THIRTY YEARS after Hiroshima and more than 80 years after the discovery of radioactivity, the vast majority of Americans do not have the vaguest idea what it is . Even the common units in which radioactivity is measured -- curies, rads and rems -- are as meaningless to people as the most impenetrable gibberish of advanced mathematics. This is a form of ignorance that is risky to live with.

It also produces fear of the unknown. During Three Mile Island, a tiny amount of radioactivity was discovered in the milk at neighborhood dairies. This was duly reported by the media -- 12 picocuries per liter found in local milk! -- and generated cries of panic. But wait. picocuries ? What in the hell is a picocurie? In fact, the amount of radioactivity discovered is several thousand times less than an amount that would have justified precautionary measures. But how could you know? And who could trust a government that in the past had -- perhaps knowingly -- allowed members of its armed forces and citizens of Utah to be exposed to clearly dangerous levels of radioactivity?

Even if the nuclear industry were dismantled tomorrow, radioactivity would still be inescapable. It comes at us as cosmic rays and occurs naturally in radioactive substances in the earth and in all living matter. Trying to convince a congressional committee that certain standards for the disposal of nuclear waste are too strict, Nobel laureate Rosalyn Yalow pointed out the other day that the amounts of radioactive potassium and carbon present in a normal human being would require that person -- if he or she were a dead laboratory animal -- to be disposed of as nuclear waste.

The most important source of radioactivity -- and here is where ignorance breeds risk -- is medical and dental work. As much as 99 percent of all normal exposure above the natural background comes from medicine. x-rays are the most common source, and the most grossly overused. Though most medical exposures are low doses, the thing to remember is that the effects of low levels of radiation are simply unknown. The only safe rule to follow in the face of this uncertainty is that any unnecessary exposure is unwise.

Modern medicine is here to stay and so, apparently, are nuclear reactors. Society could live with them a lot more comfortably, and more safely, if Americans had a basic knowledge of what radioactivity is, what its properties are, which parts of body are most sensitive, what annual doses are thought to be safe, and what amounts are known to be dangerous. Providing that knowledge is the responsibilty of the schools, the federal government and the scientific establishment, and none has done the job very well. Whether people will care enough to learn it remains to be seen.