The sad short answer is: there is no such thing as a safe distance.
Here are the reasons:
- The magnetic field from a powerline decreases with distance, for sure. But the magnetic field from a powerline varies from moment to moment depending on how much current is flowing in the wire at the time. It will be higher during peak electricity usage times. So the only way to know how strong the field is at a given distance, AT ANY PARTICULAR MOMENT, is to measure it with a gaussmeter. We always recommend taking multiple measurements at various times during the day.
- There could easily be additional sources of field. They might come from underground wires, ground mounted transformers, or even common sources within the home. They will add to the strength of the field emitted by the powerlines. Either field alone could be within tolerable limits, but could possibly exceed tolerable limits when combined.
- The safety or danger of a magnetic field from a powerline depends on more than just the strength of the field. Some research has shown that harmonics (higher frequency fields), radio-frequency signals in the line, and power spikes may have more to do with health effects than just the normal 60 Hz magnetic field.
- The time of day that you are exposed may be very important. Some research shows that exposure during sleep may be more harmful than exposure during waking hours as it affects the melatonin balance which is a hormone that, among other things, fights cancers cells.
- Whether you are located upwind or downwind of the powerline may also be important. Recent research has shown that the corona field around high tension lines can ionize the air around the lines. This ionized air has been thought to attract and concentrate radio-active particles and automotive pollutants that can be harmful.
- There are probably other factors which determine how much EMF your body can tolerate, such as genetic predisposition, how much exposure you receive at work or school, your age, your exposure to harmful chemicals (pesticides, preservatives, etc.) which may be activated by the EMF, your overall health, and so on.
- Most important of all, scientists simply do not yet know how much exposure is safe or harmful.
While there are official standards for exposure to electric and magnetic fields, they are based on the amount of field needed to cause immediate harm. There is plenty of evidence to show that biological effects occur at levels well below the standard limits. In the end, we are each left to decide how much exposure we are willing to accept. One rule of thumb that is used by some experts is that you should limit your exposure to 60 Hz magnetic fields which are in excess of 2.5 mG. There is not a lot of scientific evidence to support this recommendation, but it is based on the Swedish recommendation for exposure to ELF fields from computer monitors.
You should get a gaussmeter and make some measurements. At least find out if the fields from the powerline exceed the 2.5 mG guideline.