside the finished, occupied space shows the real level.

Does smoking change the risk?

Yes, sharply. At 4 pCi/L, EPA estimates that about 62 in 1,000 smokers could get lung cancer over a lifetime, against about 7 in 1,000 never-smokers ³. WHO notes that radon is much more likely to cause lung cancer in people who smoke or have smoked, and that the majority of radon-induced lung cancers are caused jointly by radon and smoking ¹.

Which number should I use?

Use your own country's reference level, then aim lower where practical. WHO recommends 100 Bq/m³ ², Health Canada 200 Bq/m³ ⁶, the EU allows no national level above 300 Bq/m³ ⁵, and EPA acts at 4 pCi/L ³.

Plan radon into your shelter from the start

A radon test result belongs in the planning of any underground shelter, alongside ventilation and moisture. Build your Phaenyx plan to see the risks around your home first.

Sources

1. World Health Organization — WHO handbook on indoor radon: a public health perspective (2009) — https://www.who.int/publications/i/item/9789241547673 (accessed 2026-10-20)
2. World Health Organization — Radon and health (fact sheet) — https://www.who.int/news-room/fact-sheets/detail/radon-and-health (accessed 2026-10-20)
3. U.S. Environmental Protection Agency — Health Risk of Radon — https://www.epa.gov/radon/health-risk-radon (accessed 2026-10-20)
4. U.S. Environmental Protection Agency — How often should I test/retest my home for radon? — https://www.epa.gov/radon/how-often-should-i-testretest-my-home-radon (accessed 2026-10-20)
5. Council of the European Union — Directive 2013/59/Euratom, Article 74: Indoor exposure to radon — https://www.legislation.gov.uk/eudr/2013/59/article/74 (accessed 2026-10-20)
6. Health Canada — Government of Canada Radon Guideline — https://www.canada.ca/en/health-canada/services/health-risks-safety/radiation/radon/government-canada-radon-guideline.html (accessed 2026-10-20)
7. Health Canada — Guide for radon measurements in homes — https://www.canada.ca/en/health-canada/services/publications/health-risks-safety/guide-radon-measurements-residential-dwellings.html (accessed 2026-10-20)
8. Health Canada — Radon: What you need to know (factsheet) — https://www.canada.ca/en/health-canada/services/environmental-workplace-health/reports-publications/radon-what-you-need-to-know.html (accessed 2026-10-20)