Radon and its Progeny

What is radon?
Radon is a dense, noble gas, with an atomic number of 86, meaning that there are 86 protons in the nucleus of a radon atom. Radon is heavier than air, and it is soluble in water. Because it is a noble gas, radon is chemically inert and electrically uncharged. It is a naturally occurring radioactive material and, has been the subject of much controversy in regard to its potential for radiological impact on homes and public buildings.

What are some other characteristics of radon?
Radon comes from the decay of primordial atoms that have been in the ground since the time the earth was formed. There are three isotopes of radon, all of which are naturally-occurring. In addition, all of them emit alpha particles when they decay, and they all have short half-lives. Radon-222 (Rn-222), commonly known as radon, is a member of the uranium series and has a 3.8 day half-life. Rn-220 or thoron, as it is referred to in historical context, is a member of the thorium decay series. It has a much shorter half-life (55.6 seconds). Radon-219 (Rn-219), historically referred to as actinon (An), is a member of the Actinium series and has the shortest half-life of the three isotopes at 4.0 seconds.

Do you mean that when a uranium atom decays, radon is produced?
Not quite. When uranium, thorium or actinium atoms decay, other radioactive atoms are formed. One of these, radium, is the immediate source of the three radon isotopes. The radium isotope of principal interest is Radium-226 (Ra-226) which decays through alpha emission to produce radon gas (Rn-222).

Where is radon found?
It might be simpler to list places where it is not present! Because it is naturally occurring, radon is found just about everywhere, including air, soil, groundwater, granite, pumice, clay, brick, and other construction materials such as concrete made from fly ash and industrial slag.

What potential hazards from radon gas are present from a radiological standpoint?
Several potential hazards have been documented. The fact that radon gas is present in soil, building materials, even though it is typically in low concentrations, means that humans present there can be exposed to the radiations emitted from its decay. Because radon emits alpha particles, the only exposure pathway of interest is inhalation. However, when radon decays, it produces four additional radioactive elements. These are colloquially known as "radon daughters", although the term "radon progeny" is gaining in usage.

How many radon daughters are there and what are they called?
There are four (4) short-lived daughters: Po-218, Pb-214, Bi-214, and Po-214. The Po-218 and the Po-214 are alpha emitters, while the Pb-214 and Bi-214 are beta/gamma emitters. Pb-210, which falls at the end of this mini-decay series, is relatively long-lived (22 year half-life). As such, it is not normally considered to be one of the radon progeny.

What characteristics do the progeny have?
Unlike radon, the progeny are not gaseous, but rather particulate in nature. They attach themselves to dust particles or other particulates that are suspended in the air. Once inhaled, they will reside in the lung (following radon decay) and irradiate lung tissue based on their decay and associated respective half-lives (which range from a fraction of a second to less than 30 minutes).

Is there a group of people that have historically received high exposures to radon gas?
Yes. Perhaps the best-studied group are uranium miners, both here in this country and abroad. In Herman Cember's textbook entitled, Introduction to Health Physics (third edition), he recounts that as early as the 16th century, a large proportion of miners died from lung cancer. In 1924, inhalation of radon gas was postulated to be responsible for this situation.

Where did the radon come from in the mines?
The locations chosen for the mining of various minerals contained high concentrations of uranium and radium in the ground. The radium decayed into radon, and the radon permeated through the soil into the air, where it was trapped in mine shafts.

Why was lung cancer thought to be the effect?
This was because alpha emission is the principal mode of decay for radon and its progeny. The short distances traveled by this form of ionizing radiation do not allow it to reach any other organs. Therefore, it is highly improbable that the inhalation of radon daughters could cause a radiological impact on any other organs than the lung.

Does a link between radon exposure and lung cancer really exist?
Yes, but it is influenced by, among other factors, the study group and the level and duration of exposure. For example, studies of uranium miners in the western United States, who were exposed to elevated concentrations of radon and its progeny, have concluded that several hundred lung cancers (in excess of the anticipated natural incidence) resulted. Statistical examinations of these data have established a causal relationship between radon and radon daughter exposures and the onset of lung cancer.

What about the risk to the general population?
Keep in mind that the lung cancers reported above were associated with very high exposure levels. Joseph Bevelacqua, in his textbook Contemporary Health Physics: Problems and Solutions, appropriately points out that these levels are two to three orders of magnitude higher than those found in typical indoor environments. In addition, relating these findings to exposures and effects in the general population requires consideration of a number of confounding factors.

What do you mean by confounding factors?
Well, one important issue is that many miners were cigarette smokers. Cigarette smoking has also been statistically linked to the appearance of lung cancer. It is also thought that the combined effects of cigarette smoking and radon inhalation, if any, would be greater than either effect taken alone. This is known as a "synergistic" effect. The contribution from this source of exposure must be somehow eliminated to truly judge the influence of radon exposures.

Are there others?
Yes. By the nature of their work, miners were exposed to silica dust, another known carcinogen. For much of the general population, smoking and silica dust inhalation are not really issues. Therefore it can be difficult to relate cancer incidence in uranium miners to the risk of effects in the general population.

How important then are exposures to radon and its daughters to the general population?
Exposures to radon and its progeny constitute a very important component in the field of environmental health physics. The National Council on Radiation Protection and Measurements (NCRP), in its Report No. 93, Ionizing Radiation Exposure of the Population of the United States,stated that exposures to radon and its progeny constitute the most significant source of natural background dose to the general public.

What fraction of the annual natural background dose is received from this component?
Radon and its daughters are estimated to contribute two-thirds of the yearly natural background in the United States. This amounts to approximately 200 millirem (mrem) of the 300 mrem annual total from all sources of natural background.

How does the contribution from inhalation of radon gas compare to that of the daughters?
According to Dan Gollnick, author of the textbook called Basic Radiation Protection Technology, the inhalation of radon gas delivers approximately one (1) percent of the dose to the lung. The remaining 99% of the dose is the result of the radon daughters.

If this is so, why do public health officials concentrate on the gaseous component?
First, it is easier to measure the level of radon gas than it is to measure the particulate daughters. Thus the assumption is made that if you can control the concentration of radon, you will control the level of the radon daughter products.

Is this a reasonable position to take?
Yes. If the concentrations of radon are low, it stands to reason that the concentration of the progeny will also be low.

How does the presence of radon gas interfere with routine radiological measurements?
Since all isotopes of radon are naturally occurring, they are always present and will decay by alpha and beta emission into their respective daughter products. It is their presence that makes it initially difficult to discern the contribution from manmade (artifically produced) radionuclides in measurements of atmospheric radioactivity.

So what can be done?
A background correction could be performed, but this is often not feasible due to the significant daily variation in radon air concentrations. The easiest solution is to introduce a time delay between sample collection and subsequent counting. After a long enough time passes, all of the Rn-222 daughters in the sample will have decayed and will not contribute to the sample count rate. The same technique, though with a longer waiting period, can be employed for Rn-220 daughters. Lastly, there are counting instruments available which can distinguish between manmade and natural contributors by identification of radiation energies.

How are radon concentrations reported?
Radon levels are typically reported in traditional units of picocuries per liter (pCi/l), representing the amount of radioactivity (pCi) in a specified volume of air (liter). Alternatively, if the international system of units (SI System) is used, the concentration would be reported in units of becquerels per cubic meter (Bq/m3).

Is there an easy conversion between the conventional and the SI units?
As a matter of fact there is. One (1) pCi/l is equivalent to 37 Bq/m3.

What units are used when reporting radon daughter concentrations?
The conventional - although some say archaic - unit is the working leve, abreviated "WL". One (1) working level is defined as any combination of the short-lived radon daughters in one (1) liter in air that has the potential to release 1.3E5 (130,000) MeV (millions of electron volts) of alpha energy. Alternatively, one (1) WL is also the amount of alpha energy associated with the short-lived daughters that are in secular equilibrium with 100 pCi of radon.

Run that last one by me again . . .
If the radon is in "secular equilibrium" with its progeny (i.e., the concentration of each daughter is equivalent to the concentration of all other daughters as well as to the radon concentration), then one WL would be equal to 100 pCi/liter. In a practical sense, however, these conditions rarely occur.

Can radon move through soil?
Yes. That is how it gets from the ground to our homes.

How does it move?
The radioactive parent of radon gas, Ra-226, is typically distributed throughout soil. When the Ra- 226 decays, the result is an alpha particle and a Rn-222 atom. These travel off in opposite directions, away from the original point of decay. (This phenomenon is known as a "recoil".) Sometimes the direction of travel is out of the soil and into the atmosphere.

Do all of the Rn-222 atoms released from Ra-226 in soil enter the atmosphere?
Absolutely not. For the radon to reach the air spaces in the soil and the atmosphere before it decays into one of its progeny, it frequently must travel some distance through the soil. Since its energy only permits it to move a very small distance (less than a millionth of a meter), essentially only radon that is formed near the surface of the soil contributes to the atmospheric radon concentration.

What determines the amount of radon that will escape?
The amount of radon escaping, or "emanating", from soil with identical Ra-226 concentrations can be quite different. For example if the Ra-226 is confined, primarily, to the surface of the first soil volume, but distributed throughout the second, there will be different amounts of emanation from them.

Are there other confounding factors?
Yes. Some of the radon atoms that are released from the surface of a particle of soil cross some air space only to embed themselves into another soil particle. They are likely to remain there until they decay. Consequently, they don't escape into the atmosphere.

More?
Yes. If the soil is damp, the recoil atom of radon is slowed and/or trapped in the soil air spaces. The result is reduced emanation in humid soils.

How does the concentration of radon in the soil compare to that in the air?
As a general rule, the concentration of radon in the soil air spaces is approximately one thousand times higher than the concentration found in the atmosphere. Once radon reaches an air space, it diffuses from a high concentration to a low concentration. However, radon is unlikely to diffuse more than one or two meters before undergoing radioactive decay. In most situations, but certainly not all, it is the bulk flow of air through soil that is driven by pressure gradients that is responsible for the movement of radon.

Can you give me a better definition of the term "emanation rate"?
Absolutely. The emanation rate is the rate at which radon escapes the soil to enter the atmosphere.

What unit is the emanation rate expressed in?
The emanation rate is usually expressed in picocuries per square meter per second (pCi/m2s).

That seems like an unusual unit. Can you explain it a bit further?
Yes. Let's look at good ‘ole dirt. Around the United States, the average Ra-226 concentration in soil is one (1) pCi per gram. The average emanation rate is on the order of 0.5 pCi/m2s. This means that over a surface area of one square meter, 0.5 picocuries of radon is being released into the air every second.

Is the emanation rate steady? Or are there factors that increase or decrease it?
A number of factors influence the emanation rate, including the Ra-226 concentration, the chemical form of the radium-bearing soil, soil permeability, soil moisture, atmospheric pressure, wind speed, and technological enhancement (i.e., manmade activities).

But it seems like the emanation rate should be related to the Ra-226 Concentration?
Yes, that's correct. All things being equal, the radon emanation rate is directly related to the Ra-226 concentration in the soil. So, if we find some soil with a Ra-226 concentration of 200 pCi/g, we would expect to see an emanation rate on the order of 100 pCi/m2s. But don't forget the factors that I mentioned previously!

Okay, so then how is the emanation rate related to soil permeability?
The more permeable the soil, the greater the emanation rate. Sandy soil, for example, can be expected to have a higher emanation rate than clay and rock. On the other hand, fissures in the clay or rock, if present, can serve to increase the rate of radon release. As you can see, soil porosity and permeability are not quite the same. Porous soils are usually more permeable, but not always. A good example is clay, which can be highly porous, but have low permeability.

How is the emanation rate related to soil moisture?
The effect of soil moisture and rain can be hard to predict. Remember, a certain amount of moisture can increase radon concentrations in the soil air spaces. At the same time, however, increasing the water content of the soil reduces air flow. The result is a situation where the radon emanation is low for very dry soil, increases with increasing moisture and then, above a soil moisture content of approximately 25%, begins to decrease again. Heavy rains that raise the water table can result in a temporary increase in radon emanation as air is `pushed' out of the soil. Later, after the soil becomes saturated, the emanation rate decreases.

How is the emanation rate related to atmospheric pressure?
A drop in atmospheric pressure that enhances the pressure gradient across the interface between soil and air might produce a slight increase in emanation.

How is the emanation rate related to wind speed?
Increasing the wind speed can significantly increase radon emanation rates as it, in effect, increases the pressure at the soil/air interface.

How is the emanation rate related to technological enhancement?
Human activities that increase the concentration of naturally-occurring radioactivity are referred to as technological enhancements. Examples that involve increased radon emanation include plowing a field and digging a foundation for a building. These actions serve to increase the release of radon, thus increasing the airborne concentration and the human exposure potential.

So, if soil is everywhere, radon is everywhere, right?
Yes.

What are typical outdoor radon and thoron concentrations?
Outdoor thoron and radon concentrations are usually similar, running somewhere between 0.1 and 0.3 pCi/l, at least in the lower 10 meters of the atmosphere. The closer to the source of the radon (the ground) that you measure, the higher the concentration. Therefore, concentrations at one meter above the ground may be approximately 20% higher than those at two meters. In fact, the upper levels of the atmosphere can be almost completely devoid of radon. Any atmospheric mixing that occurs typically reduces the radon concentrations at ground level by simple dilution.

What are typical indoor radon concentrations?
Typical indoor radon concentrations in the basement and first floor of most homes run between 0.5 and 2.5 pCi/l, and 0.3 to 1.5 pCi/l, respectively.

How do indoor and outdoor concentrations compare?
Indoor levels are usually three (3) to 10 times greater than those outdoors since the radon in a home is trapped and, unlike outdoor air, not subject to dilution.

So the higher up I go in my home the lower the radon concentration?
Yes. Concentrations are almost always higher in the basement than in the upper levels of a home. However, there is usually little difference between the concentrations in the upper levels since there is considerable air exchange between floors once you get above the basement level. It was not uncommon in older homes, for example, to circulate air from the basement to the upper floors.

What about my office where I have heavy-duty air conditioning?
A good question. Indoor radon levels in schools, office buildings etc., tend to be lower than those in homes due to the higher ventilation rates found in such buildings. In most cases, the air conditioning and heating systems in larger buildings are required by law to bring in a certain amount of fresh makeup air. However, where budgets are tight, like they are with some school systems, this might not occur. The result is an increase in the indoor radon levels.

So radon in offices is lower than in homes, right?
Well, not necessarily. The concrete foundations in offices are likely to consist of several adjoining slabs, unlike those in homes which are usually poured as a single slab. The cracks between the slabs can serve as an entry point for radon if they are not properly sealed. So you see the actual radon concentration in a building is influenced by a variety of factors, only a few of which are height above the basement floor, heating or air conditioning capability, and the type of floor slab in the building.

What are typical indoor thoron concentrations?
Typical indoor thoron concentrations run from 0.2 to 0.3 pCi/l. Because of its short half-life, the thoron concentration is unaffected by things like ventilation rate. As a result, we see comparable levels in homes, office buildings, schools, etc.

Is the radon concentration in the morning the same as at night?
No. Since the upper atmosphere represents a large pool of aged (radon free) air, anything that promotes atmospheric mixing will reduce the radon concentration near the ground. As a result, radon concentrations are usually highest in the early morning when stable atmospheric conditions prevail and lowest in mid-afternoon. The early morning outdoor maximum is typically two to four times the afternoon low.

What about the inside of a house?
Concentrations also vary over time indoors. In fact, they parallel those seen in the outdoor environment. However, the differences between the highs and lows inside the building are often greater than they are outside, varying by as much as a factor of 20 over the course of a year at a given location.

What about seasonal variations?
Seasonal variations are harder to predict due to the influence of geography, but for a given location they usually follow a similar pattern from one year to the next. In general, the maximum concentrations of radon occur between July and December; and minimum concentrations occur between March and June. The maximum (monthly average) outdoor concentration is typically three times the seasonal minimum. Thus a common assumption that indoor radon levels are always highest in the winter is an incorrect one.

How do radon concentrations differ from city to city, or state to state?
To some degree, the outdoor and indoor concentrations in various geographical areas reflect the local geology, meaning that the higher the radium concentration in the underlying rocks and soil, the higher the radon concentration will be. Different rock types, and the soils derived from them, can show great differences in their Ra-226 content.

What kinds of rocks contribute to elevated radon levels?
Rocks/deposits associated with elevated radon levels include: granites, found in the Reading Prong in New Jersey and Pennsylvania; dark shales such as the Chattanooga shale deposits in Tennessee; phosphate deposits such as those found in Polk County in Florida; and uranium-bearing sandstones like those in the Morrison Formation near Grand Junction, Colorado. These all have higher than average natural Ra-226 concentrations.

Do high radium levels guarantee high levels of radon gas?
No. A local geology rich in radium does not guarantee high radon levels. If the ground in that area is saturated with water, covered with snow, or frozen, there is no effective path for the migration of radon into the atmosphere. However, keep in mind that the ambient radon may have originated some distance upwind. There might be no significant local source of radon in your neighborhood, but the presence of granite formations upwind might result in higher than expected outdoor concentrations.

What about islands?
Since the radium content of surface water (lakes, oceans etc.) is negligible, radon concentrations on small islands and along the coast, particularly when the prevailing breezes are coming off the water, tend to be low.

Other than the Ra-226 in soil, what else can cause indoor radon?
Some of the more major sources include groundwater, the use of extracted materials from industrial processes for fill (i.e., mill tailings), the burning of natural gas, and the building materials themselves. There is also the issue of local soil/geology.

What impact does the soil/geology have on radon levels?
The major source of indoor radon is the soil and rock near the foundations of the house/building. For the most part, radon is drawn into the home from the soil because the air pressure inside a building is lower than in the soil or outdoors. Anything that decreases the air pressure inside a home might increase the entry of radon. Fireplaces, furnaces, clothes driers, fans, exposure of home to the wind, and the chimney effect are good examples. (The chimney effect is created by the tendency of warm air in the home to rise and be replaced by cooler air entering from the basement or lower levels).

So if I lower the air pressure in my house, I have to be careful about increasing the radon concentration?
No, not necessarily. Sometimes lowering the pressure in the home can actually lower radon levels by dilution, that is, by increasing the influx of outdoor air. In reality, what actually occurs in your home depends on the building construction and a host of other factors. Frankly, this can be difficult, if not impossible, to predict.

Where does radon and thoron typically enter the home?
The major entry points are cracks in the basement floor and foundation, especially at the joint between the two. Other locations are where drains, pipes, conduit etc. penetrate basement walls and floors, and those hollow channels in concrete blocks foundation walls. Thoron, on the other hand, is typically generated "inside" the home from radioactivity in building materials - it does not have to enter from the outside.

What does the groundwater have to do with radon levels?
Groundwater in the vicinity of uranium-bearing soils and rocks can end up with elevated levels of radon. Radon is relatively soluble in water and groundwater, by definition, has been in intimate contact with the particles of soil or rock in which the radon is produced. So the radon, in essence, seeps in.

What levels of radon are typical throughout the country?
Average concentrations in drinking water that is supplied from wells, rather than drawn from surface water bodies, range from a few hundred to a few thousand picocuries of radon per liter. Concentrations of up to several million picocuries per liter have been reported in parts of Maine.

Is that only true for well water?
Pretty much. This is partly due to the inability of the relatively small aquifers associated with granite-bearing rocks to serve large populations. It may also reflect different treatments and holdup times of the water supplies in large and small municipalities. However, where radon levels in the water supply are high enough, the simple act of boiling water, flushing a toilet or taking a shower can result in a measurable increase in the radon levels in a home.

So there is a relationship between air and water radon concentrations?
Yes. As a general rule, an increase of one (1) pCi/l of radon in indoor air can be expected for every 10,000 pCi/l in the water.

What happens if I drink water that has radon in it?
The primary concern, in the case of very high concentrations, is the risk of developing stomach cancer. Whether or not this risk is greater than that from inhaling the radon escaping from this water is uncertain. Some reports suggest that the risks are approximately equal. Others estimate that the overall risk from ingested radon is one-tenth that of inhaling the radon that emanates from it.

Before you mentioned industrial processes that generate materials used as fill, and you gave the example of "mill tailings". What does that have to do with radon levels?
Mill tailings are essentially the by-product (primarily Ra-226) remaining after uranium is separated from the rest of the decay chain in a uranium milling operation. Many years ago, because the risk from inhaling radon gas was unknown or considered a low risk, these tailings were used as backfill around building and home foundations. The most noteable examples are in places like Grand Junction, Colorado and Salt Lake City, Utah. While very little uranium was present in the tailings, the left-over Ra-226 concentrations could easily reach hundreds of picocuries per gram. To make matters worse, the sandy consistency of the tailings made it very permeable to radon.

What happened to homes built on mill tailings?
Thousands of homes eventually required remediation (i.e., removal of the fill material). This was performed under a program known as the Uranium Mill Tailings Remedial Action Program (UMTRAP), funded by the U.S. Department of Energy (DOE).

You also said the use of natural gas can have an impact on radon levels, right?
Yes. Natural gas, which is used as a consumer product in home cooking and heating, typically contains between 10 and 20 pCi/l of radon. So, if you use natural gas in your home and your neighbor does not, all things being equal, your home will have the higher radon concentration.

What impact do building materials have on radon levels?
There are a number of building materials which contain elevated levels of naturally-occurring uranium and/or radium. Examples are brick, concrete block, and phosphor-gypsum wallboard (a product of the phosphate industry).

Okay, we have the type of soil/rock around a home, the groundwater issue, the natural gas issue, the mill tailings issue, and others. How do these rank with respect to radon concentrations in homes?
In general, in about 95% of the homes around the country, the primary source of radon is the soil and rock that is near and under the building foundation. Groundwater, if it is brought into the house from a nearby well, makes up the remaining five (5) percent. Now that the UMTRAP program is pretty much complete, the other sources seldom play an important role.

How can I tell if a home has elevated radon levels?
The only way to definitively identify a home with elevated levels of radon is to make measurements. Certain types of homes, however, can be expected to have higher levels even without the measurements.

What kinds of homes are those?
There are some general characteristics that are typically associated with homes containing elevated radon levels. These include homes with basements rather than crawlspaces (assuming the crawlspace is vented), walls below grade, exposed earth in the basement or sumps, the use of hollow concrete block foundation walls rather than solid poured concrete, tight rather than drafty seals, and the use of water supplied from private wells rather than city services. In addition, homes exposed to the wind (e.g. homes located in the country versus the city) and those located on ridges and slopes rather than in valleys can exhibit elevated radon levels as well. (Just in case you didn't pick up on it, the soil on slopes is usually better drained and thus more permeable.)

How can radon problems in new construction be prevented?
An interesting set of recommendations can be found in the EPA's Radon-resistant Construction Techniques for New Residential Construction (EPA/625/2-91/032) and Radon Reduction in New Construction (EPA-87-009).

What are some of the EPA's recommendations?
Some of their techniques for preventing radon problems include installing a subslab drain system vented to atmosphere, placing a plastic vapor barrier under the slab, pouring the floor slab and foundation as single unit, employing re-bar in concrete to help reduce the consequences of cracking, sealing the top and bottom rows of the concrete block foundation walls, and using a high quality sealant between the slab floor and walls of foundation (if they are built as separate units).

But what if my house is already built?
In that case, you might want to look at a few documents. One is the EPA's Radon Reduction Techniques for Detached Houses, Technical Guidance (2nd edition EPA/625/5-87/019). Others are the Consumer's Guide to Radon Reduction (402-K92-003), Assessment Protocols, Durability of Performance of a Home Radon Reduction System (EPA/625/6-91/032), Sub-slab Depressurization for Low-Permeability Fill Material (EPA/625/6-91/029), Removal of Radon from Household Water (OPA-87-011) and NCRP Report No. 103, Control of Radon in Houses.  The EPA and your state's office of radon programs have even newer documents available.

What kinds of advice do these documents give me?
These advise a number of mitigation techniques, including but not limited to: removing (via excavation) high-concentration soils and other radium-containing materials that are located near foundations; increasing the ventilation in the home (especially in the crawlspace or basement), and blowing air into, rather than out of the home under the assumption that fans are used. One can also seal all cracks in basement walls, floors, sump covers, etc.; employ subslab ventilation, seal the openings between the crawlspace and the living area (i.e., pipes, vents, conduit, etc.), and install charcoal adsorber or aeration systems designed for those situations where the major source of radon is the water supply.

What do you mean by blowing air into, rather than out of, the home?
This technique avoids generating negative pressures inside the home.

What is the downslide to sealing cracks?
This is not likely to be completely effective. In fact, it may be necessary to pour a new concrete floor.

What process is typically involved in subslab ventilation?
This technique usually involves penetrating the basement floor with 6" PVC pipe and exhausting the radon-laden air outside of the home through the use of fans before it can enter the basement.

Are there any concerns associated with the use of charcoal systems?
Yes. In some cases the adsorber units concentrate radioactivity even further, creating some elevated exposure rates in their vicinity.

Where can I obtain more information about radon?
There are a number of excellent references that discuss the radon issue in great detail. Quite a few of them are listed in the "Bibliography" that is located in the "Tool Box" section of the IEM web page. If you don't find the information you need there, please don't hesitate to "Ask a CHP".

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