Call it strain. Call it pressure. Call it tension. By any name, stress is almost as certain as death and taxes.

"There is no such thing as a stress-free environment," says Dr. Philip Gold, a stress researcher at the National Institute of Mental Health. "There are situations where stress is not overwhelming, where it's something that people might call part of the fabric of a rich life."

Most people perceive of stress as negative. That's probably because, by definition, it is "a perceived threat or demand which somehow exceeds one's capabilities to easily deal with it," says medical psychologist Andrew Baum of the Uniformed Services University of the Health Sciences.

Yet stress has a positive side. Living without stress is unthinkable -- it is the emotional equivalent of friction. Imagine trying to walk without friction, or attempting to ride a bike without the resistance of the road. It simply can't be done.

The same goes for stress. Low levels act as motivators, as devices that challenge and promote interest. For example, the stress of failing a quiz can prompt a student to buckle down, study and ace the final exam.

But none of the potential positives overrides the well-known connection between too much stress and disease. From allergies and asthma to cancer and heart attack, numerous studies suggest that high levels of stress help promote illness by altering the immune system and placing an added load on the heart and blood vessels.

New research is now fine-tuning that picture. Studies are indicating that the relationship between stress and disease is not a simple cause-and-effect connection, but rather a complex equation that can change with the situation and the individual. Approach a crisis with a different outlook, throw in greater control or more flexibility, and suddenly the tension-filled situation takes on a whole new demeanor. This may explain why some people appear able to let pressures and stress roll off their backs despite seemingly impossible situations.

Among the most recent findings:

*Stress seems closely related to anxiety and depression. The biggest clue for this linkage is a brain chemical called corticotropin releasing hormone (CRH). During stress, anxiety and depression, the brain produces extra CRH, thereby setting off a series of chain reactions throughout the body.

"We know that small amounts or moderate amounts of stress are alerting," explains Gold of NIMH. "We think better, we move faster and we adapt better." But too much stress tips the scales the wrong way. Then people "can't function," he says. "They can't remember, they can't adapt to a situation. They make the wrong choices . . . They feel demoralized and lose confidence in themselves . . . It is catastrophic."

*Stresses can accumulate, but the amount of damage depends in part on the background and resources of the person under stress. In a long-term study of 200 third- to sixth-grade children, psychologists Norman Garmezy and Ann Masten of the University of Minnesota found that children with high IQs, high family incomes and positive family backgrounds perform better than average in high stress situations, such as divorce or moving. Social support of families and relatives seemed especially to benefit girls in these situations. Boys were more apt to become disruptive at high stress levels, regardless of social support.

*The way a person perceives stress can go a long way toward blunting its ill effects, even with chronic, long-term exposure to stress. Studies of people living near the Three Mile Island nuclear power plant found that residents have slight suppression of the immune system, increases in blood pressure and higher levels of stress hormones. The worst effects appear in those residents who feel powerless and blame their predicament on others, says psychologist Baum, who is conducting an ongoing study.

*Job stress by itself is "not problematic," reports psychologist Leonard Syme of the University of California at San Francisco. "But high job stress with little latitude or flexibility causes trouble." Bus drivers are a prime example, says Syme. His continuing study of San Francisco bus drivers shows that they have signficantly high rates of high blood pressure.

Bus drivers often start their shifts behind schedule and race against the clock to meet an unrealistic timetable. Since they get demerits for being late, drivers try to catch up by cutting out rest stops and lunch breaks. Added to this is heavy city traffic, passenger hassles and roving supervisors who mingle with the crowd like undercover agents -- all working to drive up blood pressure in drivers. Worldwide, bus drivers have "an increased incidence of high blood pressure, musculoskeletal disorders and gastrointestinal problems," Syme says.

But some jobs with high psychological strain take the least toll on health. The significant difference is that these jobs also give employes a great deal of control. Among the jobs that fit this bill are architect, forester, natural scientist, dentist, skilled machinist and auto repairman, reports Robert A. Karasek, an industrial engineer at the University of Southern California, following four studies of some 5,000 American and Swedish workers. By often setting their own hours and having flexibility and latitude on the job, these workers seem less vulnerable to the effects of stress.

In contrast, waiters and waitresses, assembly line workers, freight handlers and garment stitchers are among those with "high psychological demands and low control," Karasek says. People in those occupations "have a higher prevalence of myocardial infarction heart attack ," he says, than those with greater control over their jobs.

Another example of how control -- and skill -- can help cut the effects of stress is in athletics. Professional players often face high stress. In a recent basketball game against the Washington Bullets, Boston Celtic Larry Bird sunk a foul shot in the last few seconds of regulation time to tie the score and put the game into overtime. For Bird, who makes 90 percent of his foul shots, "the demands of were extraordinary," says University of California's Syme. But so, too, is his sense of control over the situation. "His skills and capabilities," Syme notes, "are very, very high as well."

In that situation, the coach and the fans are more apt to feel stressed, because they have no control over the outcome. "The coach can't grab the ball and shoot it for Larry Bird," Syme says. The fans "are forced to not do anything."

One reason people often have trouble managing stress is that the body is programmed to maintain an equilibrium. Stress is "the reaction of our bodies to any threat or change" in the status quo, says Dr. George Chrousos, a stress researcher at the National Institute of Child Health and Development (NICHD).

When the temperature rises to the 90s, the body responds by sweating to lose heat and cool off. Eat a big meal, and the body shifts the gastrointestinal system into action to digest food for immediate use and store the rest. Get angry or become frightened, and the body mobilizes resources with the primitive response known as "fight or flight."

Physical stresses include "malnutrition, pain, surgery, anesthesia, blood loss, burns, infection and inflammation," Chrousos says. Emotional stresses range from the separation of a young child from a parent to the bereavement after the death of a spouse or friend.

How a person reacts to various forms of stress depends on past experience and probably genetics. Studies of monkeys by psychologist Stephen Suomi at NICHD suggest that perhaps a quarter of animals are born with biochemical temperaments that make them "uptight."

Similar research with human infants by psychologist Jerome Kagan at Harvard University suggests that some children, too, may be born more susceptible to stress. Studies are under way to identify these youngsters early and help teach them how to be less stress-prone.

Regardless of temperament or experience, "even the most self-confident person -- if in a situation for which they have no experience and no talent -- will perceive it as stressful," says Charles Spielberger, a University of South Florida psychologist who specializes in stress and anger research.

Lack of time and the perception of lack of control also work to increase stress. One example is a study of some telephone operators in North Carolina conducted by Susan Haynes and Andrea LaCroix and the University of North Carolina at Chapel Hill.

The operators use video display terminals to answer information calls requesting telephone numbers. Each time an information call is dialed, the operators have a limited amount of time -- often just 15 seconds -- to get the requested number on the screen. As they work, their terminals record their speed. Miss the 15-second deadline three times, and the operator is fired. Like the bus drivers in San Francisco, these telephone operators show increased levels of stress plus physical effects including angina -- one of the more serious symptoms of heart disease. Operators who do not work on VDTs show much lower rates of angina and other stress-related illnesses.

Stress hormones have wide-ranging effects on the body. They are released into the body when the brain receives the signal (either through vision, hearing or smell) that danger is near. The human stress response is derived from the "fight and flight" response, which kept our primitive ancestors alive by either combatting or fleeing from an agressor.

This same reaction is triggered if you find yourself in the path of an careening car. The hypothalamus releases the hormone known as CRH. Several hundred molecules of CRH, Gold says, pour into the cleft between the hypothalamus and a the nearby pituitary gland, which lies near the base of the brain.

Release of CRH sets in motion a chain reaction in the body that leads to increased speed and strength.

In response to the CRH, Gold says, the pituitary releases millions of molecules of a chemical called ACTH (or adrenocorticotropic hormone). ACTH travels through the blood to the adrenal glands, located at the tip of each kidney. The adrenal glands then produce cortisol, another chemical, which warns glands throughout the body to prepare for stress. The whole process takes just "30 to 45 seconds," says Gold.

In experiments, when cortisol was injected into participants in a study, "they actually felt a bit of a high," Gold says. "It gives them confidence."

At the same time, part of the nervous system, known as the sympathetic nervous system, also is stimulated, and calls into action the chemical messenger norepinephrine. The result is that heart rate speeds up, many blood vessels constrict and blood pressure rises. Next, epinephrine is pumped from the adrenal glands. Blood vessels supplying muscles then dilate to provide more oxygen and energy, while those servicing the stomach and intestines constrict.

Meanwhile, the liver is instructed through chemical messages in the blood to increase blood sugar (for added energy) and to shift from storing protein to converting it to sugar.

All these reactions prepare the body to fight or to flee.

In other ways, the body gets ready to repair itself in case it can't escape -- and is injured in a confrontation.

"The hypothesis is that if you look at cortisol's effects, one of its most pronounced is to depress the immune system," Gold says. This could explain why stress can lead to disease.

Why the body suppresses the immune system in response to stress is not known, says Gold, but he suggests that the body may rein in this powerful response so it doesn't overreact, and actually attack the body, in the case of injury.

"It all makes sense," says USUHS's Baum. "So if you get into a fight, you won't bleed as much. It's a response by the body that tears apart stored energy and makes it available to the body very quickly and allows you to act very strongly."

But like a strong, pungent spice, stress works well if you add just enough to enhance the flavor of a dish. Use too much or too little and the meal is ruined.

"If you can't turn off that vasoconstriction, you become hypertensive have high blood pressure ," Gold says.

"What happens when a stressor doesn't go away real quickly?" asks Baum. "What we're finding is that people remain in these stress states for long periods of time. At Three Mile Island, it's been more than five years. It can't be good for the body to remain in a heightened state of arousal."

Continuing studies of people who have lived near the nuclear reactor since the 1979 radiation leak there show increased levels of cortisol, epinephrine and norepinephrine in their urine -- an indication that they remain in a chronically stressed state. "We don't know what the effects are because the elevation isn't all that high."

Baum and his colleagues are finding unusual immune system suppression among the residents near the plant when compared with a control group in Frederick, Md. "We have found some evidence that blood pressure has increased on a fairly consistent basis," he says. Approximately 10 percent of the TMI area population has developed high blood pressure since the accident, he says, "noting that this is a very young sample."

All this points to the conclusion that "we're beginning to get some evidence of some kind of physical dysfunction that's accompanying these long-term stressors," Baum says. The findings are similar to, though much less severe than, the effects of post-traumatic stress disorder seen in Vietnam veterans, Baum says.

And they fit with some of the patterns that researchers Gold and Chrousos have seen in some of their severely depressed patients. If a person is continually stressed, and becomes depressed, then physical changes may occur in the brain that leave the person more prone to being depressed again, says Gold, who bases his theory, in part, on animal studies. If this process occurs again and again, depression may then appear without the stress.

Near TMI, Baum is finding biochemical evidence that people "are more stressed, more anxious and seem to be more depressed, particularly when there are periods of uncertainty up there, when they are not quite sure what is happening at the reactor," he says.

Anger, stress and anxiety are "normal emotional reactions," notes University of South Florida's Spielberger. "Darwin observed them in both animals and people and considered them an innate response."

Those reactions "were adaptive for our cavemen ancestors," he says. "But the problem in modern society is that you can't run away from a lot of threats. You can't attack or get angry.

"People have got to learn other ways of coping with these primitive emotions. I think if people had a better grasp of that, then they could recognize when they were anxious or angry and could use relaxation or other coping techniques to adapt."