The people who live around the Three Mile Island nuclear power plant have become the newest human guinea pigs in a health experiment that has been going on-without a final result-for more than 30 years.

The question to be settled is what are the long-term health risks to individuals exposed to low levels of radiation?

It is a question that today is being sharply and bitterly debated by scientists and doctors both inside and outside government.

The debate has the same overtones that accompanied the cigarettes and cancer controversy-but with a difference.

The Pennsylvanians did not volunteer to be dosed with low level radiation.

Nor did other human subjects in this experiment-the Japanese survivors of Hiroshima and Nagasaki; the Marshal Islanders exposed to radioactive fallout in 1954; the GI's and Utah residents exposed by the 1950s Nevada nuclear tests.

Little more than a month ago, Health, Education and Welfare Secretary Joseph A. Califano Jr. declared that no one knows fore sure how to estimate the risks of cancer from low level radiation. But he termed finding some answer high on the public health agenda.

Until the past few years, however, the government and the public seemed willing to accept standards in existence for almost 20 years. These said low doses of radiation-below 5,000 millirems a year-would not cause significant health problems even over the 10-20-30-year periods during which some cancers develop.

In recent years, However, a series of highly publicized findings, base on studies of limited groups exposed at different times to low radiation levels, have sharply challenged the existing theories. They have tended to show cancers developing 10 to 20 years later among individuals exposed to dose levels recorded far below the 5,000 millirem per year figure.

Eight GIs with exposure records indicating 1,500 millirems or less during a 1957 nuclear weapons test called Smoky later developed leukemia. A Center for Diesease Control study determined eight was more than twice the leukemia cases that should normally have been found.

A Utah State scientist studied Utah children exposed to fallout that drifted 100 miles from the same 1950s Nevada weapons test. He found they had twice the number of leukemia cases than children who had lived in the same areas before and after the tests.

A statistical survey of workers at the government's nuclear facility at Hanford, Wash., turned up a slight increase of some types of cancers, although the workers had absorbed less than 5,000 millirems.

Opponents of nuclear power seized upon these studies and added them to their anti-nuke arsenal. On the other side, supporters of nuclear power produced doctors and scientists to attack the findings, or the manner in which the studies had been done and even the qualifications of the researchers.

Radiation, absorbed into the body, can kill human cells. Radiation can also alter cells. But not all absorbed radiation causes damage and some of the damage to cells is repaired by normal mechanisms.

The degree of harm from radiation thus relates to a series of complex factors-the type of radiation, the extent of the dose and length of time of exposure, the portion of the body exposed and the tissue or organs involved.

Scientists agree that women and children are more prone to radiation damage than men. And pregnant women are perhaps the most vulnerable since absorbed radiation could do severe harm to a fetus in the womb.

These considerations led pennysylvania Gov. Richard Thornburgh to suggest evacuation of pregnant women and children from the areas within five miles of the Three Mile Island plant.

Since man became aware of radiation and its hazard to human health, the amount of radiation exposure considered "safe" has been going steadily down.

At the turn of the 20th century, X-rays were an enormously popular new medical discovery. Despite evidence at the time that heavy X-ray doses were harmful, it was not until 1921 that any limits were set.

By 1925, the international standard was put at 1,000 millirems a week. In the U.S., that level prevailed until 1934, when an American-based organization, the National Committee on Radiation Protection, recommended 500 millirems a week.

In the post-World War II days, with the experience of the Japanese atom bomb victims analyzed, the U.S. recommended does level dropped again. In 1949 it was put at 300 millirems per week. Eight years later, in 1957, it was dropped sharply to 5,000 millirems in a year (the earlier weekly limit worked out to 15,600 millirems over a year).

In addition, a general population exposure level of 500 millirems in a year was set-putting it at one-tenth the exposure permitted workers in nuclear facilities.

The lowering levels were accompanied by additional incidences of cancer which turned up primarily in Japanese groups whose post-exposure health was being followed.

HEW Secretary Califano said last week that the evacuated women and children from around Three Mile Island, plus workers at the stricken facility, would be prime subjects for a proposed long-term health study.

Such a program not only will permit the people to keep track of their health, but could give the government additional information it needs to help solve the vexing low-level radiation problem.