Measuring magnetic fields

Magnetic fields within homes can vary at different locations and also over time.

Electric and magnetic fields from electricity

Everything electrical, from a toaster to a high-voltage powerline, produces electric and magnetic fields. In Australia, the electric and magnetic fields associated with the use of electricity are generated at a frequency of 50 hertz (Hz) (50 cycles per second).

Both the electric and magnetic fields are strong close to an operating electrical source. The strength of the electric field depends on the voltage (typically 240 Volts for households) and is present in any live wire whether an electrical appliance is being used or not. Magnetic fields, on the other hand, are produced by electric currents and are only present when an appliance is operating i.e. there is no magnetic field when an electrical appliance is turned off.

Do electrical sources cause any health effects?

The scientific evidence does not establish that exposure to the electric and magnetic fields found around the home, the office or near powerlines causes health effects. However, there are some epidemiological (population) studies that have reported a possible association between prolonged exposure to ELF magnetic fields at levels higher than typical and increased rates of childhood leukaemia. Other research including studies on cells and animals has not confirmed these results. On balance, the evidence related to childhood leukaemia is not strong; however people should be aware of the issue in order to make informed decisions.

For further information on electric and magnetic fields and the possibility of adverse health effects refer to the ARPANSA fact sheet Electricity and health.

How can I measure my exposure to magnetic fields from electrical sources?

The strength of the magnetic field is expressed in units of Tesla (T) or microtesla (µT). Another unit, which is commonly used is the Gauss (G) or milligauss (mG), where 1 G is equivalent to 10-4 T (or 1 mG = 0.1µT).

There are a range of different instruments that can measure magnetic field strength. The gauss meter is a hand-held device that provides a simple way of performing such measurements.

ARPANSA has two different gauss meter models available for hire, which are a Sypris Model 4080 and an EMDEX Snap. Both instruments operate in a similar manner and they are shown in the image above.

The gauss meters available from ARPANSA measure magnetic fields from electrical sources that operate in the frequencies between 25 Hz (40 Hz for the EMDEX Snap) to 1000 Hz in units of mG. The magnetic field meter is not suitable for the measurement of radiofrequency fields emitted from telecommunications sources such as mobile phones, mobile phone towers, Wi-Fi, smart meters, etc. Readings taken very close (a few cm) to other electronic devices such as mobile phones (as distinct from electrical devices such as heaters, washing machines etc) may also give false readings. Shaking or vibrating any of the units may also give false readings. Since the meters only measure magnetic fields from electrical sources they will not measure the earth's static magnetic field which has a value of approximately 500 mG.

The power switch for each of the meters is located on the side of the device. When the Sypris meter is turned on, it will perform an initial self-diagnostic test by showing all available readouts on its digital display. When the EMDEX meter is turned on it will display the battery level. Following the initial stage, both models will display the magnetic field intensity at the location where the meter is held or placed and the intensity will change if moved accordingly. For the Sypris meter, if the negative sign is still showing after the initial test that indicates that the meter is running low on power and the battery needs to be replaced (if this occurs please ring ARPANSA for assistance). For the EMDEX instrument the battery should be replaced if the meter indicates a battery level of 25% or less.

How do I perform the measurements?

Measurements of the magnetic field in the home should generally be taken in the middle of the room at about one metre from the ground or in locations where people spend a significant amount of time, for example, the bed. Measurements should also be performed several times over the course of a day. This is to allow for possible variations to electricity demand which typically peak during the evening at about 7.00 pm. Measurements can also be made at any other locations of interest.

It is important to remember, that as mentioned earlier, existing evidence relates any health effects to prolonged magnetic field exposure. Measurements taken with the gauss meter are instantaneous (i.e. measured at one point in time) and do not accurately reflect prolonged exposure levels. Details of the preferred measurement technique and instrumentation for making time-averaged measurements are available in a measurement protocol developed by ARPANSA which is available from ARPANSA Technical Report TR134.

PDF icon Technical report 134 - Measurement of residential power frequency magnetic fields

What are typical magnetic field strengths?

Magnetic fields within homes can vary at different locations and also over time. The actual strength of the field at a given location depends upon the number and kinds of sources and their distance from the location of measurement. Typical values measured in areas away from electrical appliances are of the order of 0.1 – 2.0 mG.

Magnetic fields from individual appliances can vary considerably as well, depending on the way they were designed and manufactured. One brand of hair dryer, for example, may generate a stronger magnetic field than another. In general, appliances, which use a high current (such as those which have an electric motor) will lead to higher readings. It should also be noted that different body parts will be exposed to different magnetic field levels from the same appliance, depending on how far that part of the body is from the appliance when in use. Typical values of magnetic fields measured at normal user distance from some common domestic electrical appliances are listed in Table 1.

Table 1: Typical values of magnetic fields measured at normal user distance
Appliance  Range of measurements (mG)*
Electric stove 2 - 30
Refrigerator 2 - 5
Electric kettle 2 - 10
Toaster 2 - 10
Television 0.2 - 2
Personal computer 2 - 20
Electric blanket 5 - 30
Hair dryer 10 - 70
Pedestal fan 0.2 - 2

 * Note: Levels of magnetic fields may vary from the range of measurements shown.

What about homes near powerlines?

The powerlines that are present in typical neighbourhoods are called 'distribution' lines and they operate at lower voltage than 'transmission' lines, which use very high voltages. As stated earlier, however, it is the current and not the voltage that is associated with the strength of the magnetic field. Therefore, proximity to high voltage lines will not necessarily give a high reading unless those lines are also carrying a large current. Typical values of magnetic fields measured near powerlines and substations are listed in Table 2. These are well below the exposure limit in the international guidelines of 2000 mG.

Table 2: Typical values of magnetic fields measured near powerlines and substations
Source Location of measurement Range of measurements (mG)*
Distribution line (street powerlines) directly underneath 2 - 30
Distribution line (street powerlines) 10m away 0.5 - 10
Substation at substation fence   1 - 8
Transmission line (high voltage powerlines) directly underneath 10 - 200
Transmission line (high voltage powerlines) at edge of easement 2 - 50

 * Note: Levels of magnetic fields may vary from the range of measurements shown.