Once in a while a set of findings comes along that could explode like a bomb in the public arena. One is that life expectancy in the United States has been going way up. Not just those marginal numbers about infant survival, either.

In the last dozen years, the gain in life expectancy for adults has even surpassed that made between 1940 and 1954 with the introduction of antibiotics. In fact, the recent rate of gain exceeds that of any other time in this century.

In the 1940s, for instance, a typical 45-year-old American could expect to live to age 72. The most recent data show a typical 45-year-old living to age 77 -- an increase in life expectancy of more than 18 percent.

The increase at the older ages are even more dramatic because of the lowered death rate from heart disease and stroke. A person is 65 today can expect to survive to age 81.

The life-expectancy projections for the rest of this century may turn out to be even more significant. Prof. Eileen Crimmins of the California Institute of Technology has studied the relevant medical data and says it is likely that "we have entered a new era of mortality decline, one concentrated at the older ages." She projects that someone who is 65 years old in the year 2000 will survive to age 87.

Her estimates are pretty high, but they are not even close to those made by the respected French demographer, Jean Bourgeois-Richot who says that 65-year-olds in Western countries should expect to live almost to age 100 by the year 2000. A recent report from the World Health Organization verifies the direction of the changes, if not the actual numbers.

All of this leads to a striking, if subversive, notion: We live in a healthy society, getting still healthier. That you will notice is directly contrary to the notion that we live in an unhealthy society, getting still unhealthier.

We have heard about air pollution, water pollution, noise pollution, toxic wastes, carcinogens, low-level radiation, nuclear hazards, DDT, cyclamates, ozone layers and stress -- to just begin a long, long list. We have heard that "the enemy is us." American economic effort has been described as "the Gross National Pollution."

This perceived unhealthiness has, of course, generated a small army of lawmakers making laws. After all, we live in a responsive society; sooner or later, our legislators respond to perceived conditions.

Now the perception will change -- or at least it should. We have to accept the fact that we live in a healthy society, getting healthier. Are we any longer capable of facing up to the good news?

First to face that music should be environmentalists. Have they perhaps built their castle on sand? They have in recent years helped change industry, politics, culture and birth rates by drumming home one simple notion: Modern society is hazardous to your health.

That notion was accepted by legislators, professors, students and a lot of people who read the article in the paper that said that designer jeans cause cancer. The result has been bewildering array of environmental laws and regulations, some quite sensible, some bizarre.

But the question before the house is not whether there are some environmental conditions that seriously harm us. Of course there are, and there ought to be laws and tough ones.

The real question is: What kind of laws would we have if prevailing opinion urged us to improve an already healthy society instead of screaming "stop poisoning the planet"?

A case in point: Would the Clean Air Act have been passed in the form that it was a decade ago if lawmakers hadn't been health-mongered by environmental activists hit-listing the "dirty dozen" legislators while predicting that a Silent Spring of poisonous degadation was just over the smoggy horizon?

That law costs the public and private sectors about $30 billion a year. Yet, eminent scientists, in and out of government, today ask whether there is any -- repeat, any -- good evidence of important health or life-expectancy gains due to the clean-air regulations.

Pittsburgh cleaned up its air decades before the rest of us. A good thing, too. But there has been no evidence that the once-smoky city improved its health any faster than the rest of the United States.

What sketchy evidence exists begs this question: Is the Clean Air Act worth the whole $30 billion, or could some of that money be spent on other things with better payoffs? (The sharp gains in life expectancy began well before the new environmental laws took hold.)

So now, a new premise: What will be the ripples from the idea that we live in a healthy society, getting healthier?

One such ripple will be prosaic and difficult: There will be greater chaos in the Social Security system as estimates have to be changed to account for increased life expectancy. Brace yourselves for screeds that denounce this heartless society for maltreating its elderly. Better yet, put away more of your own bucks for your own retirement.

Other ripples operate at another level. We were told that ours was a sick society. We were even warned that it might be irresponsible to bring children into such a world. We were told that we should lower our expectations. We were told that America was over the rim, a decaying, polluted suburb of history.

We were told a lot of things that weren't so. We should each, typically, have many extra years left to think about that.