Agency for Toxic Substances and Disease Registry 
Radon Toxicity
Exposure Pathways

Sources of Radon Exposure

• Radioactive decay of uranium through radium produces radon, which can move from soil into the air. It decays into a series of progeny, some of which are short-lived and emit alpha and beta particles and gamma rays.

Radon gas is derived from the radioactive decay of radium, a ubiquitous element found in rock and soil. The decay series begins with uranium-238 and goes through four intermediates to form radium-226, which has a half-life of 1,600 years. Radium-226 then decays to form radon-222 gas. Radon's half-life, 3.8 days, provides sufficient time for it to diffuse through soil and into homes, where further disintegration produces the more radiologically active radon progeny ("radon daughters"). These radon progeny, which include four isotopes with half-lives of less than 30 minutes, are the major source of human exposure to alpha radiation (high-energy, high-mass particles, each consisting of two protons and two neutrons). This alpha radiation produces damage that, if not repaired, results in cellular transformation in the respiratory tract, which can lead to radon-induced lung diseases or cancer.

• Radon, a colorless, odorless gas, is both chemically inert and imperceptible to the senses.
• Its infiltration into buildings is the main source of indoor radon; however, building materials and the water supply can also be sources.

Radon itself is imperceptible by odor, taste, and color, and causes no symptoms of irritation or discomfort. There are no early signs of exposure. Only by measuring actual radon or progeny levels can people know whether they are being exposed to excessive levels of radon. Radon seeps from the soil into buildings primarily through sump holes, dirt floors, floor drains, and cinder block walls, and through cracks in foundations and concrete floors (Figure 1). When trapped indoors, especially during a temperature inversion that reduces its escape from the building, radon can become concentrated to unacceptable levels. When radon escapes from the soil to the outdoor air, it is diluted to levels that offer relatively little health risk.

• Although concrete slab basements allow for less soil gas entry than do unfinished dirt-floor basements, both types of surfaces could permit entry of radon.

Figure 1. Sources of Radon and Common Entry Points

Radon gas can enter a building by diffusion, but pressure-driven flow is a more important mechanism. Negative pressure in the home relative to the soil is caused by exhaust fans (kitchen and bathroom), and by rising warm air created by fireplaces, clothes dryers, and furnaces. In addition to pressure differences, the type of building foundation can affect radon entry. Basements allow more opportunity for soil gas entry, but slab-on-grade foundations (no basement) allow for less. In most cases, the increase of indoor radon due to home "tightening" for energy conservation is slight compared to the amount of radon coming from the soil.

Typical building materials, such as concrete block, brick, granite, and sheet rock, contain some radium and are sources of indoor radium. Normally, these construction materials do not contribute significantly to elevated indoor radon levels. In rare cases, however, building materials themselves have been the main source of radioactive gas. Building materials contaminated with uranium and vanadium mill tailings in Monticello, Utah, and uranium mill tailings in Grand Junction, Colorado, were an important source of radon because they contained elevated concentrations of radium. (Tailings are the sandlike material remaining after minerals are removed from ore.) Also, concrete made from phosphate slag in Idaho and Montana and insulation made from radium-containing phosphate waste from the state of Washington have been found to emit high levels of radon.

Radon might enter into homes via the water supply. With municipal water or surface reservoirs, most of the radon volatilizes to air or decays before the water reaches homes, leaving only a small amount from decay of uranium and radium. However, water from private wells might be another matter. Groundwater that comes from deep subterranean sources and passes over rock rich in uranium and radium, such as that found in northern New England, might dissolve some of the radon gas produced from radium decay. As the water splashes during showering, toilet flushing, dishwashing, and laundering, radon is released into the air and can result in inhalation exposure. Radon can also be present in natural gas supplies.

Challenge

Hazard Assessment

Respiratory Dose and Units of Measure

• Radon and its progeny can be detected only by testing.

Because the health effects of radon are insidious and have a long latency period, it is important to measure exposure to the gas empirically. Techniques for measuring radon are discussed in the Radon Detection section. Included here is a review of the basic unit of radon measurement and the factors that are used to estimate radiation dose from air concentration information and physical parameters. (Note that this subsection is on dose and units, and not on risk.)

The relationship between exposure to radon and the dose of radiation from decay products that reaches target cells in the respiratory tract is complex. Some factors that influence the pulmonary radiation dose include the following:

• Characteristics of inhaled air radon. Progeny that are attached to dust particles (the attached fraction) deposit much more efficiently than free or unattached progeny; of the attached progeny, only those adhering to the smallest particles are likely to reach the alveoli.
• Amount of air inhaled. The amount and deposition of inhaled radon decay products vary with the flow rate in each airway segment.
• Breathing pattern. The proportion of oral to nasal breathing will affect the number of particles reaching the airways. Oral breathing deposits more of the larger particles in the nasopharyngeal region. Regardless of the breathing pattern, the smaller the particle, the deeper it penetrates into the lung and the more likely it is to deposit there.
• Architecture of the lungs. Sizes and branching pattern of the airways affect deposition; these patterns may differ between children and adults and between males and females. Preferential deposition of larger particles occurs at all branch points because of inertial impaction.
• Biologic characteristics of the lungs. The radiation dose occurs in those areas where mucociliary action is either absent or ineffective in removing the particles. Particles moving with the mucous flow cause essentially no radiation dose to tissue because of the short range of alpha particles in fluids.

It is possible, therefore, that two environments with the same radon measurement (e.g., a dusty mine and a home environment) might cause different deposition patterns and, therefore, deliver different doses of alpha radiation to a person's lungs. Likewise, two persons in the same environment might receive differing doses of alpha radiation to the target cells in the upper portion of their lungs because of differing breathing patterns and pulmonary architecture.

• EPA recommends remediation for homes with airborne radon levels at or above 4 pCi/L.
• In early 2000, EPA proposed municipal drinking water levels tied to state plans to remediate radon in indoor air.

If particle size distribution is not known, an assumed distribution, along with the average measured air concentration, is used to estimate deposition within the lung and the resulting radiation dose. The higher the average radon level a person experiences, the higher the radiation dose. Radon gas can be collected on activated charcoal filter media, or the attached progeny can be collected on mesh filters. Radon measurements are expressed in picocuries per liter of air, where a picocurie is equivalent to the amount of progeny in which 0.037 atoms disintegrate per second. EPA has recommended that remedial action be taken to lower the amount of radon in homes if the level measured in air is 4 pCi/L or greater.

Risk Estimates

Even conservative estimates based on current knowledge suggest that radon is one of the most important environmental causes of death. EPA and the National Cancer Institute estimate that approximately 15,000 deaths annually in the United States are due to lung cancer caused by indoor radon exposure. It has also been estimated that approximately 14% of the 164,100 cases of lung cancer diagnosed annually are attributable to radon.

• For a lifetime exposure at the EPA recommended guideline of 4 pCi/L, EPA estimates that the risk of developing lung cancer is 1 to 5%, depending on whether a person is a nonsmoker, former smoker, or smoker.
• The overall risk of radon exposure is related not only to its level in the home, but also to the occupants and their lifestyles.

For a lifetime exposure at the EPA recommended guideline of 4 pCi/L, EPA estimates that the risk of developing lung cancer is 1 to 5%, depending on whether a person is a nonsmoker, former smoker, or smoker. The National Research Council estimates the risk as 0.8 to 1.4%.

Many factors influence the risk of lung cancer due to radon exposure; among these are age, duration of exposure, time since initiation of exposure, cigarette smoking, and other carcinogen exposures (Table 1 and Table 2). In assessing the risk of radon in a home or office, it is important to consider not only the average level of radon, but also the occupants and their lifestyles. Are there any smokers? Any children? How much time is spent in the home? Where do occupants sleep? The highest radon levels are typically found in the lowest level of the house. If well water is the major source of radon, upper floors can be affected more than lower floors. In colder climates, radon levels are often higher in the winter and lower in the summer.

Table 1. Radon Risk Evaluation Chart if You Smoke

Table 2. Radon Risk Evaluation Chart if You Have Never Smoked

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