"Dear Arjun"

Dear JCW,

The term was a new one to me, but I see that it was included in 2 letters to the editor in the May 3 1996 issue of Science magazine:

After some thought on this new term I wondered if someone had confused "sieverts" and "kelvin" to erroneously create the new term, "selvins." Sure enough, two weeks after the letters were printed, Science ran a correction, admitting the error was introduced during editing.

But as long as we're on the subject, you might ask:

Dear Arjun:

Just what is a sievert, and where can I get one?

Many years ago, before the invention of Dramamine, sea sickness sufferers endured nightmarish journeys when their commutes required boat travel. At first the problem was not well understood. It was thought that they simply suffered from vertigo which happened to strike at inopportune times during ocean travel. Eventually the term "Sievert" was adopted to describe this condition of "vertigo at sea."

These days, sievert means something a bit different. A sievert (abbreviated Sv) is a unit of equivalent absorbed radiation dose equal to 100 rems. Sieverts measure the biological damage done by ionizing radiation. When high energy electromagnetic radiation (called X-rays and gamma rays), electrons, positrons, neutrons, or alpha particles (helium nuclei) strike living cells they deposit energy in those cells and cause damage. The amount of energy deposited is measured in grays or rads (1 gray = 100 rads). The biological damage depends on the kind of radiation. Generally neutrons and alpha particles cause more damage per unit of energy deposited. The radiation dose measured in energy units is multiplied by a "quality factor" to account for this variation in biological damage to yield a unit called sieverts (or rems). So we have:

sieverts = grays multiplied by the quality factor;
rems = rads multiplied by the quality factor.

One sievert is a large radiation exposure. A sudden exposure to one sievert would produce observable symptoms (also called non-stochastic effects) such as hair loss, nausea, and lowered white blood cell counts. Radiation doses below 0.1 to 0.5 sievert (10 to 50 rem) do not produce immediate observable effects, but increase cancer risk and risk of genetic damage. The first observable effect is lowered white blood cell count and occurs at about 0.1 sievert. Radiation doses delivered slowly (over a period of days or longer) generally do not produce non-stochastic effects. Natural background radiation from all sources at sea-level is about 0.001 sievert per year.

Cumulative low dose radiation can add up to quite large doses without producing non-stochastic effects. The effects of such low dose radiation are calculated in terms of cancer risk to exposed populations by adding up all the individual doses received within a particular population group (such as people living in an area or workers in a particular plant). Such total population doses are measured in "person-sieverts," simply to indicate that doses of many individuals are being added up, without consideration for differences in individual response.