Radiation, whether from X-rays or radioactive materials or a nuclear reactor, "ionizes" or casues changes in some of the atoms inside our cells.
Large enough doses of radiation (rougly more than 200 "rads" - see below for an explanation of this and other terms) may cause rapid death from radiation sicknes.
Though they lack hard proof, scientists almost universally agree that the addition of any amount of radiaton, even the smallest, is bad, in that it may cause future cancers or mutations in a small number of persons. But scientists also agree that the use of modest amounts for useful purposes - like diagnostic X-rays, providing the X-rays are really necessary - may be of more potential benefit than potential harm.
A dose of ionizing radiation received from any source is usually measured in:
Roentgens, a measure of the quantity of X-ray (or gamma ray) radiation in the air or
Rads (for "radiation absorbed dose"), a measure of the amount of any kind of ionizing radiation absorbed in body tissue, or
Rems (for "roentgen equivalent, man") a measure of the quantity of any ionizing radiation with the same biological effectiveness as one rad of X-rays.
A roentgen, rad and rem are nearly equivalent, for most practical purposes. All are often expressed in subdivisions, like the "millirem" (one-thousandth of a rem).
The average chest X-ray delivers a dose of 25 millirads (to the skin).
But a chest X-ray is delivered quickly to only part of the body.
Any doses at a site like Middletown, Pa., where many people are in the area for long periods, must be expressed in dose rates over time to have any real meaning. An individual living in a really hard-hit area might receive the equivalent of many chest X-rays, far more than any prudent practitioner would administer.
The dose in a Middletown incident, whether large or small, will also be delivered to all parts of the body, not just one part. A whole-body dose is of more pontential harm than a limited dose.