Pylons and the weak link

The link with cancer and neurological disorders is elusive. Picture / Amos Chapple

The first study suggesting an association between childhood leukaemia and power-line magnetic fields was published in 1979.

That finding, in Denver, Colorado, has spawned numerous studies trying to pin down a link to cancer and possibly neurological disorders such as Alzheimer's.

Despite years of study, the link to childhood leukaemia remains weak yet consistent, say mainstream researchers, and the link with other conditions even weaker.

The question has gained new importance for the people who live near the potential routes - the final decision on the route will be made in June - of Transpower's proposed $500 million line from near Lake Taupo to South Auckland.

At 400,000 volts, it will be New Zealand's highest-voltage overhead power line. The largest at present is 220,000 volts. Home supply is about 230 volts.

It will also use huge pylons, some up to 67m above the ground. The average height of the 400 to 500 pylons will be 55m.

They have to be so high to keep the heavy electric cables at least 11.5m above the ground at their lowest points That is to maintain safety from the powerful electric and magnetic fields generated around the cables by the electricity that flows through them.

This has also led to plans to exclude homes and most other buildings, except farm sheds, from a 65m-wide easement along the path of the line.

But despite these safety measures, Transpower makes no health claims about the line. "It upsets people when we say this, but we don't feel we can have an opinion on that," says communications manager Chris Roberts. Instead the state-owned power grid owner relies on its compliance with National Radiation Laboratory guidelines and the advice of occupational physician and Auckland University lecturer Dr David Black.

"They classify these magnetic fields as a 2B carcinogen," says Black. "There is data of a possible carcinogenic effect but you can't decide. It's like coffee and pickled vegetables [and car exhaust]."

He is referring to findings on the cancer risks of so-called extremely low frequency (ELF) fields from electric wires and motors - findings of the International Agency for Research on Cancer as part of a World Health Organisation project.

Class 1 substances, for which there is strong evidence of a cancer-causing effect, include tobacco and asbestos. Class 2A, "probably carcinogenic", includes diesel exhaust and sun lamps. Class 2B is possibly carcinogenic. Class 3 is unclassifiable, where the data is inadequate for an evaluation. Class 4 substances are probably not carcinogenic.

The electric field around overhead cables, which causes their buzzing sound, was clearly demonstrated by protesters against the Transpower scheme in Hunua who brandished fluorescent light tubes which lit beneath 110,000 volt cables. They claim these existing cables are harmful to people's health.

The Health Ministry's National Radiation Laboratory says electric fields under New Zealand's presently highest voltage lines can reach 3000 volts/metre - well within guidelines. The laboratory says: "Studies of people and animals who have been exposed to ELF electric fields show that, at the strengths normally encountered in the home and under power lines, [they] do not cause health problems."

Greater concern is usually expressed about the magnetic fields of high-voltage lines. A study led by Otago University epidemiologist Dr John Dockerty, who is also a Government adviser, found no statistically significant association between magnetic fields in the home and childhood leukaemia.

But Clevedon resident Dr Laura Bennet, an Auckland University physiologist , says a doubled risk is a strong link and slightly higher than the link between passive smoking and cancer. She acknowledges the causal evidence is inconsistent, but notes it took many years for that kind of evidence to back ill-health associations found with asbestos.