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. 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 which says that there is not a threshold at which there is an increase in the rate of occurrence of stochastic effects with increasing dose. At the same time, radiology and other practices that involve use of ionizing radiation bring benefits, 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 ALARP principle. Computed tomography, better known as CT scans or CAT scans have made an enormous contribution to medicine, however not without some risk. The ionizing radiation used in CT scans can lead to radiation-induced cancer. Age is a significant factor in risk associated with CT scans, and in procedures involving children and systems that do not require extensive imaging, lower doses are used.