The most useful fact to know is that the magnetic field from a power line falls off very fast as you move away from it. If you double your distance from the line, the field strength drops by about three-quarters of what it was. That is because the field spreads out in three dimensions, so the strength is not just cut in half but reduced dramatically. This quick drop is the main reason that a line that produces a strong reading right under it may be barely measurable a few hundred feet away.
The field itself comes from the flow of current in the line. Voltage does not create the magnetic field. A line can have very high voltage, but if little current is flowing, the magnetic field will be low. What matters is the load, the amount of power being carried at that moment. The load on a transmission line changes throughout the day. It is higher in the morning and evening when people use more electricity, and lower at night. So the field under that line changes too. A single reading taken at one time of day does not tell you what the field is at other times.
Every wire that carries alternating current makes a magnetic field around it. That includes the big transmission lines along the highway, the lines on your street, and the wiring inside your walls. In the United States, these fields are measured in milligauss. In a survey of 992 American homes, the average reading room by room, away from appliances, was less than 1 milligauss. Right beside a large transmission line at average load, the field is in the tens of milligauss. But as you walk away, it drops fast. The field will be lower at the edge of the yard than directly under the line.
Inside a house, the magnetic field is often shaped by wiring problems more than by the line outside. Current flowing on a water pipe, or a circuit where the outgoing and returning currents do not cancel each other, can raise the field across a whole room. A licensed electrician can find and fix that. So one reading in one spot does not tell the whole story.
On health, the International Agency for Research on Cancer classified extremely low frequency magnetic fields as possibly carcinogenic in 2002. That classification was based mostly on studies that linked higher average exposure to childhood leukemia. Very few children live with fields that high. Those studies cannot show cause and effect. For comparison, the international reference level for the public is 2,000 milligauss. That is far above anything you would measure in a typical home. If you have questions about health, talk to your doctor. If you have questions about your utility’s lines or about wiring in your house, talk to your utility or a licensed electrician.
This is covered in more detail at power line magnetic fields by distance.