Radon

Characteristics of Radon-222

Fate and Transport

Monitoring in the Environment

Measurement

Measurement Methods

Control and Prevention

Harmful Effects

Absorption, Distribution and Organic Sites of Toxicity

Radon Dose

Radon Biomarkers

Risk Assessment

Molecular Action and Genetic Effects


Radon for Skeptics

Radon for Children

Absorption, Distribution, and Organic Sites of Toxicity:


Inhalation Route

  • The primary route of exposure to radon and its progeny is inhalation.
  • As described in the previous sections, charged radon particles can easily bind to available surfaces, including walls, floors, clothing, and aerosolized particles such as dust and other particulates. Once bound to aerosolized particles, the charged progeny can easily be transported throughout the environment via wind action, and more importantly, can be inhaled by respiring animals and humans. The radon progeny can be inhaled either as free particles or particles that are attached to dust. Because they are ionized, the progeny preferentially attach to the respiratory epithelium, particularly the bronchi, the site of most lung cancers. Most of the radon gas inhaled will be exhaled (due to the relatively long half-life of radon gas) before it can decay and deposit a significant radiation dose to the lung tissue.

  • The radon progeny deposit on the mucus lining of the respiratory tract through:
    -impaction
    -sedimentation
    -diffusion
  • The radon daughters act as soluble substances and are released from the dust particles after they undergo solvation.
  • The depth at which a radon progeny can incorporate into the respiratory tract is related to the size of the aerosolized particle it is attached to. Studies have shown that this depth can vary by a factor of two with changes in particle volume and weight. The radon progeny are eventually cleared from the respiratory tract through mucociliary action or engulfment by macrophages, but because of their short half-life, the progeny can release alpha particles before being removed.
  • The total respiratory radon progeny deposition is 18-51% of the inhaled amount.
  • The larynx and trachea deposition is approximately 22% of the inhaled amount

Ingestion Route

  • Exposure to radon by the oral route occurs from dissolution of radon in drinking water.
  • Of the total radon dissolved in water, approximately 30% to 70% may be lost by aeration and would be available for inhalation.
  • Ingestion of water with dissolved radon gas in it can release charged radon daughters which attach to the stomach lining when the stomach contents are agitated.

  • If the radon is in the uncharged gaseous phase, it can be absorbed into the blood stream through the stomach or intestinal walls and distributed throughout the body. The majority of radon absorption following ingestion in water occurs in the stomach and small intestine, and only 1% to 3% of the ingested radon remains to enter the large intestine.
  • Once radon has entered the blood stream it is distributed among the organs according to the blood flow to them and the relative solubility of radon in the organs as compared to the blood. Radon dissolved in blood that enters the lung will equilibrate with air in the gas-exchange region and be removed from the body. Greater than 90% of the absorbed dose is eliminated by exhalation in less than one hour.
  • Since radon is highly fat soluble and actively transported through the body on the coattails of lymphocytes, common organs of destinations are the liver (5%), kidney (1.6%), lungs (90% - primarily for elimination), and other adipose tissue stores. Once the dissolved gas decays and becomes charged, it can bind and decay further within various body tissues, and emit harmful, mutagenic and cytotoxic, alpha particles.
  • Long-lived radon progeny have been detected in excreted urine.
  • The absorption of radon following ingestion of a meal high in fat is delayed (deals with the high solubility of radon in fat).

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