As many as 21,000 people die prematurely east of the Mississippi River every year because of pollutants exhausted into the air by power plants burning coal and oil.

These people die at least one and as many as 15 years short of their expected lifespans, due mostly to heart and lung failure brought on by chronic respiratory diseases such as bronchitis and emphysema. The lung diseases are believed caused by sulfur dioxide gas and microscopic sulfate particles, combustion products of coal and oil that settle in the lungs of people inhaling them.

These are some of the conclusions of scientists at the Brookhaven National Laboratory in New York and the Carnegie-mellon University in Pittsburgh, who have been at work for two years on a portion of a still-secret energy study for the National Academy of Sciences.

Financed by a $3 million grant from the Energy Research and Development Administration, the study was designed to take a long and comprehensive look at the pros and cons of nuclear power and the alternatives to it. The study has involved the work of more than 200 scientists and economists and more than 50 institutions coast-to-coast.

The Brookhaven and Carnegia-Mellon findings warn that if the nation turns to coal as it principal alternate source of electricity, it can expect as many as 35,000 premature deaths by the year 2010 instead of the estimated 21,000 taking place right now. The study predicts the 35,000 deaths from lung disease even if electric power plants install expensive devices to scrub out 80 per cent of the sulfur exhausted by their smokestacks.

One of the most striking findings of the study is that the stack gases exhausted by power plants in the Midwest are roughly 10 times more harmful to people on the East Coast than to Midwesterners living within 50 miles of the power plants.

The main reason is that the tall (up to 900 feet) smokestacks built in the last 10 year s to carry air pollutants away from the power plants discharge them so high they're caught by prevailing westerly winds and brought to the East Coast. The study identifies power plants in Ohio, Illinois and western Pennsylavania as polluters of New York and New Jersey.

Another reason is that the sulfur dioxide that makes up most of the sulfur exhaust coming out of the tall stacks stays in the atmosphere a longer time. This gives the sulfur dioxide more oppportunity to combine with moisture, aerosols and dust to form sulfates, which scientists now believe are more toxic than the sulfur dioxide.

"When somebody breathes in a sulfate it could be in the form of something like sulfuric acid," Brookhaven's Ronald Meyers said in a telephone interview. "Humans have a chance to exhale the sulfur dioxide gas. Things like sulfuric acid stick to the lungs and cause more damage."

The Brookhaven-Carnegie-Mellon part of the study discusses the pollutants exhausted into the air by 266 power plants in the eastern half of the United States because they burn more sulfur-bearing fuels and because the population is denser than it is in the west.

The study says that the 266 power plants polluted the air in the year as recent as 1973 with 17 million tons of sulfur dioxide. About 75 per cent of the sulfur pollutant came from coalburning plants, the rest from oil burners. The plants most responsible for the pollution are in Illinois, Indiana, Michigan, Wisconsin an Ohio.

The study emphasizes that its figures of 21,000 premature deaths from sulfur pollutants is nothing more than an estimate. They study says it can only infer that the deaths were due to air pollution and not some unexplained cause.

But scientists involved insist it is the most careful study done so far on the health effects of air pollution. They say they have analyzed death rates and air pollution in more than 100 Eastern U.S. cities where they have identified increases in the death rate with increases in air pollution.

Scientists said they have also monitored weather and wind patterns in the same cities and have shown that increases in death rate came at the same time that winds carried more air pollution to the cities suffering higher death rates.

The study will recommend that if U.S. power plants are ordered to switch to coal as President Carter has suggested, they be ordered to burn coal with the lowest sulfur content. The study will also recommend that the Enviromental Protection Agency insist that all power plants burning coal be equipped with scrubbers to take out at least 80 per cent and as much as 90 per cent of the sulfur before it leaves the smokestack.

"The implications for the increased use of coal are grim," the study states. "We recommend that these implications be viewed and studied at the highest levels of government."