Request an Inspection

Call us at (415) 785-7986 or click the button below to schedule your building health assessment.

Request a Site Visit

Sign up for our Quarterly newsletter

Subscribe to our e-mail newsletter to receive helpful updates and articles from Healthy Building Science.

Sign Up For Newsletter

We value your privacy.
Your email is never shared or sold.

New home purchase? Magnetic Field Guidelines

magnetic field guidelines

Magnetic Field Guidelines for the New Home 

magnetic field guidlines

High Voltage Distribution lines

EMF is a broad umbrella term encompassing many aspects of electromagnetic issues that can affect human health. Subcatagories of EMF issues include RF Radiation (also known as ElectroMagnetic Radiation [EMR]), Interior/Exterior Magnetic Fields, Interior/ Exterior Electric Fields, Transient Voltage Fluctuations (also known as Dirty Electricity or ElectroMagnetic Interference [EMI]), Transient Current Fluctuations (also known as Dirty Magnetism) and Net Current (also known as Stray Current).

While almost all of these can be mitigated to large degree through various practices, shielding materials and other changes to the home, there is one type that is insidious and extremely difficult to mitigate against, making it not cost effective to attempt. Avoidance is the best strategy. This aspect of EMF is specifically called Exterior-sourced Magnetic Fields. Before purchasing a new home, consider the following magnetic field guidelines.

These are generated by overhead high voltage power lines (Transmission Lines and Neighborhood Distribution Lines). It is not typically possible to visibly determine the likely strength of the magnetic fields from such as source and, as such, an accurate gaussmeter must be used (a cost-effectively accurate gaussmeter is the Tenmars TM-192).

Magnetic Fields from Transmission Lines

These power lines are used to power cities and are typically between 250,000 and 750,000 Volts. They are capable of carrying the cumulative amperage of an entire city or large sections of very large cities. Depending on the time of the day, the current draw from the city can be very large, and while the high voltage diminishes the amperage, the associated magnetic fields can cast out over a large area because the “Hot” and “Neutral” conductors of the transmission lines are 15 feet or more apart. The further the conductors are apart, the larger area over which the magnetic fields will cast out. If there is very little amperage, such as with a transmission line which is feeding a small town or a small section of town, the magnetic fields will be very minimal even with the large distance between the conductors.

High Voltage Neighborhood Distribution Lines

magnetic field guidlines

Neighborhood Transmission Lines

These are typically between 7000 Volts and 15,000 Volts and run through neighborhoods supplying upto hundreds or thousands of homes. They typically carry the cumulative amperage of a neighborhood or other small section of a city or town. While they do not supply as many people as the transmission line, the lower voltage has associated with it a larger amperage. With overhead distribution lines, with the “Hot” and “Neutral” conductors being roughly 5’ apart the magnetic fields are capable of being cast out over a large distance, but the strength of this magnetic field is greatly dependent on the number of homes that are ‘downstream’. If the distribution line is near the end of the electrical branch, the distribution line might only be supplying 10 or 20 homes and so the cumulative current to that point is very little and there will be very minimal amounts of magnetic fields. If the distribution line is closer to the electrical ‘trunk’ (closer to the substation) and there are very many homes that are being supplied, the magnetic fields due to the distribution line can be immense.

My experience with homes towards the end of the electrical branch showed magnetic fields across the property entirely below 1.0 milliGauss (mG) (upper limit of Low Concern as described by the International Institute of Building Biology and Ecology [IBE]) even with the distribution line in the backyard. The worst case I have experienced were with homes distribution lines near the substation where magnetic fields were above 5.0 milliGauss (mG) (lower limit of High Concern as described by IBE) over 100 feet from the distribution line.

Magnetic Fields – Time of Day, Season

Typically the highest electrical draw and current load that cities in California experience is during the early morning and the evening due to the lighting demands of the residence. Northern latitude cities, such as Chicago, would likely be similar with the lighting and heating demands during the early morning and evening. If you are unable to test the magnetic fields at the open houses during these times, you might measure during the midday and double (or triple for Chicago as the heating requirements are higher) the value measured to ‘guesstimate’ the early morning and evening magnetic field levels. It will not be the most accurate measurement but should work for weeding out unacceptable homes from acceptable ones.

Another aspect of northern latitude cities, when compared to California, is that they will have higher heating and lighting requirements in the early morning and evening during the Winter than during the midday. During the Summer, with their additional heat and humidity, during the afternoon when the heat is at its highest many air-conditioning units will likely be running and so the highest magnetic fields of the day will be during the afternoon. Keep this in mind when performing the testing.

Other Points to Consider

There are other interior sources of magnetic fields to be aware of to bear in mind while testing.

  1. Have the lights of the home turned off when testing, some wiring errors that may be easy to fix can cause large magnetic fields through the home to be present with the lighting on.
  2. Test further away from appliances. Magnetic fields are typically present within 3 feet of appliances that are actively operating
  3. Test further away from electrical panels and subpanels. Magnetic fields can be strong within 3 feet to 5 feet of most electrical panels and subpanels.

Magnetic field strength due to the powerlines would steady climb as you approach the side of the house due to them and should steadily drop the further you recede from them. If you find it climbing drastically on a particular spot of the home, this is just likely due to an appliance, wiring error, subpanel or the like, and can likely be easily fixed. The overall general ambient levels of the magnetic fields spreading across the property due to the powerlines is the unmitigatable concern.

If a storage shed is close to the powerlines and a moderate magnetic field concern while the rest of the house is of low or no concern, this should be considered acceptable. Intermittent exposure (such as going to a shed or a storage room in the house) is not considered very much a concern except by ultra-sensitive individuals. The areas of most concern are typically the Bedrooms, Living Room, Kitchen and Home Office (areas with long duration exposure to the magnetic fields present).

IBE Concern Levels

Magnetic Field Strength (milliGauss, mG)


None <0.2
Slight (Low) 0.2 – 1.0
Severe (Moderate) 1.0 – 5.0
Extreme >5.0


Leave a Reply



Subscribe to our e-mail newsletter to receive helpful updates and articles from Healthy Building Science.

You have Successfully Subscribed!