Power lines—do their magnetic fields pose a risk? In 1979 an epidemiologic study all...

Power lines—do their magnetic fields pose a risk? In 1979 an epidemiologic study alleged a correlation between childhood leukemia in Denver neighborhoods and nearby high-voltage electric power lines. Since then there have been many studies exploring health risks caused by magnetic and electric fields from high-voltage power lines. Epidemiologic and animal studies that hinted at problems are not supported by independent tests at other laboratories. No mechanism linking power lines and cancer has been found. In recent years, large-scale scientific studies have had negative results, and scientific and medical societies have issued official statements that power lines are not a health risk. Power lines produce both electric and magnetic fields. The interior of the human body is an electrical conductor, and as a result, electric fields are greatly reduced in magnitude within the body. The electric fields inside the body from power lines are much smaller than electric fields normally existing in the body. However, magnetic fields are not reduced in the body. Earth’s magnetic field, approximately 5 x 10^-5 T, is very small and not regarded as a health threat. Thus, it is interesting to compare Earth’s magnetic field to fields produced by high power lines. The magnetic field B produced at a distance r from a straight wire with an electric current I is

B = (2 x 10^-7 T ^. m/A)I/r

The magnetic field from a high-voltage power line located 40 m above the ground carrying a 100-A current is much smaller than

Earth’s u B field.

Wires that provide electric power for household appliances also produce electric and magnetic fields. The current in the wire for a 500-W space heater is about 5 A. With the wire located several meters from your body, the magnetic field of such an appliance is somewhat smaller than Earth’s magnetic field. By comparison, laboratory mice lived for several generations in 0.0010-T magnetic fields (20 times Earth’s magnetic field) without any adverse effects. During the last three decades, electric power use has increased the magnitudes of the field created by power lines to which Americans are exposed by roughly a factor of 20. Yet during that same time interval, leukemia rates have slowly declined. It seems unlikely that magnetic fields produced by home appliances or high-voltage power lines are a hazard.

Which answer below is closest to the ratio of the power line field at the same location?

(a) 0.001 (b) 0.01 (c) 0.1 (d) 10 (e) 100