Absolute risk.

An expression of excess risk based on the assumption that the excess risk from radiation exposure adds to the underlying (baseline) risk by an increment dependent on dose but independent of the underlying natural risk.

Absorbed dose.

The mean energy imparted by ionizing radiation to an irradiated medium per unit mass. Units: gray (Gy), rad. 1 Gy = 100 rad.

Action level.

A concentration of radon in air that the Environmental Protection Agency recommends should be the maximum concentration in homes; i.e., concentrations above this level should be reduced or mitigated to this level. In 1998, EPA recommends an action level of 148 Bqm-3 or 4 pCiL-1.


The amount of radionuclide radioactivity defined as the mean number of decays per unit time. Units: becquerel (Bq), curie (Ci). 1 Bq = 2.7 × 10-11 Ci.

Activity Median Aerodynamic Diameter (AMAD).

The diameter of a unit-density sphere with the same terminal settling velocity in air as that of the aerosol particle whose activity is the median for the environment.

Additive effects.

Equal to the sum of effects from two agents when acting alone.


Solid or liquid particles that are dispersed in a gaseous medium, are able to remain suspended for a relatively long time, and that have a high surface area to volume ratio.

Alpha particle.

Two neutrons and two protons bound as a single particle that is emitted from the nucleus of certain radioactive isotopes in the process of decay or disintegration. Is positively charged and indistinguishable from a helium atom nucleus.

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