How Radon Gas Gets into Homes

Radon Gas is Found in Homes all Over the U.S.

Radon ElementRadon is an invisible and odorless radioactive gas. Elevated levels of radon have been found in homes all across the country. Any home in any state may have a radon problem: new and old homes, well-sealed and drafty homes, and homes with or without basements. Radon gas gets into all types of buildings, including office buildings and schools.

You and your family receive the greatest radiation dose in your home. That's where you spend most time - 70 to 75 percent, more for small children. The average person receives each year more radiation from radon than from all other natural or man-made sources combined. Over the years, the accumulated radiation exposure may exceed the exposure of uranium miners.

The Never-Ending Supply of Radon Gas

Radon gas is produced during the natural disintegration of radioactive heavy metals uranium and thorium, which are dispersed throughout the Earth's crust. As the atoms of radioactive heavy metals disintegrate, they change into lighter and lighter radioactive heavy metals until they end up as stable, non-radioactive lead. But at each step of this radioactive decay chain the atom nuclei emitradiation - alpha and beta particles, or gamma rays (more energetic than x-rays).

EPA Radon Map GuideRadon-222 – We will never run out of uranium or thorium. Radon levels on Earth have not changed since the last Ice Age. The radioactive half-life of Uranium-238, when a half of its atoms disintegrates, is 4.5 billion years. The half-life of Thorium-232 is 14.1 billion years. Their decay chain continuously produces radium, which decays into radon isotopes Radon-222 (most common in homes) and Radon-220 (Thoron).

Radioactive decay makes life on Earth possible by heating its core. Japanese scientists have measured the antineutrinos produced by the decay of uranium and thorium deep inside the Earth. The decay generates about 19 million megawatts of heat, about half of all the heat generated inside the planet. The other half comes from gravitational and chemical sources. (New York Times, 7/28/05)

Heavier than Air – Radon is the heaviest known gas, nine times heavier than air. It is an aberration – the only gas in the long decay chain of heavy metal elements. Its disintegration begets a decay chain of radioactive heavy metals polonium, bismuth, and lead. After 22 years, a half of Radon-222 atoms ends up as Lead-206, a stable non-radioactive element.

Radon Exists in the Soil Gas

Dig up the top 6 feet of an acre of land and you will find about 50 pounds of uranium. Uranium and its daughter products radium and radon are found in nearly all rocks and soils. Most contain only 1 to 3 parts per million (ppm) of uranium but some, like granites, dark shales, light-colored volcanic rocks, and sedimentary rocks with phosphate, may contain as much as 100 ppm. Thorium, which is even more common, also produces radium.

When radium atoms disintegrate into alpha particles and atoms of radon, 10 to 50 percent of the radon atoms escape from the mineral grain into the underground "soil gas," which also carries biological decay gases and moisture. In most areas of the United States, the soil gas contains between 200 and 2,000 pCi of radon per liter. As radon slowly diffuses from the ground into the ambient air, its flux varies widely but, typically, one square foot of soil emits 130 pCi of radon each hour (0.4 pCi/m2s). In the United States, the resulting outdoor radon level averages 0.45 pCi/L. But the stacks of radon mitigation systems emit undiluted soil gas with radon concentrations 2,000 times higher, averaging over 1,000 pCi/L.

As radon gas moves through underground fissures, it usually decays into solid particles after several feet. But it travels much farther in dry, permeable soils, like gravel or course sand. Radon is soluble in water and underground streams can carry it long distances. This unpredictable underground movement of radon gas explains why homes in low-radium areas also have high radon levels and why radon levels can vary several-fold between adjacent houses.

Sources of Indoor Radon-222

Soil – Radon from soil is by far the main source of indoor radon! Radon gas naturally moves into the permeable disturbed soil and gravel bed surrounding foundations and then, into the buildings through openings and pores in concrete.

Private Water Wells – If you have a private water well, radon from the water gets released into the air in your home during showering and when washing dishes or laundry. Inhalation of the radon gas released into air is much more dangerous than its ingestion with drinking water. As a rule of thumb, each 10,000 pCi/L of radon in water increases its level in indoor air by 1 pCi/L. In the U.S., the average radon concentration in surface water is 10 pCi/L but 750 pC/L in well water. However, levels exceeding 20,000 pCi/L are not uncommon. To check the level of dissolved radon, you can purchase our Radon-in-Water Test Kit.

Rock, Brick, and Concrete – Building materials like rocks, bricks, and concrete also give off radon by emanation, including some marble countertops although the recent publicity is over-blown. They produce Radon-222 and Thoron (Radon-220) but only 0.1 - 0.3 pCi/L each in a typical basement. The very short half-life of Thoron (96 seconds) reduces its typical concentration in homes to 0.3 pCi/L.

Water Migration – Radon is soluble in water and also gets indoors by "water migration." Water is drawn indoors by the capillary action of the pores in concrete or pushed by hydrostatic pressure (seepage). The higher temperature and lower pressure indoors release the dissolved gas. Water in sump pits and floor drains also releases radon. The typical basement or floor slab lets in 15 or 10 gallons of water and vapor each day.

Outside Air – Radon from outside air also settles in basements of homes because it is nine times heavier than air. Radon released by tailings from uranium mines was found to travel hundreds of miles and settle in homes. But more commonly, radon gas released from soil is drawn into basements over the top of the foundation, through bulkhead doors, or uncaulked basement windows above window wells. It can be even drawn into the attic underneath uncaulked siding and then sink into the house.

Porosity of Concreteporosity of concrete

Concrete cures by reacting with water - hydration. But almost half of the water added to the concrete mix for workability is surplus and has to evaporate. As the surplus water in newly poured concrete pushes to the surface, it leaves behind a network of capillaries (pores).

The pores constitute 12 to 18% of the concrete by volume. Their diameter is much smaller than a human hair but the pores are much larger than radon atoms or water molecules. They let in radon gas, water vapor, liquid water, and other gases.

The RadonSeal® Solution

RadonSeal® Deep-Penetrating Concrete Sealer reacts with cement and seals the capillaries (pores) against radon atoms, as well as the larger water molecules. Instead of removing radon from underneath the slab by a fan, it simply seals the concrete and leaves the gas in the ground. Sealing the pores greatly reduces radon diffusion and advection through the slab and basement walls. At the same time, it blocks water seepage and reduces water vapor transmission.

Compared to fan mitigation systems, RadonSeal does not depend on the permeability of soil, reliability of mechanical equipment, or the power grid. It does not spew a plume of heavy, radioactive gas above the house. And if a sub-slab depressurization system is already installed, RadonSeal will lower its energy losses (in treated air drawn through the slab) and will reduce the radon level further. RadonSeal never has to be applied again because it cures as a mineral and seals the concrete permanently.

Typical Openings in Basements

Radon, soil gasses, and water vapor will easily pass through any openings, cracks, gaps, drains, or thin concrete (rat slabs) in the basement. In combination to sealing the concrete all openings have to be sealed off or caulked to stop the transmission of radon, as well as soil gas and water vapor. Common openings in basements:

  • SUMP PITS – Make sure sump pit covers are sealed air tight. EPA recommends caulking around the perimter of the cover.
  • FLOOR DRAINS – If a floor drain is not in use, cover or use a floor drain plug.
  • CRAWLSPACES – Seal the concrete, fill voids, and repair cracks in attached crawlspaces.
  • BATHROOM ROUGH-INS – Gravel areas, or thin concrete.
  • FLOOR-TO-WALL JOINTS – Seal gaps in floor-to wall joints (around floating slabs).
  • CONCRETE BLOCK FOUNDATIONS – Hollow, uncapped concrete blocks. Use closed-cell expansion foam to fill the top.
  • CRACKS – Repair cracks in floors or walls.
  • EXPANSION CONTROL JOINTS – Fill saw cuts, or control joints in floors.
  • SUPPORT COLUMNS – Seal around hollow lolly (support) columns going through the floor.
  • PIPE PROTRUSIONS – Seal around gaps around pipe or conduit penetrations.

DIY Concrete Foundation Crack Repair Kits – For sealing cracks in poured concrete walls, we recommend polyurethane foam injection. Polyurethane remains flexible for years, as the house settles and concrete continuously moves. Hydraulic cement is too rigid and will soon get loose.

CrackWeld Concrete Floor Repair Kit – For sealing cracks in slabs - route them out with a grinder and deep-fill with self-leveling polyurethane caulk available in stores. Or avoid the grinding and repair them permanently with our CrackWeld Floor Repair Kits. Caulking the surface only is inadequate - the gap fills up with radon gas which then easily by-passes the caulk through concrete. Surface caulking will soon detach as moisture degrades its bond to the concrete surface.

Accumulation of Radon Inside Homes

Modern houses tend to build up radon, because the building envelope is almost airtight while the foundation is "leaky" to soil gas. The soil gas infiltration from basements ranges from less than 1% to over 20% of the total "fresh air" infiltration into homes (typically 3-5%). Opening basement windows to increase ventilation may help by removing the radon gas but it wastes energy and in some cases, can actually increase the radon level by increasing suction through the porous concrete. The best solution is sealing the concrete to ensure the foundation is more airtight than the building envelope.

The heavy radon gas accumulates in basements and on lower floors. Heating and air-conditioning, natural air movement, the "chimney effect," as well as diffusion of radon atoms through the floors and walls distribute radon throughout the house. This, in turn, draws in more radon from the ground by concentration-driven diffusion, until an equilibrium radon level is established on each floor in the house.

According to the Iowa Residential Radon - Lung Cancer Study (2000), the median first floor radon concentration in one-story homes is, on average, 60% of the basement level. But for two-story homes, the median radon levels at first and second floors are 51% and 62% of the basement level, respectively. However, the radon levels very greatly and there have been cases of radon concentrations being the highest at the top floor of multistory buildings.

How Buildings Draw Radon from the Ground

The "Stack Effect" – Soil gas carrying radon moves to the permeable gravel and disturbed ground around the house, particularly when "bottled up" by frozen or rain-soaked ground. The air pressure inside homes is slightly lower than in the ground (typically 0.7-1.4 psi vacuum), which draws in radon gas from several feet away. The warm air inside buildings moves naturally upwards and this "stack effect" reduces the air pressure on lower floors. Strong winds create a vacuum in the downwind side of the building by the Bernoulli effect.

Home furnace radonCombustion Appliances – Furnaces, hot water heaters, fireplaces, exhaust fans, and vents reduce the indoor pressure further and draw in more radon gas. A typical clothes dryer may exhaust 100 cfm of air and a kitchen fan 750 cfm. This may de-pressurize the house so much, say below 5 Pa, that it will reverse chimney flow and cause combustion spillage from appliances, particularly in tight homes.

The resulting pressure-driven flow (advection) draws in radon gas through openings or cracks and through the pores in concrete. This pressure-driven infiltration of radon is only a part of the radon inflow or radon would be easy to mitigate by just sealing all openings or simply pressurizing the basement.

Radon is also pulled in by the difference in radon concentration indoors and in the soil (diffusion). Radon tries to equalize the indoor concentration and its atoms easily penetrate through the pores in concrete. The diffusion flow through an intact concrete slab driven by the concentration gradient is several times higher than the pressure-driven advective flow. This explains why the method of pressurizing the basement does not work.