A deadly radiation dose deposits surprisingly little energy in the body
The gray measures radiation by energy absorbed: one joule per kilogram of matter. A whole-body dose of about five grays is lethal for half of those exposed, yet for a 75-kilogram adult that amounts to only 375 joules. The danger lies not in the quantity of energy but in how ionising radiation breaks molecules.
Measuring radiation used to mean measuring its effect on air. Soon after Wilhelm Röntgen discovered X-rays in 1895, doctors embraced them for spotting broken bones, and the hazards became apparent. An international commission founded in 1928 defined the roentgen by how much electric charge X-rays produced in a cubic centimetre of dry air. That was a big step toward standardisation, but it said nothing directly about what happened inside tissue.
The British physicist Louis Harold Gray, who studied neutron damage to living tissue, helped change that. In 1940 he and colleagues proposed a unit tied to the energy actually deposited in tissue rather than to exposure in air. In 1953 the rad, equal to 100 ergs per gram, was recommended. When the metric system was reorganised as SI, the equivalent unit became the joule per kilogram, and in 1975, a decade after Gray died, it was named the gray. One gray equals 100 rad, which is why radiotherapists still often talk in centigrays.
A gray alone does not capture biological risk, because different radiation types harm tissue differently. For X-rays and gamma rays, one gray corresponds to one sievert, the unit for estimating probable health effects, but for alpha particles a single gray counts as 20 sieverts. Standards bodies insist on the two distinct names precisely so the quantities are not confused, even though both reduce to joules per kilogram.
Doses span a huge range. A pelvic CT scan delivers around 6 milligrays. Curative radiotherapy for solid tumours typically totals 60 to 80 grays, aimed tightly at the target and usually split into small daily fractions of about 2 grays. Industry uses the unit as well, to control food irradiation, sterilisation and the radiation hardening of materials.
Source: Gray (unit)
