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Most of the public’s exposure to natural radiation comes from radon, which can be found in homes, schools and office buildings. The illustration shows the sources of radon that can accumulate in buildings.

Most radon in homes comes from radon in the soil that seeps into homes through cracks in the foundation or slab. The amount of radon in the soil varies widely and depends on the chemical makeup of the soil. There can be large house-to-house differences in soil radon concentrations. The only way to know is to test.

Radon is also found in the water in homes, particularly homes that have their own wells rather relying on municipal water. When the water is agitated, as when showering or washing dishes, radon escapes into the air. However, radon from water in the home generally contributes only a small proportion (less than 5%) of the total radon in indoor air in most housing. Municipal water systems hold and treat water, which helps to release radon, so that levels are very low by the time the water reaches our homes. But people who have private wells, particularly in areas of high radium soil content, may be exposed to higher levels of radon.

The EPA estimates that the national average indoor radon level in homes is about 1.3 pCi/l of air. We also estimate that about one in 15 homes nationwide has levels at or above the level of 4 pCi/L, the level at which the EPA recommends taking action to reduce concentrations. Levels greater than 2,000 pCi/L of air have been measured in some homes. The only way you can know if there is radon in your home is to test for it.

Continuous Radon Monitoring

This device measures radon and produces results in pCi/L. This detection category includes devices that record real-time, continuous measurements of radon gas over a series of minutes, and then report the results in hourly increments. Air is either pumped (in active mode) or diffuses (in passive mode) into a counting chamber, which is typically a scintillation cell or ionization chamber. The RDPs are filtered out. Alpha particles are counted from radon (active mode) or radon and its RDPs (passive mode). The result using this type of detector is normally available at the completion of the test in the home or building without additional processing or analysis. These detectors are usually deployed for a minimum of 48 hours.
When an alpha-scintillation cell is used, the room air is continuously collected in a scintillation cell, and the RDPs are filtered out. The alpha particles cause the cell’s scintillation material coating to release light. The “glows” are then counted by a photo-multiplier tube.

When a pulsed ion chamber is used, the ions are created from the alpha radiation. The ions are detected by the electrometer. The test produces results in short-term averages.

When a solid-state silicon detector is used, the alpha particles from the radon and its RDPs impact a silicon chip. The impacts produce electrical pulses. The pulses are measurable and counted, and the counts are averaged. This test is passive only. It needs a power supply, and it has relatively low efficiency.

Activated Charcoal Absorption

These devices utilize an airtight container filled with activated charcoal and covered with a screen and filter. The detector is opened in the area to be sampled and exposed to the air for a specified period of time. Radon present in the air adsorbs onto the charcoal, adsorption being a process by which gases or vapors condense to create a thin film. At the end of the sampling period, the container is sealed and then sent to a laboratory for analysis using a scintillation detector. Charcoal detectors may be subject to effects from drafts and high humidity. These detectors are normally deployed for measurement periods of two to seven days.

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