Radiation measurement is an important part of understanding radiological and nuclear hazards within the CBRNE framework. CBRNE stands for Chemical, Biological, Radiological, Nuclear and Explosive threats. In radiological and nuclear incidents, different units are used to describe the amount of radiation absorbed by a material or person and the potential biological effect of that radiation. The four terms rad, rem, gray (Gy), and sievert (Sv) are closely related, but they do not measure exactly the same thing. Rad and gray primarily describe absorbed radiation dose, whereas rem and sievert are used to express the biological effect or health risk associated with radiation exposure. The rad is an older unit of absorbed radiation dose. The term rad originally referred to “radiation absorbed dose.” It measures the amount of ionizing radiation energy absorbed by a material per unit mass. One rad is equal to the absorption of 100 ergs of radiation energy per gram of material. In modern SI units, absorbed dose is expressed in gray. The relationship between the two is straightforward: 1 gray equals 100 rad, and 1 rad equals 0.01 gray, or 10 milligray (mGy). Thus, if a person or an object absorbs a dose of 50 rad, the corresponding absorbed dose is 0.5 Gy. The gray is the SI unit of absorbed dose. One gray represents the absorption of one joule of ionizing radiation energy by one kilogram of matter. Therefore, 1 Gy = 1 J/kg. The gray replaced the rad as the internationally preferred unit for absorbed dose. It is important to understand that absorbed dose alone does not completely indicate the biological damage caused by radiation. Different types of radiation can produce different biological effects even when they deposit the same amount of energy in tissue. For example, alpha particles, beta particles, gamma rays, X-rays and neutrons interact with biological tissues in different ways. A particular absorbed dose of alpha radiation can cause substantially greater biological damage than the same absorbed dose of gamma radiation because alpha particles have a much greater relative biological effectiveness for many purposes. Consequently, another set of units—rem and sievert—is used when the biological significance of radiation exposure needs to be considered. The rem is an older unit used to express radiation dose in terms of its biological effect. The name comes from “roentgen equivalent man.” It is a unit of equivalent dose that takes account of the type of radiation involved. The corresponding modern SI unit is the sievert. The basic conversion is 1 sievert = 100 rem, while 1 rem = 0.01 sievert. Therefore, 50 rem corresponds to 0.5 Sv, and 100 rem corresponds to 1 Sv. The sievert is the SI unit used for equivalent