Radiation is silent, invisible, odorless -- and everywhere. Every person on earth is being constantly bombarded with low levels of natural radiation from cosmic rays and tiny amounts of radioactive elements in soil, water, rock, food and the human body itself. But the level of this "background" radiation -- about 100 millirems -- is so low that its effect on human health cannot be measured.

At very high doses, such as apparently occurred last week at the Chernobyl nuclear power plant in the Soviet Union, radiation can be lethal. But at low doses, such as from background sources or the expected fallout in the United States from the Chernobyl accident, radiation is much less harmful.

All radiation has some effect on the body. An expert panel of the National Academy of Sciences concluded in 1980 that there is no threshhold below which radiation has no adverse effect on human health. But the exact nature and extent of the effect of low-level radiation is still unknown.

"The main thing for the public to understand is that with radiation, it's not all or nothing," said Dr. Niel Wald, chairman of the department of radiation health at the University of Pittsburgh School of Public Health. "Many people think wrongly that if it's radiation, you're harmed; and if it's not, you're not."

Normal background radiation comes from three basic sources:

*Cosmic rays and other high-energy radiation from the sun and outer space. A five-hour transcontinental flight at 39,000 feet, above the protective layer of the earth's atmosphere, exposes a passenger to about 2.5 millirems of cosmic radiation.

*Terrestrial radiation from naturally occurring radioactive elements, including radium and uranium in soil and rocks. Granite, for example, gives off minute but measurable amounts of radiation.

*Natural radioactive substances in all living things, including food and human tissues. These "internal emitters," such as potassium-40, carbon-14 and radium-226, "have been around for millions of years," said Neal Nelson, a radiobiologist in the Environmental Protection Agency's Office of Radiation Programs. "They're in everything -- trees, frogs, everything."

The average American, Nelson said, is exposed to 77 millirems a year of natural radiation -- 53 millirems from cosmic and terrestrial radiation and 24 millirems from radioactive materials in food and the body itself.

Actual exposure varies with altitude, climate, soil and occupation. A typical resident of mile-high Denver near the Rocky Mountains, which contain uranium, receives 104 millirems a year of cosmic and terrestrial radiation. A typical resident of the District, by comparison, receives only 49 millirems a year.

Medical and dental X-rays add between 50 and 100 millirems per year for the average American. Exposure from a chest X-ray, for example, averages about 15 millirems.

Other non-natural sources -- mostly fallout from above-ground nuclear bomb tests before 1963 -- account for less than 10 millirems of exposure per year. These sources include tiny amounts of radiation from nuclear power plants and even tinier amounts from color television sets, electron microscopes, airport inspection machines and glow-in-the dark watches.

Background radiation is estimated by the National Academy of Sciences to cause about 1 percent of all cancers. But whatever cancers are caused by background radiation are statistically "lost" in the vast numbers of cancers that occur each year, and cannot be distinguished from other cancers.

"There's no way to tell a cancer caused by radiation from any other cancer," EPA's Nelson said. "There's no little red tag on it."

The needle-in-a-haystack challenge of identifying radiation-caused cancers is illustrated by the accident at the Three Mile Island nuclear power plant in Pennsylvania in 1979. Scientists from the Food and Drug Administration, the Nuclear Regulatory Commission and EPA tried to estimate the health effects of exposure to the radiation leak.

Among the 2 million people living within 50 miles of Three Mile Island, the scientists concluded, the leak would cause one fatal cancer and one nonfatal cancer or genetic defect. But since about 325,000 fatal cancers are expected to occur "normally" in a population of 2 million, finding the additional one or two deaths is impossible.

There are two main kinds of radiation: nonionizing, such as microwaves and radio waves, and ionizing, which comes from radioactive materials or X-ray machines.

Ionizing radiation alters normal atoms by knocking out one or more electrons to create charged atoms called ions. These highly unstable ions can damage human cells.

Even if it doesn't kill cells, ionizing radiation can short-circuit their information systems, damaging their ability to grow or reproduce. If the damage is too great for the body to repair, it causes radiation sickness or -- sometimes years later -- cancer.

Radioactive substances emit radiation in several forms, known as alpha, beta or gamma rays and X-rays. Gamma rays and X-rays penetrate the body easily; alpha and beta rays don't, but they can damage internal organs if ingested or inhaled. Milk contaminated with radioactive iodine, for example, gives off beta particles that can damage the thyroid gland.

Some radioactive substances stay radioactive longer than others. Iodine-131 loses half its radioactivity in eight days. But cesium-137 and strontium-90, which tend to concentrate in the bones, have a half-life of about 30 years.

Exposure to low levels of radiation causes no immediate symptoms in an individual, but may increase the statistical risk of getting cancer. Doses as low as 10 rads can damage the lymph nodes and limit growth of bone marrow, the body's "blood factory," increasing the risk of infection. But only above 50 rads do immediate symptoms show up.

Above 50 rads, radiation sickness is common, with symptoms of diarrhea, nausea and vomiting. Whole-body doses above 120 rads can be fatal within 60 days, and doses above 300 rads kill about half of those exposed. Above 700 rads, the entire population would die.

The very young, the very old and the chronically ill are most at risk from radiation. A fetus is especially vulnerable because its growth depends on rapid reproduction of cells with which radiation can interfere. Because radiation can kill cancerous cells as well as normal cells, radiation is used to treat some tumors. Some cancer patients receive massive doses of radiation -- as much as 10,000 rads. But comparisons between controlled therapeutic doses and accidental exposures can be misleading because anticancer radiation is usually aimed at a single organ or part of an organ. The effect is not the same as "whole-body" exposure to the fallout from a nuclear accident.

Evidence from studies on animals suggests that a high dose in a short period of time has more effect than a lower dose given over a long period of time, though EPA's Nelson cautioned that the relationship between dose and duration of exposure is not well understood at low levels of radiation.

For example, a person exposed to 100 rads from a bomb blast or nuclear power plant meltdown is likely to develop radiation sickness. That same person, exposed to 10 rads per year over a 10-year period, would not show any symptoms.

The difference could be compared to the difference between getting hit once by a speeding bullet and being repeatedly pelted with hand-tossed BB shot. It is still not known how many people were exposed to serious harm by radioactive fallout from the Chernobyl accident. But experts say only those at or near the plant could have been exposed to doses high enough to produce immediate symptoms of radiation sickness.

Fallout in North America from the Soviet accident will be measurable, said Pittsburgh's Wald, but its health effects will not be.

"It's more of a scientific or intellectual concern than an actual health risk," said Dr. Erwin Hirsch, professor of surgery at Boston University School of Medicine and an expert on radiation and trauma.

The farther away from the Chernobyl accident, the lower the exposure -- even down-wind.

"If I were in Kiev right now, I'd be very concerned about which way the wind was blowing," John Wallace, chairman of the department of atmospheric sciences at the University of Washington in Seattle, said last week. "But I don't think it really matters as far as we're concerned."

By the time the fallout reaches North America, Wallace said, "whatever concentrations they're experiencing in the Ukraine will be orders of magnitude less.

"The atmosphere is pretty kind to us."