At the top of every ski slope, economist Lester Lave faces a choice.

He can loosen the screw on each of his ski bindings so they will release more easily, reducing his risk of serious injury but possibly cutting short an exhilarating run. Or he can throw caution to the wind and tighten the screw, knowing that the binding will release only under great force.

It's only natural that Lave, an expert in risk assessment and president of the Society for Risk Analysis, should see his favorite sport as an exercise in risk management - a classic trade-off between fun and safety.

"Every time, at the top of the hill, I make that choice," said Lave, professor of economics at Carnegie-Mellon University in Pittsburgh. "A quarter turn to the right or left."

Usually the method works. But twice he took spectacular falls that resulted in serious injury - a broken shoulder and a cracked vertebra.

Love Canal, Bhopal , Tylenol, Gadhafi, Challenger, Chernobyl. Cancer, AIDS, toxic shock. Saccharine, asbestos, vinyl chloride. Acid rain, yellow rain, red tide, Agent Orange. VDTs, PCBs, DDT, EDB. This is the age of risk.

Or is it?

"We live in a less and less risky society, and yet we're more and more anxious about it," said Edward Burger, director of Georgetown University's Institute for Health Policy Analysis.

A Harris poll found that more than three of four Americans believed life was riskier in 1980 than 20 years earlier.

"How extraordinary!" wrote Aaron Wildavsky, political scientist at the University of California at Berkeley and co-author of "Risk and Culture." "The richest, longest-lived, best-protected, most resourceful civilization, with the highest degree of insight into its own technology, is on its way to becoming the most frightened. Has there ever been, one wonders, a society that produced more uncertainty more often about everyday life?

"It isn't much, really, in dispute - only the land we live on, the water we drink, the air we breathe, the food we eat, the energy that supports us. Chicken Little is alive and well in America."

Nearly 2 million Americans planning trips abroad this year changed their minds in February, apparently out of fear of terrorist attacks, according to the U.S. Travel Data Center. And that was "before" the U.S. bombing of Libya and the Soviet nuclear accident at Chernobyl.

Statistics suggest they're overreacting - more Americans drowned in their bathtubs last year than were killed or injured in terrorist attacks. But perception of risk is more than a numbers game.

After all, this is a society where an estimated 1,000 Americans die each day from smoking-related causes, and about 10,000 die each year because their seat belts weren't buckled - but the artificial sweetener cyclamate was banned in 1969 as a potential human carcinogen.

What matters to most individuals is not the size of a risk, but whether it is seen as acceptable, said Chris G. Whipple, an expert in risk assessment with the Electric Power Research Institute.

"You don't know whether they think the risk is one in two or one in a billion," Whipple said. "You just know they're canceling reservations."

The risk Lave takes on a ski slope is

statistically much greater than his risk of being caught in a terrorist attack - he's planning a family trip to Italy this month - or being exposed to radiation from a nuclear accident. But skiing is a different kind of risk.

It's voluntary, well-known and fun. Though the risk of breaking a bone is considerable, the risk of dying in a ski accident is almost nil. And the skier has at least the illusion of being in control.

"Most people I know feel safer when they're driving an automobile than when they're sitting in the back of an airplane," Whipple said. "The numbers say they're wrong."

The risk per mile of dying in a car is about 10 times that of dying in a commercial airplane.

The effort to define that difference is called risk analysis, which puts percentages and dollar signs on chancy human behavior and "acts of God."

A couple of decades ago, risk analysis was an arcane concept. Now it's a growth industry (there are 1,100 members of the Society for Risk Analysis) called upon by businesses, utilities, labor unions, insurance companies, hospitals and government agencies for help in estimating and controlling risks from toxic chemicals, accidents and other hazards.

In part it's an offshoot of the consumer and environmental movements of the 1960s and 1970s, signaled by two influential books, Ralph Nader's "Unsafe at Any Speed" and Rachel Carson's "Silent Spring." These movements spawned unprecedented government regulation of automobile safety, air and water pollution, food additives, chemicals and consumer product safety, which in turn prompted demands for better estimates of risk and benefit.

"We have tended to do our risk management in an insanely simplistic fashion," Lave said. "(We said,) `My God, it's a carcinogen - ban it!' "

"Basically the lawyers for both sides called up the most extreme scientists they could find on both sides, and they fought it out in adversarial proceedings," said Elizabeth Anderson, former director of the Environmental Protection Agency's office of health and environmental assessment.

"Thinking about uncertainty is very difficult for people - for experts as well as laypeople," said Paul Slovic, a psychologist and expert in risk analysis at Decision Research in Eugene, Ore.

Experts know a lot about the technical aspects of, say, nuclear power or food chemistry. But they tend to define risk narrowly, in terms of factors that can be reduced to numbers, such as how many people died last year from a given hazard.

The public, on the other hand, often cares less about numbers than about other factors, such as whether a hazard is new or old, known or unknown, controllable or uncontrollable, voluntary or involuntary, and whether it is potentially catastrophic. Fear of nuclear radiation, for example, is heightened by each of these considerations.

In short, the experts worry mainly about quantity of risk, while laymen worry about quality of risk.

"There's a kind of wisdom and foolishness on both sides," Slovic said.

Experts can offer the public numbers and estimates to put risks into perspective. But public perceptions can warn the experts against using sheer statistics to compare risks as different as living near a nuclear reactor and drinking diet sodas.

In general, according to another study by Slovic and his colleagues, people overestimate rare but highly publicized causes of death - such as botulism and tornadoes - and underestimate common causes that kill one person at a time - such as asthma, emphysema and stroke.

Homicides and stroke were seen as about equally common causes of death, when in fact stroke kills 11 times as many. People estimated that tornadoes and asthma kill about 500 Americans a year, when in fact tornadoes kill about 100 a year and asthma kills about 3 ,000.

The way to size up a risk is to compare it with other, more familiar risks, said Bernard Cohen, a physicist at the University of Pittsburgh. He has compiled a life expectancy reduction (LER) scale that ranks hazards according to how much they shorten the average American's life.

Heart disease tops the list by lopping 2,100 days, or nearly six years, off the average American's life. Cigarette smoking, cancer, being overweight and being unmarried all have an LER of several years. By contrast, tornadoes shorten the average life span by a single day.

Risk assessment is at best an imperfect science. Estimating the risk of a potentially cancer-causing pesticide or food additive, for example, depends on a myriad of factors, some of which are unmeasurable or even unknown.

Risk estimates themselves run the risk of losing credibility when they fail to acknowledge the uncertainty by including a possible range of error.

Many risk analysts see the public's preoccupation with risk as ironic. Some sociologists suggest that leisure-oriented baby-boomers have the "luxury" of time to worry about risk. Whatever the reason, Americans today seem less willing than their forbears to accept risk as a price of living.