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Indoor Air Quality

Indoor Radon Concentration & Mitigation Simulator

Estimate the indoor concentration of Radon-222 seeping from granite or sedimentary soils into basements and slabs, using a steady-state mass balance with soil type, basement geometry, air change rate and mitigation method. The tool reports compliance with EPA 4 pCi/L (148 Bq/m³) and WHO 100 Bq/m³, the BEIR VI lifetime lung-cancer risk, and the relative effectiveness of SSD, mechanical ventilation, passive vent and sealing.

Parameters
Source radon concentration
Bq/m³
Radon activity in soil gas beneath the slab / crawlspace
Soil type
Granite has the highest U-238 content and is the strongest radon source
Building volume V
m³
Air change rate ACH
1/h
Tight, high-performance envelopes have the lowest ACH
Mitigation method
SSD = sub-slab depressurization; HRV = heat-recovery ventilation
Basement
A basement gives the largest soil-contact area and the highest radon influx
Results
—
Indoor radon (Bq/m³)
—
Indoor radon (pCi/L)
—
Exceeds EPA 4 pCi/L
—
Required reduction (%)
—
Lifetime lung cancer risk (%)
—
Recommended action
—
House cross-section — radon entry paths and SSD pipe

Schematic of radon entering through slab cracks into the basement, with an SSD pipe and fan venting above the roof when selected. Particle colour scales with the indoor concentration.

Indoor radon vs air change rate ACH
Concentration by mitigation method
Theory & Key Formulas

$$C_{indoor} = \frac{S \cdot e_{mit}}{V\,(k_e + \lambda)},\qquad \text{EPA: } C \leq 4\ \text{pCi/L}$$

S: source influx (Bq/h); e_mit: residual fraction after mitigation (0-1); V: building volume (m³); k_e: ACH (1/h); λ: radon decay constant 7.6×10⁻³ 1/h.

$$S = C_{src} \cdot f_{soil} \cdot f_{base} \cdot 5,\qquad C_{pCi/L} = C_{Bq/m^3}/37$$

Source influx scales as soil concentration × soil factor × basement factor × an area constant of 5. To convert to pCi/L, divide Bq/m³ by 37 (1 pCi/L ≈ 37 Bq/m³).

$$\text{Lifetime risk} \approx 0.18\,\% \times C_{pCi/L},\qquad \text{Reduction} = \max\!\left(0,\ 1 - \frac{4}{C_{pCi/L}}\right)$$

Lifetime lung-cancer risk uses the BEIR VI slope of 0.18%/(pCi/L) in a linear no-threshold (LNT) model. The required reduction is the percentage cut needed to bring the house to the EPA 4 pCi/L action level.

Indoor Radon Concentration & Mitigation — EPA 4 pCi/L Action Level

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I keep seeing "radon" warnings on real-estate listings. Is it really a serious problem at home? My parents' place has a basement, so I'm a bit worried.
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Yes, it really is. Radon-222 is a noble-gas radioactive decay product of uranium-238 → radium-226 with a 3.8-day half-life. It seeps from soil and rock through micro-cracks in slabs and gaps in the foundation, accumulating in basements and ground floors. According to US CDC, radon is the second leading cause of lung cancer after smoking, with about 21,000 deaths per year in the United States and roughly 84,000 worldwide per WHO estimates. For never-smokers it is the single largest cause of lung cancer.
🙋
Wow, that's a lot. So how do we tell whether a house is "dangerous"? I see 4 pCi/L in the tool.
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That is the US EPA Action Level — 4 pCi/L = 148 Bq/m³. Anything above it triggers mitigation. WHO is stricter at 100 Bq/m³ Reference Level, and Japan's MHLW uses 200 Bq/m³. Different countries draw the line differently. This tool checks EPA and WHO together: above 4 pCi/L turns red, the 2-4 pCi/L grey zone turns amber. The defaults (granite, full basement, ACH 0.5, no mitigation) give about 39 Bq/m³ ≈ 1.06 pCi/L, which sits comfortably below both limits — hence the "Good" recommendation.
🙋
OK. But if I push the source up to 2000 Bq/m³ and keep the basement, a "required reduction" number pops up. What is that?
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It tells you what fraction of the radon you need to cut to bring the house down to the EPA 4 pCi/L line. If the indoor reading is 10 pCi/L, you need to drop to 4/10 = 0.4 of that, a 60% reduction. Then you pick a mitigation method that can do at least that. Sub-Slab Depressurization (SSD) installs a suction fan beneath the foundation slab and keeps the soil side at negative pressure, typically achieving 90-95% reduction. That is why we use a residual of 5% in this tool — it is overwhelmingly the strongest single measure and is now the standard build-in option for new US homes.
🙋
How do the other methods (passive vent, sealing, mechanical ventilation) compare? The bar chart is interesting.
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Look at the "Mitigation method" chart. SSD wins by a wide margin; mechanical ventilation (HRV/ERV) is second at about 70% reduction; sealing alone only drops the level by around 30%. A common misconception is that sealing keeps radon out — in practice sealing can also "trap and concentrate" radon, and works against you unless combined with SSD. Winter heating periods drop the ACH and pile radon up further. The hot-spot geographies are well-known: in Japan the Abukuma granite belt and parts of the Kanto loam; in Europe Cornwall and Bohemia; in North America Iowa, Pennsylvania and the Colorado Plateau — several US states require radon testing at real-estate transactions.

Frequently Asked Questions

We apply the steady-state mass balance C_indoor = (S · e_mit) / (V · (k_e + λ)). S is the source influx in Bq/h, computed as source concentration × soil factor × basement factor × an area unit constant of 5. e_mit is the residual fraction for the chosen mitigation (SSD = 0.05, sealing = 0.7, etc.), V is the building volume, k_e is ACH in 1/h, and λ is the radon decay constant 7.6×10⁻³ 1/h. Output is shown in both Bq/m³ and pCi/L (= Bq/m³ ÷ 37).
The US EPA Action Level is 4 pCi/L = 148 Bq/m³, above which mitigation is recommended. The WHO Reference Level is the stricter 100 Bq/m³, and the UK PHE uses an Action Level of 200 Bq/m³ that sits between the two. Japan's MHLW guideline is 200 Bq/m³ indoors. This tool checks both EPA and WHO simultaneously, flagging anything above 4 pCi/L in red and the 2-4 pCi/L grey zone in amber.
SSD installs a suction fan beneath the foundation slab and keeps the soil side at a slight negative pressure, physically preventing radon-laden soil gas from migrating into the building. We use a residual fraction of 5% (95% reduction), which matches the representative figure in the EPA Indoor Radon Guide and AARST B-2014 standards. The other options (passive vent 50%, mechanical vent 30%, sealing 70%) are weaker — sealing alone cannot stop radon and is normally combined with SSD.
We use the BEIR VI representative slope of 0.18% lifetime risk per pCi/L, applied linearly with no threshold (LNT model). A house at 4 pCi/L gives roughly 0.72% (1 in 138) added lifetime lung-cancer probability for a 70-year occupant. WHO and US EPA rank radon as the second leading cause of lung cancer after smoking, killing about 21,000 people per year in the US and 84,000 worldwide. Smokers face 9 to 25 times the radon risk of never-smokers.

Real-World Applications

Radon testing in real-estate transactions: In US granite-rich states such as Pennsylvania and Iowa, a 90-day alpha-track detector or a 48-hour short-term test is legally recommended before sale. This simulator lets you screen your house ahead of time — entering your soil type, basement geometry and ACH to decide whether an SSD install is likely. More than half of all Iowa homes exceed the EPA limit, and "Radon-Resistant New Construction" (passive SSD piping installed at build) is now widespread.

Net-zero and Passive House design: High-performance envelopes typically run at ACH 0.5 or lower, sapping natural dilution. Sliding the ACH input down here reproduces the rapid concentration rise seen in tight new builds. The Passive House Institute (PHI) recommends combining mechanical ventilation (HRV/ERV) with a passive sub-slab vent stack (PSV) by default. Japan's 2030 ZEH mandate is starting to include radon-resistant construction in the discussion.

Underground spaces — subway, basements, spa: Subway stations, underpasses and spa facilities continuously monitor radon against the working-environment limit of 200 Bq/m³ and public limit of 100 Bq/m³, following ANSI/AARST B-2014 and ISO 11665. The steady-state solution in this tool is a useful first-order check against those limits. Note that Japanese hot-spring law regulates only the spring-mouth concentration, not the accumulated indoor air, so the two should not be confused.

CAE coupling — CFD radon diffusion: The steady-state CMC (Continuous Mass Concentration) solution here gives a volume-averaged value. In detailed design, OpenFOAM or ANSYS Fluent runs radon as a passive scalar to visualise local concentrations and ventilation non-uniformity. If the volume-averaged CFD result differs from this tool by an order of magnitude, it is a sanity check pointing to a wrong source term or boundary condition.

Common Misconceptions and Pitfalls

The first pitfall is the assumption that "a tight building keeps radon out". Envelope sealing does cut radon influx by about 30%, but it also reduces ACH, so the indoor concentration often goes up rather than down. We use a residual of 70% for sealing alone for this reason. In cold-climate homes where winter ACH falls to 0.1, a house at 2 pCi/L in summer can jump to 8 pCi/L in winter. Sealing should always be paired with SSD or mechanical ventilation (HRV) — this is also the standard pairing in the EPA I-BEAM (Indoor Air Quality Building Education and Assessment Model).

The second trap is relying on a short-term test alone. Radon concentration varies 2-3× with barometric pressure, rainfall, temperature and season. The 48-hour short-term tests used in US real-estate are quick but risk over- or under-estimating the annual average. EPA and AARST strongly recommend a 90-day alpha-track detector or a long-term Continuous Radon Monitor (Airthings View Plus, Corentium Pro, RadonEye, Pro Lab Inc. and similar) for the final decision. Treat the steady-state value from this tool as an "annual-average estimate" and always confirm with long-term measurement.

Finally, "radon is only a spa-town problem" is a dangerous misconception. Granite belts (Iowa, Pennsylvania, the Colorado Plateau, Cornwall, Bohemia, Japan's Abukuma and parts of the lower Kanto loam) are indeed the highest-risk areas, but sedimentary and alluvial soils can also exceed the EPA limit depending on basement geometry, foundation cracks and groundwater. Marie Curie's old laboratory site in Paris (Joly Curie) has documented extreme localised concentrations. Do not assume your house is fine without testing — EPA strongly recommends starting with an inexpensive $15-30 test kit. The right use of this simulator is sensitivity analysis before testing and comparison of mitigation strategies after a result comes back high.

How to Use

  1. Enter the radon source concentration from soil gas (Bq/m³), typically 20,000–100,000 Bq/m³ for granite-bearing regions.
  2. Input your basement or building volume in cubic meters; a standard 200 m² basement with 2.5 m ceiling height equals 500 m³.
  3. Set the air change rate (ACH) between 0.3–1.0 for naturally ventilated basements, or 0.1–0.3 for sealed foundations; higher values reduce indoor accumulation.
  4. Review the indoor radon concentration in both Bq/m³ and pCi/L; EPA action level is 4 pCi/L (148 Bq/m³).
  5. Check the required mitigation reduction percentage and lifetime lung cancer risk based on BEIR VII exposure model.

Worked Example

A 250 m³ basement in Pennsylvania granite terrain with soil-gas radon of 50,000 Bq/m³ and natural ACH of 0.5 yields indoor radon of approximately 8.3 pCi/L (298 Bq/m³), exceeding EPA's 4 pCi/L threshold. Installing a sub-slab depressurization system (reducing soil entry by 80%) and increasing ACH to 0.8 via heat-recovery ventilation drops indoor radon to 2.1 pCi/L (76 Bq/m³), eliminating excess risk and reducing lifetime lung cancer incidence from 1.2% to 0.3% for non-smokers.

Practical Notes

  1. Granite and limestone aquifer zones in the US (Northeast, Upper Midwest) show radon potential index >4; sedimentary formations typically <2. Measure actual soil-gas with radon probes before design.
  2. Sub-slab depressurization and sealing cracks reduce entry; increasing outdoor air exchange is costlier but ensures long-term compliance in retrofit scenarios.
  3. Radon decay products (Po-218, Po-214) attach to lung tissue; BEIR VII models assume 25 years residential exposure. Smokers face 10× greater lung cancer risk at elevated concentrations.
  4. Seasonal variation ±30% due to stack effect; measure for 48 hours minimum in non-heating months for accuracy.