What is Radon?

Learning Objectives

Upon completion of this section, you will be able to

  • Explain what radon is, and
  • Describe the main source of human exposure to alpha radiation.

German physicist Friedrich Ernst Dorn discovered radon in 1900 while researching the natural radioactive decay of radium.

Radon is a radioactive element. Two of its isotopes (radon-220 and radon-222) are progeny in two decay chains that begin with naturally occurring thorium and uranium, respectively, in rock, soil, water, and air.

  • Because radon is a noble gas, it is colorless, odorless, tasteless, and imperceptible to the senses.
  • The most common radon isotope is radon-222 (222Rn).

The growing popularity of CT scans and nuclear medicine in medical radiation have replaced radon as the primary source of ionizing radiation exposure (NCRP 2009).

Radon has no commercial uses.

Except where stated otherwise, this Case Study in Environmental Medicine uses “radon” to refer to radon-222 and its progeny.


Radon (Rn) is a radioactive gas (Lewis 2001) that naturally occurs in different forms known as isotopes. Radon is a chemically and biologically inert noble gas. Its nucleus is heavily neutron-rich, making it radioactive.

  • Radon’s half-life is 3.8 days.
  • Radon is present in air, water, and soil.
  • Radon will undergo radioactive decay in the environment.
Radon Decay

Each parent atom (thorium-234 or uranium-238) decays several times to become a radium atom (Ra-224 or Ra-226), then radon (Rn-220 or Rn-222), and several more times through a series, creating radioactive substances known as radon daughters or progeny. The atom finally decays into a stable lead atom.

As radon progeny undergo radioactive decay, radiation is released in forms that include

  • High-energy alpha particles,
  • Beta particles, and
  • Gamma radiation.

Once formed, radon’s noble gas nature releases it from chemical bonds in rock, soil, water, and building materials. Radon’s half-life provides sufficient time for it to diffuse from its origin and into the atmosphere. This allows for entry into buildings and homes, where further disintegration produces radon progeny. These progeny tend to be electrically charged and tend to attach to dust particles.

  • Radon progeny include four isotopes with half-lives of fewer than 30 minutes. These are the major source of human exposure to alpha radiation (high-energy, high-mass particles, each consisting of two protons and two neutrons).
  • Alpha radiation may—directly or indirectly—damage DNA and other cell components, which could result in radon-induced lung diseases or cancer.

Radon and its progeny are measured in different terms for environmental/residential and occupational exposures.

Environmental/residential radon is usually measured in terms of its quantity of radioactive material, or activity (in units of curies or becquerels).

  • A curie (Ci) is the amount of air, soil, or other material in which 37 billion atoms transform each second, and 1 Ci = 3.7 x 1010 Bq.
  • A Becquerel (Bq) is the amount of material in which 1 atom transforms each second.
  • Prefixes are often used with these units, [e.g., pCi or picocurie (10-12 curie)].

Occupational radon is measured in terms of “working levels” or the total amount of energy imparted to tissue from radon progeny. EPA recommends limiting indoor residential radon concentrations to 4pCi/L, which is generally about a 0.016 working level.

Radon gas has been identified as a leading cause of lung cancer, second only to cigarette smoking (ACS 2006; EPA 2009a).

  • Radon gas is responsible for an estimated 21,000 deaths from lung cancer annually (NCI 2004; EPA 2009b).
  • The risk of cancer due to radon exposure is increased for smokers, as the radiation emitted by tobacco synergizes when in
Key Points
  • Radon is the result of radium atom decay.
  • Radon gas and its progeny are imperceptible to the senses.
  • Radon progeny are a significant source of human exposure to alpha radiation.
  • In 2009, due primarily to the popularity of CT scans and nuclear medicine, medical radiation supplanted radon as the largest source of exposure to ionizing radiation.
  • Alpha radiation may alter cells, which could result in radon-induced lung diseases or cancer.
  • In 2009, radon gas was identified as the second leading cause of lung cancer.