A shooting victim feels the piercing pain of a bullet wound immediately. The victim of an accident involving hazardous chemicals often smells the poisons. But the victims of radiation exposure can neither see, hear, feel nor smell the potentially damaging radioactive rays and particles as they penetrate the skin.

Because of this, the mention of radiation often strikes fear in the public. While such concern is heightened worldwide by an event such as the Soviet power plant accident at Chernobyl, experts warn against an all-or-nothing view of radiation and its potential health effects.

Extremely high levels of radiation can cause severe or life-threatening illness within weeks, days or even minutes -- as may have happened near the Chernobyl plant. The likelihood of serious effects is reduced as the exposure levels decrease, and depends on the amount of exposure, the timing and the type of radiation involved, as well as the susceptibility of those exposed.

Possible long-range effects include higher incidence of cataracts, cancer, risks to the unborn and genetic damage that could be passed on.

Radiation has assumed mythical qualities, as much a symbol of fear as a technical word. But in crises involving nuclear technology, its meaning becomes concrete and important.

Radiation is not a single type of material, but comprises atomic particles of different types -- common, everyday particles that make up the world and our bodies. Their power to cause havoc comes when they become part of a nuclear reaction.

In a nuclear reaction, a common element such as iodine can become extremely agitated and rise to a higher level of energy in which it can pick up extra electrons, protons and neutrons. Thus it becomes an isotope of iodine, and instead of being counted as iodine-127, the natural variety, it becomes the excited and unstable iodine-131, for example.

Elements must fall back to their normal states. They do this by firing off their extra energy and particles. What they fire off is known as radiation -- particles of light called gamma rays, electrons called beta particles, or a proton and neutron pair called the alpha particle.

With time and a peaceful place to return to their usual states, radioactive materials are not harmful. But when collected in large numbers inside the fuel rods of a nuclear plant, then in an accident shot into the air, they amount to billions and billions of silent little bombs.

A cloud of radioactivity consisting mainly of iodine-131 and cesium-137 -- the type now over the Soviet Union and parts of Europe -- has particles constantly "popping off" radioactivity, some of which hit the body directly.

Other particles can fall to Earth and be picked up on the skin, inhaled or swallowed.

In either case, many of the particles do not fire off their radiation immediately, but do so eventually. With iodine, about half the radioactive particles pop off within eight days, which is the "half-life" of radioactive iodine. Cesium has a half-life of about 30 years, so its danger continues much longer.

Radioactive elements are considered relatively safe after about 10 half-lives have passed -- 80 days for iodine, 300 years for cesium.

These excited, radioactive atoms work on the body like little time bombs. They fire off their particles in the midst of cells, killing or injuring them. Or, especially in cases of medium to low doses of radiation, the particles shoot into the cells and hit the chromosomes.

Two or three hits in the right points can alter the cells' genetic instructions, ordering them to begin growing out of control, to become cancers.

It can take from two to 20 years for these cancers to develop, once the mistaken seeds are sown in the cells' instructions.

One of problems with understanding how much radiation is a health hazard is the fact that radiation is found naturally in the environment, in the form of high-energy particles from space known as cosmic rays as well as radioactive substances found in the earth. While such "background" radiation averages about 100 millirems per year, it varies with location, increasing at higher elevations.

Dr. Richard Reba, director of nuclear medicine at George Washington University Medical Center here, said that simply going from the coast to Denver to live for a year could roughly double one's exposure to background radiation.

The amount of radiation that might reach the Uhited States from the Soviet accident may be so low as to be hard to measure above the background levels, said Dr. Darrell McIndoe, a nuclear health effects specialist at St. Joseph Hospital in Towson, Md.

Figuring the amount of radiation can be done in various ways, the most common of which are rads and rems. A rad is the amount of radiation that deposits a unit (100 ergs) of energy in body tissue. A rem is a rad multiplied by a number describing how damaging the type of radiation is.

Exposure is often expressed in millirems, or thousandths of a rem.

Common manmade exposures include chest X-rays, which can give from 10 to 25 millirems.

In Sweden on the peak day of radiation measures after the Chernobyl accident, 30 millirems were counted, or 100 times normal. In the Soviet Union, however, the levels deposited near the reactor were estimated to be about 100,000 millirems.