Early reports from Bhopal, India, said 400 people were dead. In the year that has passed since poisonous methyl isocyanate gas leaked out of Union Carbide plant, it has become clear that the health effects of the disaster were far more severe than originally stated.

It now appears that at least 2,000 people, and by some estimates 10,000, died. The extent of lung damage and other health effects on the survivors are equally difficult to gauge, since many victims were poor and transient. But one widely reported estimate is that 200,000 suffered some injury from the gas, and perhaps 40,000 are still suffering severe, perhaps lifelong, effects.

In short order, the Dec. 4, 1984, accident brought 121 lawsuits seeking $100 billion in damages for more than 100,000 plaintiffs. Union Carbide, in a report March 20, for the first time suggested that the accident may in fact have been caused by sabotage.

"It's clear that the initial euphoria that this case would be quickly settled has dissipated," said Kenneth Feinberg, a Washington lawyer who specializes in such mass lawsuits. "What you have now is a war of attrition where both sides are hunkering down . . . The perception is that this is going to be a prolonged legal struggle."

Meanwhile, Union Carbide said that at at another plant in Institute, W.Va., there had been 190 chemical leaks over five years. Then in August, a cloud of toxic gas leaked from the plant, sending more than 125 people to doctors and briefly closing nearby highways. The chemical was identified as aldicarb oxime, a less toxic derivative of the gas that leaked in Bhopal.

Conflicting theories remain about why the Bhopal leak was so deadly. Studies suggest that the methyl isocyanate gas in the tanks broke down into deadly hydrogen cyanide during storage, causing the accident to be especially devastating and leading to the long-term medical complications. Had this chemical breakdown been predicted, other antidotes might have been used.

According to the National Academy of Sciences, there is a similar lack of knowledge about the health effects of the other 54,000 chemicals used in commercial products. -- Paul Berg MARGARET HECKLER Changing of the Guard at Health & Human Services

On Oct. 1, Health and Human Services Secretary Margaret M. Heckler bowed to pressure from within the administration and resigned to become the new ambassador of Ireland.

Heckler's 2 1/2-year term as HHS secretary was often rocky. And in the end, a combination of her messy divorce proceedings plus what some called a feisty and sometimes temperamental manner seemed to do her in.

Heckler was one of the first in the Reagan Administration to publicly acknowledge acquired immune deficiency syndrome as a major public health issue. Funds targeted during her administration helped pay for the research the enabled National Cancer Institute researcher Dr. Robert Gallo to be among the first to identify the HTLV-3 virus as the cause of AIDS.

Heckler also backed strong legislation for collecting child support from absent fathers. She supported Medicare aid to hospices and protection of the nation's medical research establishment. She singled out the debilitating Alzheimer's disease for special attention. Most recently, she had been planning a special initiative on breast cancer.

Now this is left to her successor, Dr. Otis Bowen, 67, a former two-term governor of Indiana and a practicing physician for more than 44 years. Bowen was sworn in Dec. 13. Like all HHS secretaries, part of his job will entail sparring with the powerful Office of Management and Budget, which often has taken a knife to various health and welfare programs. After being sworn in Dec. 13, Bowen entered his first battle with OMB: a tug-of-war over proposed Medicare cuts.