Sources of EMFs

EMFs come from a wide range of sources and vary enormously in the strength of the fields. (This page and the links from it deal with power-frequency fields - see also static fields.)

See also: electric fields , magnetic fields , units used for EMFs, instruments available for measuring fields.

The diagram shows how the magnetic field varies over a typical house. The peaks come from the various appliances, and they are superimposed on a low background field, which varies only slightly across the house.

• Appliances are present in every home and fields come from everything from your kettle to your TV - even a mobile phone.
   
• A minority of homes in the UK are close enough to an overhead power line, an underground cable or a substation for that to be the main source of the background field.
   
• But in most homes, the background field comes from low-voltage distribution wiring.  House wiring can sometimes contribute too.
  Outside the home, we can experience EMFs in schools, when using electrified transport, and at work.

How fields vary

• Fields from any source fall with distance
• Magnetic fields vary over a day and a year as loads vary. 
• Fields have also changed over longer periods of time – perhaps four-fold over the last forty years, though this is hard to quantify. 
   

Field levels

• The sort of home we live in can affect the level of fields we are exposed to. 
• It’s not just where you live that matters. As you move about, you experience fields from many other sources (such as appliances) as well as the background fields in the home. This means your personal exposure is higher than the background field in the home
• The average magnetic field in a home in the UK is about 50 nT. This is lower than in many other countries and in particular lower than in the USA
• The great majority of homes in the UK have fields less than 0.4 µT. About 1.5% of UK homes have average fields greater than 0.2 µT and about 0.4% greater than 0.4 µT. These percentages of homes with higher fields are smaller than in many other countries. More on fields greater than 0.2 and 0.4 µT
• Only about a half of homes in the UK with fields above 0.4 µT get that exposure from high-voltage power lines – in the rest, the field probably comes from the distribution system or house wiring