Radon in Buildings

The National Research Council concludes that "bulk flow of soil gas that contains radon is the main mechanism of radon entry into buildings," driven by pressure differences between indoor air and adjacent soil. The WHO handbook calls pressure-driven airflow "the most important radon transport mechanism."

Diffusion through intact concrete is relatively weak. EPA training estimates that a typical slab cuts radon flux from bare earth by a factor of 25–50 when diffusion is the only mechanism. Even a modest flow of soil gas through cracks — about 1/3 to 2/3 cubic foot per hour at 1,000 pCi/L — equals the entire diffusive contribution from a 1,600 ft² slab.

Entry rate scales with indoor–outdoor pressure difference at low soil-gas flows, rising roughly proportionally to ΔP. At very high flows, the relationship becomes ΔP^(2/3) because the soil reservoir near the building becomes depleted of radon. Diffusion matters primarily in buildings with minimal depressurization or bare dirt-floor crawl spaces.

When indoor air is warmer than outdoor air, the lighter indoor column creates a pressure gradient: upper floors are pressurized and lower floors are depressurized relative to outdoors. The pressure difference is approximately 0.04 Pa per meter of height per kelvin of temperature difference.

For a basement 4 meters below the neutral pressure plane: at 20°C indoor vs 0°C outdoor (ΔT = 20 K), depressurization is about 3.2 Pa. At −20°C outdoor (ΔT = 40 K), it reaches about 6.4 Pa. In summer with air conditioning, the pattern can reverse, creating slight pressurization that suppresses radon entry.

The neutral pressure plane (NPP) shifts toward the largest air leaks. Sealing attic bypasses lowers the NPP and reduces basement suction. EPA notes the NPP often sits "within a few inches of the top floor ceiling" in leaky-ceiling houses, placing nearly the entire house under negative pressure and maximizing basement depressurization.

The floor-wall perimeter junction — where slab, footing, and wall meet — is the dominant entry pathway in most models. LBL modeling uses a 3 mm wide L-shaped gap at this junction. At that width, gap resistance is negligible and soil/fill permeability controls flow. Gravel under the slab can increase entry through this gap by up to a factor of 5.

Cracks and joints bypass the low permeability of intact concrete. Laboratory tests (Nielson et al. 1997) found negligible advection through intact concrete (permeability ~10⁻¹⁶ m²) but significant flow through cracks, disturbed penetrations, cold joints, and masonry blocks. The smallest crack area shown to measurably increase radon advection was about 10⁻⁷ m².

Service penetrations (water, sewer, gas, electrical) leave annular gaps through slabs and walls. The utility trench backfill outside is often loose sand or gravel, creating preferential flow paths "through seemingly impermeable soils." Sumps and floor drains connected to gravel drainage or perimeter drain pipes provide low-resistance conduits directly to high-permeability soil.

Full basements have the largest below-grade surface area and are furthest below the neutral plane, resulting in high entry risk. Construction typically creates a permeable backfill zone around the foundation. Hollow-block basement walls are particularly problematic: interconnected cores act as a depressurized plenum, and open top courses provide direct pathways.

Slab-on-grade foundations have numerous service penetrations, and the slab-wall perimeter joint is "especially difficult to seal in finished homes." Because water infiltration is less concerning above grade, "little attention is paid to sealing openings to the soil."

Crawl spaces with dirt floors and block walls facilitate radon entry, especially when unventilated. Sealing or encapsulating without active suction removes dilution and can increase radon. Combination foundations (basement adjacent to slab-on-grade) are "often associated" with elevated radon due to multiple interacting zones.

Basements typically have the highest radon concentrations due to four combined factors: proximity to the soil source with largest contact area, strongest depressurization (furthest below neutral plane), lowest natural air exchange (fewer windows, less opening), and dilution as radon diffuses upward into occupied spaces.

US data (BEIR VI): first-floor concentrations averaged about 40% of basement levels, making basements roughly 2.5× higher. Second floors were about 90% of first floors. UK data: first-floor levels were about 69% of ground-floor levels. Italian hospital data: basement-to-ground-floor ratio of 1.35. A reasonable general range is 1.3–2.5× higher in basements vs ground floor.

Exhaust devices depressurize buildings: bathroom fans (24–90 cfm), clothes dryers (100 cfm), range hoods (24–225 cfm), fireplaces (170 cfm). Combustion appliances consume indoor air. Leaky return ducts in basements create additional suction.

The relationship between ventilation and radon is non-monotonic. Finnish apartment studies found that increased mechanical exhaust raised indoor–outdoor pressure differences, which "increased the entry rate of radon and counteracted the diluting effect," becoming significant above about 5 Pa. Canadian test houses showed radon concentrations higher at moderate exhaust (−5 Pa) than at strong exhaust (−10 to −20 Pa) because very strong exhaust also brings proportionally more dilution air.

UK observational study of 470,689 homes: double glazing raised geometric mean radon by 67% (95% CI: 44–89%) compared to non-retrofitted homes. Loft insulation showed +47%, wall insulation +32%. German data: mean and median radon "nearly doubled" in energy-refurbished buildings vs unrefurbished.

Irish modeling predicted increases up to 107% after retrofits, driven primarily by reduced air permeability. However, passive houses with balanced mechanical ventilation showed no significant radon increase. The pattern: tightening the upper envelope while adding balanced (not exhaust-only) ventilation tends to be neutral or beneficial for radon. Tightening windows and doors while keeping exhaust-only ventilation and an unsealed foundation tends to raise radon significantly.

Indoor radon is the product of factors that multiply rather than add. Every link in the chain varies: soil-gas concentration can vary by a factor of 250 within 30 feet due to clay lenses, sand pockets, fractured zones, and backfilled utility trenches. Coupling at the soil-building interface varies with sub-slab gravel (up to ~5× multiplier), loose backfill (up to ~7.5× for wall cracks), and openings.

Driving pressure varies with building height, NPP location, attic air leaks, HVAC configuration, and wind exposure. Air-change rates range from below 0.1 to over 1.0 per hour. Internal distribution depends on where occupants spend time and whether shafts or ducts carry radon upward.

The WHO quantifies the coupling range: most houses draw less than 1% of their air from soil, but leaky-foundation houses on permeable ground can draw more than 10%. Soil radon potential sets the maximum possible indoor level; the building determines how close it gets to that ceiling. This is why EPA measurements show that 250 buildings across three studies "showed no correlation between tightness of house and elevated radon concentrations" — source strength differences dominate.