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The magnetic field that comes from a power line or from the wiring in your walls is not the same thing as the electric field. Electric fields are present whenever a wire is energized, but magnetic fields appear only when current is flowing. That is why a lamp cord draws a magnetic field when the lamp is on and almost none when it is off. These fields are extremely low frequency, alternating sixty times a second in the United States, which is what 60 Hz means. They are part of the same family of fields as radio waves but sit at the opposite end, far below the frequencies used for broadcasting. Unlike X-rays, they carry nowhere near enough energy to break chemical bonds or damage DNA directly.

Household fields fall off quickly with distance. A clock radio or a bedside lamp may produce a noticeable field at a few inches and very little at arm's length. Wiring inside walls, older knob-and-tube circuits, service panels, and the transformers on utility poles all create fields that weaken sharply as you move away. Power lines behave the same way. The field directly beneath a distribution line can be larger than what you find in most rooms, but it drops off rapidly with distance and is usually small by the time you are across the street. Transmission lines carry higher voltages and can produce stronger fields near the right of way, though the field at the edge of the corridor is typically much lower than directly underneath.

Two units are used for these measurements, and they describe the same field. One milligauss equals one tenth of a microtesla, so a reading of ten milligauss is the same as one microtesla. In everyday life, background levels in homes are often in the range of a fraction of a milligauss to a few milligauss, and readings close to appliances can be much higher for short distances. Public documents sometimes use one unit and sometimes the other, which can make comparisons confusing, so it helps to convert before comparing.

Exposure limits are set differently by different bodies. The International Commission on Non-Ionizing Radiation Protection publishes guidelines for the general public that are stated in microtesla and are far above ordinary residential levels. In the United States there is no single federal standard for magnetic fields from power lines. Some states have adopted their own siting rules for transmission lines, and the federal government has published information and research summaries rather than a nationwide exposure limit. The Federal Communications Commission handles radiofrequency exposure, which is a different part of the spectrum, and questions about that belong with the FCC. Questions about a specific power line, its measurements, or its right of way belong with the utility.

Large reviews of the published research, including work by the World Health Organization and the National Institute of Environmental Health Sciences, have found that the evidence for health effects from ordinary residential levels is weak and inconsistent, while noting that some studies reported a modest statistical association between long-term exposure to higher-than-usual fields and childhood leukemia. That association has not been explained by a known mechanism, and reviews have generally concluded that the evidence is not strong enough to establish causation. This is not a reason for alarm, and it is not a promise of safety either. If you have a personal health concern, that question belongs with a doctor. If you are dealing with a lease, an easement, or a property dispute, that belongs with a lawyer. Devices marketed as shields or stickers that claim to block magnetic fields or protect health should be treated with skepticism; magnetic fields at these frequencies are not blocked by paint, fabric, or small gadgets, and no such product has been shown to prevent any health effect.

There is a fuller write-up at power line magnetic fields.