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A 60-hertz alternating current running through any wire makes a low-frequency magnetic field around it. In most homes the levels are low. A survey of 992 US homes found the average reading in a room, away from appliances, was under 1 milligauss. But sometimes a whole room reads higher, and the source is inside the walls.

Two wiring problems cause this. One is a neutral current that has nowhere to go but a water pipe. In a properly built circuit, the current going out on the hot wire comes back on the neutral wire, and the two fields cancel each other. If the neutral path is broken or shared with a pipe, the returning current flows on the copper water line. That current makes a field that can spread across several rooms. The other problem is a wiring error where the hot and neutral wires run on separate paths, so the fields do not cancel. A licensed electrician can find and fix both.

A gaussmeter can help you look. You can buy one online or borrow one from a library. Walk the meter around the room slowly. Hold it at chest height and away from your body. Watch the number change as you move. Near a refrigerator or a wall switch the reading will jump. That is normal. The question is whether the background level in the middle of the room stays low. If it stays high no matter where you stand, the source is likely in the walls or under the floor.

The meter reads in milligauss. One microtesla, the unit used outside the US, equals 10 milligauss. That helps when you read a European study. The international reference level for the public is 2,000 milligauss, far above anything you will see in a house.

On health, the International Agency for Research on Cancer classified extremely low frequency magnetic fields as possibly carcinogenic in 2002. That classification came mostly from studies that linked higher average exposure to childhood leukemia. Very few children live with fields that high, and those studies cannot show cause and effect.

If your meter shows a steady reading above a few milligauss across a room, call a licensed electrician. Tell them you suspect a neutral current on a pipe or a wiring error. They can check with their own meter and fix the circuit. Do not try to open a panel or change wiring yourself. If you are worried about health effects, talk to your doctor. If the problem is outside the house, call the utility. The FCC covers radio frequency issues, not 60-hertz fields. The electrician is the right person for inside wiring.

For more on this, see house wiring magnetic fields.