The World Health Organization recommends that indoor radon levels stay below 100 Bq/m³ (becquerels per cubic meter), with an upper reference limit of 300 Bq/m³ for homes where national conditions make the lower target difficult to achieve. For workplaces, the international safety standard sets a maximum reference level of 1,000 Bq/m³. These thresholds exist because radon is the second leading cause of lung cancer worldwide, after smoking.
What the WHO Reference Levels Mean
The WHO’s recommended reference level of 100 Bq/m³ is not a safe/unsafe dividing line. There is no known threshold below which radon exposure carries zero risk. Instead, lung cancer risk rises in a linear fashion: for every 100 Bq/m³ increase in long-term average radon concentration, your lifetime risk of lung cancer goes up by roughly 16%. That percentage applies whether you’re going from 50 to 150 or from 200 to 300.
The 100 Bq/m³ figure represents the level the WHO considers achievable in most countries through building codes and mitigation. The 300 Bq/m³ upper limit acknowledges that some regions have geological conditions (granite bedrock, volcanic soil) that make it impractical to get every home below 100. Countries are encouraged to set their own national reference levels somewhere in that 100 to 300 range based on local geology, housing stock, and resources.
How These Compare to U.S. and Other Standards
If you’re in the United States, you’re more likely familiar with measurements in picocuries per liter (pCi/L). The conversion is straightforward: 100 Bq/m³ equals about 2.7 pCi/L. The EPA’s action level of 4 pCi/L translates to roughly 148 Bq/m³, which falls between the WHO’s ideal target and its upper limit. Canada uses 200 Bq/m³ as its guideline. The European Union recommends that member states set reference levels no higher than 300 Bq/m³.
In practical terms, the WHO guidelines are stricter than what most countries actually enforce. If your home tests below 100 Bq/m³, you’re within the WHO’s preferred range. Between 100 and 300, you’re in territory where mitigation is worth considering. Above 300, virtually every international body agrees action is needed.
Why Radon Is a Serious Health Concern
Radon is a naturally occurring radioactive gas that seeps into buildings from the ground. It’s colorless and odorless, so you can’t detect it without a test. When you breathe it in, its radioactive decay products lodge in lung tissue and emit alpha particle radiation, which damages cells over time. This damage accumulates with years of exposure, which is why the WHO focuses on long-term average concentrations rather than short spikes.
The 16% increase in lung cancer risk per 100 Bq/m³ is based on large pooled studies of residential radon exposure across multiple countries. To put that in perspective: someone living for decades in a home at 200 Bq/m³ faces roughly 32% higher lung cancer risk than someone at near-zero exposure, all other factors being equal.
The Interaction Between Radon and Smoking
Radon and smoking don’t just add risk, they multiply it. Based on the BEIR VI risk model used by the U.S. EPA, at a radon level of 4 pCi/L (about 148 Bq/m³), the lifetime risk of dying from radon-induced lung cancer is 7 per 1,000 for people who have never smoked. For ever-smokers at the same radon level, that figure jumps to 62 per 1,000.
At higher exposures the gap widens further. At 10 pCi/L (roughly 370 Bq/m³), never-smokers face a risk of 18 per 1,000. For ever-smokers, it climbs to 150 per 1,000, meaning about 1 in 7 will develop fatal radon-related lung cancer over a lifetime. This synergistic effect is one reason the WHO emphasizes radon reduction as a public health priority: in populations where smoking rates remain high, even moderately elevated radon levels cause significant harm.
How to Test Your Home
Radon levels vary dramatically from house to house, even on the same street, because they depend on local soil composition and how your specific foundation interacts with the ground. The only way to know your level is to test. Short-term test kits (typically 2 to 7 days) give a quick snapshot and are widely available at hardware stores or through local health departments. They’re useful as a screening tool, but because radon concentrations fluctuate with weather, season, and ventilation, a short test can over- or underestimate your actual exposure.
Long-term tests, lasting 90 days to a full year, provide a much more reliable picture of your average exposure. Since the WHO’s risk estimates are based on long-term averages, this type of test aligns best with the science. Place the detector in the lowest livable level of your home, ideally a ground-floor room or basement you actually use, with doors and windows closed as much as is practical during the test period.
Reducing Radon if Levels Are High
If your test results come back above the WHO reference level for your country, mitigation systems can typically reduce indoor radon by 50% to 99%. The most common and effective approach is sub-slab depressurization: a pipe is installed through the foundation floor and connected to a fan that draws radon-laden air from beneath the building and vents it outdoors before it can enter your living space. Installation usually takes a day and costs vary by region, but for most homes it’s a straightforward renovation.
Other strategies include sealing cracks and gaps in the foundation, improving ventilation in crawl spaces, and installing radon barriers during new construction. For new builds, adding a radon-resistant membrane and ventilation piping at the construction stage is far cheaper and easier than retrofitting later. Some countries now require these features in building codes for high-risk areas.
After mitigation, retesting is essential to confirm that levels have dropped to an acceptable range. Most systems include a simple visual indicator (a U-tube manometer) that lets you verify the fan is running, but periodic retesting every few years ensures the system continues to perform as expected.