Monitoring of radiation levels in residential areas near the Three Mile Island nuclear power plant has been so haphazard that the exact cumulatve dosage to residents may never be known.

That prospect was raised yesterday by congressional and executive branch sources who said an organized monitoring system had not been put into place as of yesterday afternoon.

A spokesman for Metropolitan Edison Co., which operates the plant, said late yesterday that there had been monitoring by the company, the Nuclear Regulatory Commission, the Pennsylvania Department of Environmental Resources and even the state police.

He conceded, however, that organizing their findings had been a problem.

The cumulative radiation does is important in determining whether low levels-such as those being observed outside the Three Mile Island facility-will have an effect on the health of an individual in the future.

The Harrisburg accident comes at a time when scientists in and out of government are debating the longterm health effects of low-level radiation.

A series of recent controversial health studies have turned up increased cases of leukemia and other cancers among individuals exposed to extremely low radiation levels, sometimes 20 years before the appearance of thedisease.

The Metropolitan Edison spokesman said yesterday that, because the radiation levels measured outside the plant were so low, cumulative dosage levels were not that important.

He added that the filters on the plant's stacks took out the most dangerous radioactive particles-iodine 131-and permitted only radioactive gases into the atmosphere. Those gases, he said, would pass through an individual's body without causing damage to human cells.

Pennsylvania Gov. Richrd Thornburgh, however, yesterday cited "continued presence of radioactivity in the area and the possibility of further emissions" as the reasons he advised preschool children and pregnant women to evacuate the area five miles from the plant.

In Washington, Sen. Edward M. Kennedy (D-Mass.) made it known that he plans to ask the Department of Health, Education and Welfare to set up a program that will keep track of all those persons exposed to radiation as a result of the Three Mile Island accident.

Kennedy said he plans to make his request during a hearing of his Senate subcommittee on health scheduled for this afternoon.

Until yesterday, officials at the Environmental Protection Agency, which has federal responsibility for setting radiation standards and monitoring them for the general populace, had stayed out of Harrisburg.

One official said the agency had asked the state if it needed monitoring help and had been turned down.

"We have no jurisdiction," was the way one EPA official put it yesteerday afternoon.

Another, with some exasperation, said, "It's unbelievable, isn't it, but we can't get into it unless the air mass [with radioactive material] starts to drift north" across a state line.

Yesterday, however, it was learned the EPA was sending a monitoring airplane it maintains in Nevada to Harrisburg. The plane, one source said, will begin its monitoring duties sometime today.

EPA Administrator Douglas M. Costle is scheduled to appear at the Kennedy subcommittee hearing to explain his agency's role in the Pennsylvania situation.

Kennedy also plans to question officials of the NRC to determine what facts have been gathered on individual exposures to date.

The radiation monitoring of discharges from Three Mile Island has been particularly difficult because the material involved is gaseous. As such it blows in different directions and cannot be reconstructed at a later date.

And, once a person has been exposed to this type of radiation, there is no way tell afterward how much he received. Harmful radiation can destroy or damage cells, but there is no way to detect such damage.

Government scientists have a similar problem today in trying to reconstruct radiation levels that occurred more than 20 years ago at nuclear weapons tests in Nevada. The reconstructions whether low-does levels absorbed at that time by soldiers maneuvering after those tests had some causal relationship with cancers that developed much later.

Harold Denton of the Nuclear Regulatory Commission said last night that, based on his agency's calculations, the total exposure to residents of the surrounding area amounted to 1,000 man-rems.

Referring to studies that show 10,000 to 20,000 man-rems would result in one to two excess cases of cancer in furture years, he said that one could estimate that the exposure in the Three Mile Island area would cause one-tenth to two-tenths of one cancer case that would not otherwise have occurred.

But he admitted that the 1,000 manrem ifgure he mentioned was a "crude estimate."

Another parallel can be found in the situation of the Utah residents who lived 60 to 100 miles away from the Nevada test sites.

According to one recent study, children exposed to some radiation from those tests developed higher rates of leukemia than those not exposed.

Again, scientists are trying to determine what possible radiation doses those children absorbed.

At today's hearing, kennedy said he plans to explore what HEW Secretary Joseph A. Califano Jr. the health implications of low levels of radiation of the type that have emanated from the Harrisburg plant.

The atomic Industrial Forum Inc., trade association of the nuclear power industry, has a three-day conference on low-level radiation beginning Monday.