(PAEC), is measured in units of energy per unit volume of air (joules per cubic meter (J m-3).

The development of a PAEC from indoor radon concentration depends on air movement and aerosol conditions within a room. PAEC can depend on whether the radon entered a room from soil or from water during bathing. The relationship between indoor radon concentration and PAEC is expressed in terms of the equilibrium ratio (ER). For a room without any depletion of radon or plate-out of decay products, ER = 1.0. In domestic environments, ER ranges from 0.3 to 0.7 with a nominal value of 0.4 (Hopke and others 1995a).

The alpha-particle dose to lung tissues depends on PAEC and on the time that a person spends in a given location. A combination of PAEC and time is a measure of exposure expressed in joule-seconds per cubic meter (J s m-3).

Absorbed Dose from Indoor Radon

A person in a room will inhale radon decay products that are suspended in air. Some activity can deposit and accumulate in the respiratory airways, depending on breathing patterns and the aerodynamic size of the particles with which the decay products are associated. Because of the short half-lives, the radon decay products that are deposited in the lung will almost certainly decay completely in the lung. The radiations emitted within the lung during these decays can deposit energy in the body. However, this radioactivity is very near the lung epithelium, so alpha particles in particular can transfer copious amounts of energy to vulnerable cells. That is why radon decay products are characterized in terms of PAEC.

Radon gas itself is also inhaled. Most of it is exhaled immediately and therefore does not accumulate in the respiratory system, as do radon decay products. Because the radon does not get close to radiosensitive cells, the absorbed dose from alpha particles is small. However, some of the radon that reaches the interior region of the lung is transferred to blood and dispersed throughout the body. Radon and the decay products formed inside the body can deliver a radiation dose to tissues and organs.

On some occasions, water is consumed immediately after leaving the faucet before its radon is released into the air. This water goes directly to the stomach. Before the ingested water leaves the stomach, some of the dissolved radon can diffuse into and through the stomach wall. During that process, the radon passes next to stem or progenitor cells that are radiosensitive. These cells can receive a radiation dose from alpha particles emitted by radon and decay products that are created in the stomach wall. After passing through the wall, radon and decay products are absorbed in blood and transported throughout the body, where they can deliver a dose to other organs.

Ingested water eventually passes through the stomach into the small intestine, where the remaining radon and decay products are released from the water

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