Radon (Rn): The Complete Practical Guide to Testing, Risk, and Mitigation
Radon is a naturally occurring radioactive gas that can seep into buildings from soil and rock and accumulate indoors, especially in lower levels like basements. It’s colorless, odorless, and tasteless, so the only reliable way to know your radon level is to test. The reason radon matters is health: it is widely recognized as a major cause of lung cancer risk, especially for people who smoke.
What Is Radon and Where Does It Come From?
Radon (often referring to radon-222) forms naturally from the breakdown of uranium in soil and rock. Outdoors, it disperses quickly. Indoors, it can enter through:
- cracks in slabs and foundation walls
- gaps around pipes and sump pits
- crawl spaces and construction joints
- well water (in some cases, radon can be released from water into indoor air)
Because indoor radon can vary by house design, soil conditions, weather, and ventilation, any home can have elevated radon, even within the same neighborhood.
Why Radon Is a Serious Health Topic
Health agencies consistently highlight radon as a major lung cancer risk:
- The CDC notes radon is the second leading cause of lung cancer deaths after smoking, and estimates (with EPA/Surgeon General) that it contributes to ~21,000 lung cancer deaths per year in the U.S.
- Risk increases significantly for smokers exposed to radon compared with non-smokers at the same radon level.
Even “moderate” radon levels can matter over long exposure periods—so testing and mitigation are the practical steps.
Radon Units Explained: pCi/L vs Bq/m³
You’ll see radon reported in two main units:
- pCi/L (picocuries per liter) — common in the United States
- Bq/m³ (becquerels per cubic meter) — common internationally
A helpful reference point comes from EPA’s action guidance:
- 4 pCi/L ≈ 150 Bq/m³, and 2–4 pCi/L ≈ 75–150 Bq/m³.
Radon “Action Level” and What Your Results Mean
EPA guidance (U.S.)
EPA recommends fixing the home if radon is 4 pCi/L (150 Bq/m³) or higher. EPA also recommends considering fixing the home if radon is between 2 and 4 pCi/L.
WHO reference level (global public health perspective)
The WHO Handbook on Indoor Radon proposes a reference level of 100 Bq/m³, and discusses how national programs may adapt based on local factors.
Radon Result Interpretation Table
| Radon Result | Meaning (Practical) | Typical Next Step |
|---|---|---|
| Below ~2 pCi/L (≈75 Bq/m³) | Lower risk, but not “zero risk” | Optional long-term confirmation, especially in winter |
| 2–4 pCi/L (≈75–150 Bq/m³) | Elevated | EPA says “consider fixing”; do a follow-up test |
| ≥4 pCi/L (≥150 Bq/m³) | High | EPA recommends mitigation (“fix the home”) |
(Use your country’s official guidance where applicable; the table above follows EPA’s published thresholds.)
How to Test for Radon Correctly
Radon levels fluctuate daily and seasonally, so correct testing method matters.
Short-term vs long-term radon tests
- Long-term tests measure radon for over 90 days and give a better estimate of the year-round average.
- Short-term tests are used when you need quicker results; CDC describes short-term kits as measuring radon over 2–90 days (device dependent).
EPA’s “A Citizen’s Guide to Radon” emphasizes that long-term tests provide a better estimate of year-round exposure and notes common long-term device types like alpha track and electret detectors.
Where to place a radon test (best practice)
Most official guidance aligns on these principles:
- Test the lowest level of the home/building that is regularly occupied (or could be used)
- Keep the device away from drafts, exterior doors/windows, and high humidity areas (unless the kit is designed for it)
- Follow kit instructions for closed-house conditions (if required)
Understanding Why Radon Levels Change
High-ranking radon education content usually explains these drivers:
- Season: levels often trend higher during heating season (windows closed, stack effect)
- Weather and pressure: changes can alter soil-gas flow into buildings
- Ventilation: more fresh air exchange can reduce indoor radon
- Building tightness and pathways: cracks, sumps, and penetrations increase entry routes
- Foundation type: basements/crawl spaces can influence accumulation
This is why “one quick test” can be a starting point—but long-term testing is the best way to estimate real exposure.
Radon Mitigation: What Works Best
Most effective and most common method: Sub-slab depressurization
EPA and other technical references describe sub-slab depressurization (also called active sub-slab suction) as the most common and typically the most reliable radon reduction approach for many basement and slab-on-grade homes.
How it works (simple explanation):
- A fan creates suction beneath the slab
- Radon-laden soil gas is pulled into a pipe and exhausted safely outdoors
- This reduces radon entry through cracks and openings
Other mitigation measures (often used together)
- Sealing obvious entry points (supportive, but usually not enough alone)
- Improving ventilation in some structures (case-by-case; may be less effective than SSD for many homes)
- For crawl spaces: specialized approaches like membrane + depressurization may be used (depending on site)
EPA’s consumer mitigation guide lists multiple soil suction methods (subslab, drain-tile, sump-hole, block-wall suction) and highlights active subslab suction as a common reliable method.
FAQ
What radon level is considered dangerous?
EPA recommends fixing homes at 4 pCi/L (150 Bq/m³) or higher, and considering mitigation for 2–4 pCi/L.
What is the WHO recommended radon reference level?
WHO proposes a reference level of 100 Bq/m³ (with discussion of national considerations).
How long should I test for radon?
Long-term tests measure radon for over 90 days and better represent the year-round average.
What is the most effective radon mitigation method?
For many basement and slab-on-grade homes, active subslab suction / sub-slab depressurization is widely described as the most common and usually the most reliable method.
Is radon really a big lung cancer risk?
CDC states radon is the second leading cause of lung cancer deaths after smoking and estimates radon contributes to ~21,000 lung cancer deaths annually in the U.S.
