More than 1,200,000 Americans may be exposed to highly or moderately serious health hazards because they live near 645 toxic waste disposal sites studied in detail by the Environment Protection Agency.

EPA prepared this latest estimate for Sen. Edward M. Kennedy (D-Mass.), whose Senate Health Subcommittee will consider the study today.

Given the fact EPA thinks some 30,000 dump sites may pose significant health problems, this means "millions of Americans" -- far more than anyone has estimated -- are taking "involuntary health risks every day, simply because of where they live," Kennedy said yesterday.

The latest figures in EPA's continuing survey of disposal sites show that:

Of the 645 sites so far classified, 108 are rated as highly serious in potential hazard, and 629,603 people could be exposed to their dangers, ranging from fires, fumes and contaminated water supplied to actual explosions of buried chemicals.

Another 106 sites are of medium seriousness, and 603,638 people could be exposed to them. Another 171 (with 400,762 people nearby) are of low seriousness, and 166 sites (with 342,745 people nearby) are still of uncertain seriousness.

Ninety-four sites, with 32,513 people in the vicinity, cause "no potential hazard."

"EPA is adding to these figures every day," said a Kennedy aide. "These re just the latest figures on their computers."

EPA has said that there are some 50,000 dump sites throughout the country being improperly operated or maintained, that as many as 30,000 may pose "significant" health problems and 1,200 to 2,000 may pose the most "substantial" or worst problems.

"The degree or magnitude of the risk is not really known," Dr. David P. Rall, director of the National Institute of Environmental Health Sciences, said last month. The 30,000 sites with significant health potential "contain a large number and variety of chemicals which may produce cancer, disorders of the central nervous system, reproduction disorders and many other illnesses," he said.

In a statement prepared for today's opening of his subcommittee hearing, Kennedy called the total scope of the problem "staggering" and said "we don't know where all the dump sites are" even yet, or what chemicals have been dumped.

Kennedy called an environmental emergency response act sponsored by Sen. John C. Culver (D-Iowa) "an important first step," but the Massachusetts senator said he will introduce amendments to public health legislation to identify populations at risk, screen for illnesses and develop treatments.