Magnetic Fields (ELF) and Cancer

The two get bundled together as "EMF," and the difference matters.

An electric field comes from voltage. Voltage is the pressure pushing electrons along a wire. More voltage means a stronger field. Electric fields are measured in volts per meter. They exist whether or not a device is switched on. Walls and other objects shield them easily.

A magnetic field comes from current actually flowing. More current means a stronger field. Magnetic fields are measured in microteslas, written µT. That is millionths of a tesla. They appear only when current is moving, which usually means the device is on. Power lines are the exception. Current runs through them all the time, so their magnetic fields never stop. Magnetic fields also pass straight through buildings, living things, and most other materials.

That is why the magnetic component is the one studied in relation to health. It is the one that reaches people indoors.

Extremely low frequency, or ELF, means frequencies up to 300 cycles per second. That unit is the hertz. For comparison, radiofrequency fields run from 3 kilohertz to 300 gigahertz. Both sit in the non-ionizing part of the spectrum. Neither can damage DNA or cells directly.

ELF sources are the electrical system itself. Power lines, house wiring, and appliances such as shavers, hair dryers, and electric blankets. The Earth's own magnetic field is a natural example. It is the one that turns a compass needle.

Here is the counterintuitive part. Magnetic fields near many household appliances are higher than fields near power lines. Appliances still contribute less to a person's total exposure, for two reasons. Most are used for only short stretches. And field strength collapses with distance. Fields drop precipitously at about 1 foot from most appliances. People usually sit 12 to 20 inches from a computer screen, and at that range the fields are dramatically lower again.

Research in children has concentrated on leukemia and brain tumors. Those are the two most common childhood cancers. Three questions have been studied. Living near power lines. Measured magnetic fields inside the home. And parents' high magnetic field exposure at work.

A 1979 study pointed to a possible link between living near power lines and childhood leukemia. Later studies gave mixed results. Most found no link at all. Some found one only in homes with very high magnetic field levels, and those levels are present in very few homes.

Three combined analyses give the clearest picture. The pattern in them is worth reading closely.

• A pooled analysis of nine studies found a twofold increase in childhood leukemia among children exposed to 0.4 µT or more. Fewer than 1% of the children in those studies reached that level.
• A meta-analysis of fifteen studies found a 1.7-fold increase at 0.3 µT or more. A little over 3% of children reached that level.
• A pooled analysis of seven studies published after 2000 found a 1.4-fold increase at 0.3 µT or more. Fewer than one half of 1% of children reached that level.

Notice two things at once. The relative increases are real, and they point the same way. But the number of children actually exposed at those levels is tiny.

So few highly exposed children exist in the data that none of the three analyses could produce a stable dose-response estimate. NCI spells out what that leaves open. The findings can be read three ways. As a steady rise in risk with exposure. As a threshold effect at 0.3 or 0.4 µT. Or as no significant increase at all. NCI's own summary of the highest-exposure finding is one word: unclear.

Other lines of evidence in children came back empty or mixed. Studies of household appliance use found no consistent link to childhood leukemia. Studies of parents' exposure before conception, or during pregnancy, have been inconsistent. NCI says that question needs further evaluation.

One question does have a clean answer. Combined data followed more than 3,000 children with acute lymphoid leukemia across eight countries. ELF magnetic field exposure was not linked to survival, or to risk of relapse. Whatever the fields may or may not do to risk, they do not appear to change outcomes once leukemia is found. Our page on childhood leukemia covers what does drive those outcomes.

Most studies of ELF exposure in adult homes show no relationship with breast cancer in women. A few individual studies suggested one. Only one of those produced a statistically significant result.

The workplace history is a lesson in method. Several studies from the 1980s and early 1990s reported higher-than-expected rates of leukemia, brain tumors, and male breast cancer in electrical occupations. Power station operators and telephone line workers were the examples. But most of those results rested on job titles. They did not rest on any measurement of what those workers actually breathed or stood near. Later studies added real exposure measurements. Those have generally not found leukemia, brain tumor, or female breast cancer risk rising with magnetic field exposure at work.

In 2002, IARC appointed a Working Group to review all the evidence on static and extremely low frequency electric and magnetic fields. Its conclusions split three ways, and the split is the point.

• ELF magnetic fields: possibly carcinogenic to humans, Group 2B. The stated basis was limited evidence from human studies on childhood leukemia — not animal evidence.
• Static magnetic fields: Group 3, not classifiable as to carcinogenicity to humans.
• Static and ELF electric fields: Group 3 as well.

That review is IARC Monographs Volume 80.

In 2015, the European Commission's Scientific Committee on Emerging and Newly Identified Health Risks reached a similar place by a different route. It found that ELF studies overall show a raised risk of childhood leukemia above estimated daily average exposures of 0.3 to 0.4 µT. In the same breath, it said no mechanism has been identified, and that experimental studies do not support the finding.

That missing mechanism is the reason this topic has stayed unresolved for four decades rather than being settled in either direction.

No mechanism by which ELF fields could cause cancer has been identified. Non-ionizing fields cannot damage DNA or cells directly, so any explanation has to be indirect. The main idea put forward involves melatonin. ELF exposure might lower melatonin levels, and there is some evidence that melatonin may hold back the growth of certain tumors. That remains speculation. Animal studies have given no indication that ELF exposure is linked to cancer. That is unusual for a 2B agent, since animal data is often what pulls one into that group.

So why keep studying it? NCI gives the honest reason. Power lines and electrical appliances are everywhere, in every home and workplace. When exposure is that universal, even a very small rise in risk would matter clinically. That is a statement about population arithmetic. It is not a statement about danger to any one household.

For the neighboring question about radiofrequency exposure, see cell phones and cancer. For what a Group 2B label is and is not saying, see hazard versus risk.

• NCI — Electromagnetic Fields and Cancer
• IARC Monographs Volume 80 — Non-ionizing Radiation, Part 1: Static and Extremely Low-frequency Electric and Magnetic Fields