The average American's cholesterol level has dropped 5 to 10 per cent since the early 1960s and this may have helped cause a sharp drop in deaths from heart and blood vessel diseases, officials of the National Heart, Lung and Blood Institute said yesterday.

Cholesterol is a fatty, waxy substance that tends to clog arteries and cause heart and blood vessel ailments. In the last 30 years there has been a vast amount of publicity about the dangers of overeating high fat and high cholesterol foods, and there have been profound changes in the average American diet.

"What we think we are seeing now are the effects of great changes in the lifestyles of men and women who have been reading and listening," Dr. Robert I. Levy, director of the heart institute, said in an interview.

"We can't prove or be sure yet that diet and other changes - like 25 per cent less smoking among middle-aged men - have contributed to the dramatic decrease we are seeing in cardiovascular deaths," he said.

"But it's looking very much as though this is the case, and this is very encouraging."

Levy said that "there is no doubt today" that a high blood cholestorol level is "a major risk factor" for heart disease although it may take several years to prove beyond doubt whether lowering cholesterol levels can reduce heart attack deaths.

Also important, he said, has been a national campaign against high blood pressure that began in 1972. Persons with high blood pressure have a high incidence of stroke and heart attacks.

Since 1972, screening campaigns have found early, previously unnoted high blood pressure in 8 million persons. There has been a 50 per cent increase in patients under treatment, with most of them now on drugs or diets to keep their blood pressure down.

Health officials first began noticing the drop in heart and stroke deaths about two years ago. In 1973 in the United States there were 1,062,000 deaths from heart and blood vessel disorders. Last year, with a larger and older populations, there were 975,000 such deaths. On a per capita basis, there were 15 per cent fewer such deaths than in 1960, Levy said.

He called the drop in the average adult's cholesterol level a surprise that has been noted only in recent months with analysis of data on 70,000 persons under observation at 12 lipid (blood fat) research clinics launched in 1971 and 1972 by the heart institute, which is part of the National Institutes of Health in Bethesda.

Levy and colleagues told a news conference that the cholesterol drop may be largely confined to persons with higher education and higher paying jobs. Men and women of higher status tend to have lower cholesterol levels than other Americans, except for low-pay workers who perform heavy exercise. Many doctors say exercise may also reduce cholesterol and cut heart attack risk.

The figures on the cholesterol decline are also based so far only on observation of whites. The lipid clinics are just starting to analyze their early data and aren't ready yet to report on the 10 per cent of their patients who are black.

Levy and his co-workers advised a "prudent diet" to cut heart attack risk - on low both in cholesterol and the "saturated fats" that are most prevalent in meat, animal fats, butter and eggs.

The heart doctors also reported that cholesterol and other lipid levels are increased by as much as three times in women who take birth control pills. They said their doctors would probably advise many such women to use other contraceptives or stop taking hormones, as well as to eat prudently.

Most doctors advise patients to try to lower their cholesterol if it is over 250 or so milligrams per 100 milliliters of blood plasma. "But some people say the ideal level might be about 200 milligrams," said Dr. Basil Rifkind. "The average U.S. level is 210 and we still have too many heart deaths," he said.

One 1962 study showed an average cholesterol level of 225.