radiation exposure. the highest risk of lethality is during the preimplantation period. this is up to day 10 postconception. malformations generally occur after organogenesis. this is the phase of development where the three germ layers (the ectoderm, endoderm, and mesoderm) form the internal organs of the fetus. the estimated dose threshold is 0.1 gylow-linear-energy-transfer (let) radiation, and this period generally occurs from day 14–50. animal data supports the idea that malformations are induced at a dose of around 100 mgy. another risk is reduction of intelligence quotient (iq). the most sensitive period is weeks 8–15 postconception. iq reduces by 30 iq points/sv, which can lead to severe intellectual disability. malformations begin to occur at a dose threshold of at least 300 mgy. cancer can also be induced by irradiation, which generally occurs from day 51-280 of pregnancy. most x-rays occur during the third trimester of pregnancy. there is sparse information on radiation exposure from the first trimester of pregnancy. however, data suggests that the relative risk is 2.7. relative risk is a measure of probability of an outcome in one group versus the other. in this case, the risk of cancer formation in the first trimester is 2.7 times higher than the risk of cancer formation in the third trimester. in addition, the united nations scientific committee on the effects of atomic radiation calculated excess relative risk in the first trimester. it is 0.28 per mgy. excess relative risk is the rate of disease in an exposed population divided by the rate of disease in an unexposed population, minus 1.0. this means that the risk of cancer from irradiation in the first trimester is 28% higher than in the third trimester. benefits of radiation in medical imaging and therapy there are multiple benefits from using radiation from medical imaging. screening imaging exams are used to catch cancer early, reducing the risk of death. it also reduces the risk of having serious life-limiting medical conditions, and avoiding surgery. these tests include lung cancer screening, breast cancer screening, and more. radiation is also used as therapy for many different types of cancer. about 50% of all cancer patients receive radiation therapy. radiation therapy destroys cancer cells, stopping them from growing. aside from cancer, many types of medical imaging are used to diagnose life-threatening diseases, such as heart attacks, pulmonary embolism, and pneumonia. exposure rate constant the gamma ray field can be characterized by the exposure rate (in units of, for instance, roentgen per hour). for a point source, the exposure rate will be linearly proportional to the source's radioactivity and inversely proportional to the square of the distance, f = γ×α / r2 where f is the exposure rate, r is the distance, α is the source activity, and γ is the exposure rate constant, which is dependent on the particular radionuclide used as the gamma ray source. below is a table of exposure rate constants for various radionuclides. they give the exposure rate in roentgens per hour for a given activity in millicuries at a distance in centimeters. exposure rate constants for various radionuclides r•cm2 / hr•mci radionuclide exposure rate constant cobalt-60 12.838 molybdenum-99 1.03 technetium-99m (6 hour) 0.720 palladium-103 (unfiltered) 1.48 silver-110m (250 day) 14.9 caesium-137 3.400 iodine-125 (unfiltered) 1.46 iridium-192 (unfiltered) 4.69 radium-226 8.25 radiation measurement quantities the following table shows radiation quantities in si and non-si units: ionizing radiation related quantities quantity unit symbol derivation year si equivalent activity (a) becquerel bq s−1 1974 si unit curie ci 3.7 × 1010 s−1 1953 3.7×1010 bq rutherford rd 106 s−1 1946 1,000,000 bq exposure (x) coulomb per kilogram c/kg c⋅kg−1 of air 1974 si unit röntgen r esu / 0.001293 g of air 1928 2.58 × 10−4 c/kg absorbed dose (d) gray gy j⋅kg−1 1974 si unit erg per gram erg/g erg⋅g−1 1950 1.0 × 10−4 gy rad rad 100 erg⋅g−1 1953 0.010 gy equivalent dose (h) sievert sv j⋅kg−1 × wr 1977 si unit röntgen equivalent man rem 100 erg⋅g−1 × wr 1971 0.010 sv effective dose (e) sievert sv j⋅kg−1 × wr × wt 1977 si unit röntgen equivalent man rem 100 erg⋅g−1 × wr × wt 1971 0.010 sv although the united states nuclear regulatory commission permits the use of the units curie, rad, and rem alongside si units, the european union european units of measurement directives required that their use for "public health ... purposes" be phased out by 31 december 1985.