Low Level Radiation


Low level radiation is present as both naturally occurring radiation and man made radiation. The impact of the naturally occurring radiation is hard to quantify; no studies have shown an increase in health problems in areas where natural radiation levels are higher. The impact of man made radiation is likely to remain insignificant until it rises above the naturally occurring radiation in the environment.


Natural Radiation

Natural radiation exists in the environment and is due to several processes:

  • Cosmic rays are high energy photons or particles that enter the Earth's atmosphere. At sea level, the atmosphere has attenuated most of their energy but they still contribute a significant portion to a person's yearly dose
    • Cosmogenic radionuclides are produced through interactions of cosmic rays with atoms in the atmosphere or on the ground. These include 14C, 3H and 7Be
  • Terrestrial Radiation occurs due to the decay of 238U, 235U and 232Th and their daughter products in the earth's crust.
    • Radon gas is usually considered as a separate entity as it makes up a large portion of the yearly dose.
  • Internal radiation typically arises from ingestion of 40K, a long lived isotope of potassium that decays via beta emission (and thus has minimal exposure from the crust).

Interestingly, Australia has a low level of radon gas in homes, meaning that the contribution to total dose is much less than in other parts of the world.

Man Made Radiation

About 18% of radiation exposure to the general population is due to man made sources:

  • 15% from medical sources (11% x-ray and 4% nuclear medicine)
  • 3% from consumer products (smoke alarms etc)
  • 0.3% from occupational exposure (mining, energy)
  • Under 0.3% from fallout

Hazards of low level radiation

In general, low levels of radiation do not cause deterministic effects as they fall below the threshold dose required to induce these effects. The exception is growth retardation, microcephaly or mental retardation in persons exposed in utero to doses over 0.1 - 0.2 Sv. Aside from this, the main risk to the populace is stochastic effects, such as induction of cancer and hereditary effects.
The ICRP uses the term detriment to describe stochastic effects. The detriment for developing cancer is about $4.6 \times 10^{-2}$ per Sv. The risk of hereditary effects is much lower at $0.1 \times 10^{-2}$ per Sv. For the average radiation worker, the risk of developing an adverse effect is about $2 \times 10^{-4}$ per year.