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-The original source of radon-222 is from the natural radioactive decay of uranium-238 and radium-226 (uraniumradiumradon), which are commonly occurring elements in the crust of the earth.
-Certain soils and rocks that contain high levels of uranium also store natural deposits of radon:
-Radon is continually being formed in soil and released to air as a result of the extended half-lives of uranium and radium and their abundance in the earths surface. Atmospheric radon is not an issue of health concern because the radon is rapidly diluted to low levels by circulation throughout outdoor air.
-Sections 307 and 309 of the Indoor Radon Abatement Act of 1988 (IRAA) directed EPA to list and identify areas of the U.S. with the potential for elevated indoor radon levels. EPA's Map of Radon Zones assigns each of the 3,141 counties in the U.S. to one of three zones based on radon potential:
-Zone 1 counties have a predicted average indoor radon screening level greater than 4 pCi/L (picocurries per liter) (red zones)
-Zone 2 counties have a predicted average indoor radon screening level between 2 and 4 pCi/L (orange zones)
-Zone 3 counties have a predicted average indoor radon screening level less than 2 pCi/L (yellow zones)
There are no sinks for radon, and it is estimated that only negligible quantities escape to the stratosphere. As a result, the ultimate and sole fate of radon-222 is transformation or degradation through radioactive decay. Radon decays only through normal radioactive processes, meaning, an atom of radon emits an alpha particle resulting in an atom of polonium-218, which in turn, also undergoes alpha particle emission to produce other radon progeny.
-The radon concentration in the soil is a function of:
a.) the radium concentration
b.) the soil moisture content
c.) the soil particle size
d.) the rate of exchange of soil-entrapped air pockets with the atmosphere
*the normal soil-gas radon measurements are in the range of 270-675 pCi radon-222/L of air.
Transport Throughout Soil
-Emanation is the process by which radon is transported from a solid to a gas or liquid medium. After radon is produced at the soil particulate level from the radioactive decay of radium, it is released into small air or water containing pores between soil and rock particles.
-This transportation of radon throughout soil is primarily accomplished through alpha recoil and the mechanical flow of air and water throughout the soil.
-Alpha recoil is defined as the process by which an atom (radon) recoils in the opposite direction from the path of particle ejection following the radioactive decay of its parent atom.
-Transportation throughout soil and within these pores is also somewhat facilitated by diffusion and convection. The diffusion constants for radon in air and water, 10-2cm2/sec and 10-5cm2/sec, respectively, indicate that diffusion of radon is a relatively slow process and the movement of radon is therefore not significantly effected by this mechanism.
-After radon is released into the pore spaces, the efficiency of its eventual release into ambient air, termed exhalation, is a function of:
a.) the soil porosity
b.) the concentration of radon in the soil/gas pore
c.) meteorological factors, including, precipitation and atmospheric pressure.
-Following the release into ambient air, the dispersion of radon is primarily determined by atmospheric stability, including vertical temperature gradients and direction of winds force, and turbulence.
Presence and Transportation Through Water
-Groundwater that is in contact with radium-containing rock and soil will be a receptor of radon emanating from the surroundings.
-In groundwater, radon transportation is determined primarily by:
a.) diffusion patterns
b.) the direction of the waters mechanical flow.
-The solubility of radon in water is relatively low, and with its short radioactive half-life, much of it will decay before it has the opportunity of release from the groundwater.
-When radon-containing groundwater reaches the surface by natural or man-made forces, the radon will inevitably be outgassed into the atmosphere.
-Although the majority of radon present in groundwater will decay prior to its arrival at the surface, groundwater is nevertheless considered the second most prominent source of environmental radon and has been estimated to contribute approximately 5x108 Ci radon-222 per year to the atmosphere.
-Radon is also minimally released from water located at or slightly beneath the oceans surface.
-Radon concentrations in surface water supplies are usually relatively low. Since municipal water supplies are typically aerated, this results in diminished radon levels.
-Rural household wells may have potentially high levels of radon contamination. Deep aquifers have highly variable radon levels. Levels depend on:
a.) uranium content of the rock
b.) distribution of the aquifer relative to the rock
c.) groundwater flow patterns.
*areas with granite-based aquifers may have highly variable levels.
Anthropogenic and Miscellaneous Sources
-Tailings from uranium mines and residues from phosphate mines both contribute to global radon in estimated amounts of 2 to 3x106 Ci radon-222 per year. Despite the fact that these sites are rather rare, emanation rates to air may potentially be substantial. Former research has estimated that 20% of the radon formed in tailings is released and that emanation rates can be as high as 1,000 pCi radon/m2/second.
-Coal residues, such as fly ash, combustion products, and natural gas also contribute to the atmospheric radon levels, however, only in negligible quantities.
Transportation Into and Throughout Indoor Environments
-Under certain circumstances the concentration of radon in a building can be increased significantly over its normal outdoor level.
-Most buildings have a confined air space with limited air movement and only a slow exchange with outside air. Consequently the concentration of any particulates or gases released into the building atmosphere will tend to increase above the concentration normally found in outside air.
-Radon can enter a building in a number of ways and once inside, the concentration of its particulate progeny will increase as the radon decays. Thus, high concentrations of radon located in soil gas within soils with high transport efficiency (i.e. loose, porous, dry soil) can lead to elevated radon concentrations in buildings.
-Because radon-222 is a gas, it can readily travel vertically throughout several meters of permeable soils before decaying. The major sources of radon-222 in indoor air are
a.) soil gas emanations from soils and rocks
b.) off-gassing of waterborne radon-222 into indoor air (initial entry through utilities such as water or natural gas)
c.) building materials
d.) outdoor air.
-Radon entry via the most important path, radon released soil-gas emanations, occurs primarily by bulk flow of soil-gas driven by small pressure differences between the lower part of the house interior and the outdoors.
-The pressure variability is primarily due to differences in indoor versus outdoor temperature and the effects of wind.
-The radon gas percolates up through porous soils under the home or building and enters through gaps and cracks in the foundation or insulation and through transport by means of pipes, sumps, drains, walls or other openings.
-The primary limiting factors for radon-222 gas migration out of the soil and its subsequent accumulation in a dwelling are:
a.) its half-life of 3.8 days
b.) the geochemical composition and geophysical properties of the soil under and around the house
c.) the nature and type of the building foundation
d.) the construction techniques implemented
e.) the level above the ground
f.) the dimensions of the rooms
g.) the ventilation rate
h.) meteorological and seasonal parameters
i.) living and working conditions
-Turbulent or heated water (flowing in wash basins, showers, washing machines, flush toilets, etc.) that has absorbed radon gas is also a source of elevated radon levels in the home, as these activities liberate dissolved radon into the home atmosphere. The amount released depends on the radon content of the water (which varies widely between regions) and the amount used (70 to 250 gallons in a typical household per day). On the average, 70% of radon contained in household water is released into indoor air
-Transport of radon indoors is primarily a function of:
a.) the rate of attachment of radon daughters to particles
b.) the concentration and size of particles
c.) the rate of deposition
d.) the ventilation rate within the enclosure (typically, radon concentration and ventilation rates display an inverse relationship)
-A major complication of modeling both radon and radon daughter transport indoors is that the ventilation rate acts both to increase flow of radon into the enclosure and to remove radon atoms and radon daughters from the enclosure. Ventilation rates will also generate variability in the radon concentrations between different areas of the enclosure.
A. Cracks in concrete slabs
B. Spaces behind brick veneer walls that rest on uncapped hollow-brick foundation
C. Pores and cracks in concrete blocks
D. Floor-wall joints
E. Exposed soil, as in a sump
F. Weeping (drain) tile, if drained to open sump
G. Mortar joints
H. Loose fitting pipe penetrations
I. Open tops of block walls
J. Building materials such as some rocks
K. Water (from some wells)
1.) Although radon is chemically inert and electrically uncharged, the radon progeny formed via the radioactive decay series are in fact electrically charged and readily attach themselves to microscopic dust particles present throughout an indoor environment.
2.) These dust particles are frequently inhaled into the lungs or ingested (from drinking water) into the gastrointestinal tract.
3.) The inhaled particles immediately attach to the lung alveoli while the ingested radon progeny are absorbed into the bloodstream and ultimately transported to the lungs.
4.) The deposited progeny readily undergo subsequent radioactive decay processes by emitting alpha radiation which slowly penetrates the inner lung surface, disrupts DNA structure within lung cells, and can potentially induce lung cancer.