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Radon gas may be more common in areas with radium-rich soils, but any home in any state may have a radon problem. High radon levels have been found in all states in new and old homes, well-sealed and drafty homes, and homes with or without basements.
Radon gas gets into any type of building - homes, offices, or schools and can build up to high levels. But you and your family receive the greatest radiation dose at home. That's where you spend most of your time - typically 75% or more for small children. Over the years, the radon exposure accumulated at home may exceed the cumulative radiation exposure of some uranium miners.
Radon is produced by the natural breakdown of radioactive heavy metals dispersed throughout the earth's crust. Radioactive decay makes life on earth possible - it heats the earth's core. As the atoms of radioactive heavy metals disintegrate, they change into lighter heavy metals until they end up as stable, non-radioactive lead. At each step of the radioactive decay chain, the nuclei emit particles or busts of energy - radiation.
The decay of Uranium-238, the most common radioactive heavy metal, will never end. Its radioactive half-life, when a half of its atoms decays, is 4.5 billion years. And, the half-life of Thorium-232, also very common in rocks and soils, is 14.1 billion years. Their radioactive decay produces radon isotopes: Radon-222 (the most common in homes) from Uranium-238 and Radon-220 (Thoron) from Thorium-232. The humankind is assured of a never-ending supply of radon.
Radon is an aberration - the only gas in the decay chain of heavy metals and the heaviest gas in nature. A daughter product of radium, radon emits alpha particles as it begets the decay chain of radioactive heavy metals polonium, bismuth and lead. They emit alpha and beta particles and gamma rays during their decay. After 22 years, a half of Radon-222 atoms ends up as Lead-206, the final stable element.
Dig up the top 6 feet of an acre of land and you will find, on average, 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. The flux of radon gas into ambient air varies widely throughout the U.S., but on average one square foot of soil emits 140 pCi of radon each hour.
When underground radium disintegrates 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 is also rich in methane and moisture. In most areas in the United States the soil gas contains between 200 and 2,000 pCi of radon per liter. The gas emitted by radon mitigation stacks typically contains 1,000 pCi/L of radon.
Radon gas moves through invisible underground fissures, but usually it decays into solid particles after several feet. But it travels much farther in dry, permeable soils, like gravel or course sand. Radon is also soluble in water, and underground streams can carry it long distances. The underground movement of radon gas explains why high radon levels are also found in homes in low-radium areas, and why radon levels can vary several-fold between adjacent houses.
Radon gas is drawn into buildings from the soil surrounding the foundation. The infiltration of radon-rich soil gas (200-2,000 pCi/L) ranges from 1% to 20% of the total air infiltration into homes. Radon from soil is the main cause of the radon public health problem.
If you have a well, radon may be present in the water and released into the air in your home when water is used for showering, washing dishes and other household uses. Inhalation of radon released from water is much more dangerous than its ingestion. As a rule of thumb, each 10,000 pCi/L of radon in water increases its level in indoor air by 1 pCi/L. Building materials like rocks, bricks, and concrete also give off radon, but they rarely cause radon problems by themselves.
The indoor radon level depends on the difference between the exhalation rate of air from the building and the infiltration rate of radon-rich soil gas (average 1,000 pCi/L) through the foundation. A "leaky" building envelope reduces the indoor concentration, if the inflow of radon gas cannot keep up, but the energy losses in heated or cooled air would be very high. Opening the windows may be counter-productive, because it can increase the suction of radon gas from the ground and the radon level indoors.
Conversely, an almost airtight building will have a high radon level, if the basement or slab is relatively "leaky" to radon gas. In some energy-efficient homes up to 20% of the total air infiltration is soil gas. In order to avoid the unhealthy soil gas and radon, the basement or foundation slab should be always sealed tighter than the building envelope.
In the nature, radon gas, being nine times heavier than air, diffuses only sluggishly from the ground and dissipates in the air. However, air pressure inside buildings is lower than the pressure in the ground. Buildings, just like a vacuum, pull in highly concentrated radon gas from several feet under the ground.
The warm air inside buildings moves upward like inside a stack. This "stack effect" further reduces the indoor air pressure at lower floors. Moreover, large combustion appliances, like furnaces, hot water heaters and fireplaces, as well as exhaust fans and vents, draw air from inside the house and also increase the pressure differential.
The ground around the house may be frozen or soaked by rain, which "bottles up" radon in the ground. But the gravel and disturbed ground underneath the house remain warm and permeable, and attracts radon gas from the surrounding soil.
The heavy radon gas accumulates in basements and on lower floors. The flow of air and diffusion of radon atoms through floors and walls then distribute it through the house. This, in turn, draws more make-up radon gas from the ground until equilibrium at a higher radon level is established throughout the house. Modern air-tight houses tend to build up radon concentrations indoors, if the foundation is more "leaky" to soil gas than the building envelope to air infiltration.
There are three basic entry routes from the ground into your home:
Openings and cracks provide a pathway to radon, but can be usually sealed off or caulked:
Concrete is naturally very porous and cannot block radon gas, which collects in the disturbed ground or aggregate around the foundation. Radon is a single atom gas, unlike oxygen O2 or nitrogen N2, and its atoms are extremely small; their diameter is only 0.268 nm (millionths of a millimeter). Radon easily penetrates through the pores in concrete, as well as mortar, paint, linoleum, tarpaper, insulation, drywall, wood paneling, wallpaper, and many plastics, and through floors and walls inside the house.
Radon is readily soluble in water and "takes a ride" indoors together with moisture. Concrete, just like a wick, draws in water with dissolved radon from the surrounding ground by the capillary action of its pores. Once inside, radon gas is released due to the lower pressure and higher temperatures indoors, and the water usually evaporates. The basement may appear dry due to good ventilation, but moisture carrying dissolved radon is still getting in. The average basement absorbs 18 gallons of water each day.
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