This matters to anyone who has heard about power lines and health and wants to know what the numbers actually mean. The fields people talk about come from ordinary 60-cycle alternating current, the kind that runs through every wire in your house and every transmission line along the road. Any device that carries current makes a low-frequency magnetic field around it. That includes the big lines on tall towers, the smaller line on your street, the wiring inside your walls, and most appliances when they are plugged in and running.
In the United States these fields are measured in milligauss. Most other countries use microtesla. The conversion is simple. One microtesla equals ten milligauss. That is handy when you are reading a study from Europe and need to compare it to home measurements. A typical home has very low levels. One large survey of almost a thousand US homes found that the average reading in rooms away from appliances was less than one milligauss. That is a small number.
Right next to a large transmission line carrying its typical load, the field is in the tens of milligauss. It drops fast as you walk away from the line. The ground underneath a high-voltage line might show fifty milligauss or so. At the edge of the right of way it is usually much lower. Inside a house the surprises often come from the wiring, not from the line outside. If current is flowing on a water pipe, or if a circuit has its outgoing and returning currents out of balance, the field can spread across a whole room. A licensed electrician can track that down and fix it.
In 2002 the International Agency for Research on Cancer classified extremely low frequency magnetic fields as possibly carcinogenic. 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. They show an association, not proof. For comparison, the international reference level for the public is set at two thousand milligauss levels in homes. That is far above what you will measure in any home.
If you are worried about a specific reading, talk to an electrician who knows how to find stray currents and wiring problems. If you are concerned about health, talk to a doctor. If you have a question about a power line near your property, call your utility. The Federal Communications Commission handles radio frequency exposure, not the low-frequency fields from power lines. The numbers are straightforward. The levels in your home are almost always very low. The limits set for public exposure are much higher than anything you are likely to encounter.
There is a fuller write-up at power line magnetic fields.