<unk>  is an acronym for an important principle in radiation protection and stands for "As Low As Reasonably Achievable". The aim is to minimize the risk of radioactive exposure or amount of dose while keeping in mind that some exposure may be acceptable in order to further the task at hand.
This compromise is well illustrated in radiology. The application of radiation can aid the patient by providing doctors with a medical diagnosis, but the exposure should be reasonably low enough to keep the statistical probability of cancers or sarcomas (stochastic effects) below an acceptable level, and to eliminate deterministic effects (eg. skin reddening or cataracts). An acceptable level of incidence of stochastic effects is considered to be equal for a worker to the risk in another work generally considered to be safe.
This policy is based on the principle that any amount of radiation exposure, no matter how small, can increase the chance of negative biological effects such as cancer, though perhaps by a negligible amount.  It is also based on the principle that the probability of the occurrence of negative effects of radiation exposure increases with cumulative lifetime dose. These ideas are combined to form the linear no-threshold model. At the same time, radiology and other practices that involve use of radiations bring benefits to population, so reducing radiation exposure can reduce the efficacy of a medical practice. The economic cost, for example of adding a barrier against radiation, must also be considered when applying the<unk>  principle.
There are four major ways to reduce radiation exposure to workers or to population:
Shielding. Use proper barriers to block or reduce ionizing radiation.
Time. Spend less time in radiation fields.
Distance. Increase distance between radioactive sources and workers or population.
Amount. Reduce the quantity of radioactive material for a practice.
ALARA
CHHIP
EVAR
G-LOC
NNK
ISREC
HM 
ISFET