When radioactivity is taken into the body, the dose received is due to the energy imparted to internal organs such as the lung, thyroid, or bones.¹ A dose conversion factor (DCF) coverts an amount of radioactivity (expressed in curies or becquerels) into a dose (expressed in rems and sieverts). The DCFs used for regulatory purposes are derived from a combination of experimental data and mathematical models. The DCF for a given radionuclide depends upon the half-life of the radioactive material and the type of radiation emitted (alpha, beta, gamma). It also depends on how easily that radioactive material passes through the body. For inhaled materials, this is indicated by the solubility of the radioactive material. For ingested material, this is indicated by the uptake fraction; the fractional amount taken up by the blood from the small intestine.

Solubility refers to how likely a material is to dissolve in water. Once absorbed, insoluble material generally spends more time in the body, and therefore does more damage. This explains why, for most radionuclides, insoluble forms have greater DCFs than more soluble forms. Similarly, radionuclide forms with smaller uptake fractions will spend less time in the body, resulting in less damage and a smaller DCF for a given intake of radioactivity.

Radiation standards for workers and non-workers are expressed as whole-body dose equivalents. But in reality, the body is rarely if ever uniformly irradiated, and certain parts of the body or organs are often more affected than others. This is because radionuclides taken into the body are distributed unequally among organs (for example, radioactive iodine concentrates in the thyroid, inhaled plutonium disproportionately affects the lungs, and strontium is deposited in bones). In addition, it is possible that only a portion of the body might be exposed to an external radiation source.

The effective dose equivalent is a way of converting the actual complicated process of radioactive intake into a simplifed concept of a uniform whole-body dose--that is, an equivalent of what an actual localized dose means to the overall body. It is a way of quantifying the increased chance of harm expected as a result of the dose, measured mainly by excess fatal cancers and hereditary disease. Effective dose is meant to allow the comparison between different types of radiation exposure and exposure to different organs.

To determine if a person has received a radiation dose above a recommended limit, a single DCF, pertaining to a particular organ or the "effective" DCF, is used. The organ chosen is then referred to as the "standard-setting organ." The effective dose is calculated by taking doses to individual organs and converting them to equivalent whole-body doses by using weighting factors. These figures are then added up to calculate the total dose. These weighting factors are shown in the box below. So, for example, a dose of 20 rem to the thyroid is equivalent to an effective dose of 0.6 rem.

It is possible to have single or continuous intake of radiation. Single intakes tend to happen in unusual circumstances, such as accidents. A continuous intake could result from living near a nuclear facility that regular releases radioactivity into the air or water.

Even when intake of radioactivity into the body takes place in a very brief period of time (as in a single intake), the material remains in the body for some time and hence the dose from that material is imparted over a period of time. The period of time depends on the half-life of the material that is taken into the body and the amount of time it stays in the body (dominated by the smaller of these numbers). The dose expected over the effective lifetime of the radioactivity in the body is the committed dose. The committed dose equivalent, established by the ICRP, is the dose equivalent deposited over the 50 years after the intake of the radionuclide.