Radiation Exposure

Exposure to radiation causes a dose. Various organs respond differently to exposure to radiation, so dose is categorized to specify which part of the body received the dose:

• Internal vs. External
• Penetrating vs. Shallow
• Extremity vs. Whole Body
• Individual organ dose (e.g. thyroid, eyes, etc...)

Regardless of how the dose is categorized, radiation dose is reported in units of millirem (mrem), or the SI unit for dose, milliSieverts (1mrem = .01mSv).

Background Radiation is the term which describes non-occupational exposure to radiation sources. The level of background radiation can vary depending on geographic location, occupation and other factors. The average exposure from background radiation for a person living in the U.S. are:

• 360 mrem average exposure from all background radiation sources in the U.S.
  • 300 mrem from natural sources
    • radon gas from naturally occurring radium deposits in the soil
    • cosmic radiation
    • internal exposure from naturally occurring radioisotopes, such as C14 and K40
  • 60 mrem from artificial sources
    • medical, consumer products, fallout from nuclear tests

Occupational radiation exposure is limited by federal and state regulations. The annual limits for radiation exposure to occupational radiation workers include:

• 5,000 mrem to whole body
• 50,000 mrem to any organ/tissue
• 15,000 mrem to the eye
• Others dose limits apply to specific organs, extremity doses, etc...
• These limits are below the levels at which health effects have been seen

Potential health effects from radiation exposure are determined by the level of dose.

Acute Health Effects
• Includes immediate radiation sickness, etc…
• These effects are observed at levels much greater than the regulatory limits
• The threshold for acute effects is about 400,000 mrem for sickness

Chronic Health Effects
• Health effects occur long after exposure has ceased
• Includes cancer, genetic effects, birth defects
• These effects are only observed at levels much greater than the regulatory limits, though no lower threshold has been determined for chronic health effects. See "ALARA" below

As Low As Reasonable Achievable (ALARA)

Because no lower threshold has been determined below which chronic health effects can be ignored, all users and licensees of radioactive material must keep their dose As Low As Reasonably Achievable.

The ALARA concept itself grows out of the assumption that any radiation exposure carries with it some risk. The ALARA effort is related to balancing the assumed risks of radiation exposure against the benefit of performing the task which may cause the exposure. ALARA also assumes that any unnecessary radiation exposures are considered as excessive.

A fundamental principle of the ALARA concept would be that anyone planning to use ionizing radiation receive enough training to keep their dose as low as reasonable achievable.