The strength of magnetic fields is measured in gauss. Those radiating from power lines and appliances are measured in milligauss, or thousandths of a gauss. A nearby power line can radiate fields of 5 to 40 milligauss.

By comparison, at a distance of one foot, home appliance's radiate fields from about 1 to 280 milligauss, the highest figure being for an electric can opener. All magnetic fields drop sharply with distance.

The Earth's magnetic field, to which humans are constantly exposed, is about 500 milligauss. This is often hundreds of times larger than the man-made ones people worry about.

Magnetic fields that have been linked to childhood leukemia in some epidemiological studies are 3 to 4 milligauss, roughly one one-hundredth of the strength of the Earth's magnetic field. The studies that suggest such causation have often been criticized as ambiguous or flawed.

Despite the murkiness, citizen activists and local governments have forced electric utilities to move power lines, to install shielding and to cancel electrical- plant upgrades out of fear that power lines cause cancer. The annual cost of reducing power-line fields in the United States is estimated at $1 billion to $3 billion.

In the debate, those who see a danger tend to say that remedial action is necessary, no matter how small the risk, because it is best to err on the side of caution.

The federal government sets no standards on exposure levels for electromagnetic fields, leaving the issue in legal limbo.

The world's largest group of physicists, the American Physical Society, has declared it can find no evidence that the electromagnetic fields that radiate from power lines cause cancer.

After researching the power-cancer issue since 1989, the society last year embarked on a study meant to serve as the basis for a society position.

The society said groundless public fears about a possible link between power lines and cancer were diverting billions of dollars from "more serious environmental problems."

Many physicists are skeptical of a link between power lines and cancer because the fields are so weak. Both electric and magnetic fields are produced whenever electricity flows through a wire, but fears center on magnetic fields because only they penetrate the human body