Dear WDDTY:

The power line article states that there is a "significant increased risk at 200-300 nanotesla". I find this amazing as the Earth's natural magnetic field is 50,000 nanoteslas, some 250 times larger.

Simon Best goes on to say that "unless you live near a power line, this isn't going to be a major problem since they can be shielded by trees and walls". The problem with EMFs is that, because electrical wiring is all pervasive, they cannot effectively be shielded by trees and walls. Also, by my limited understanding, EMFs are more of a threat than magnetic fields.

He also says that only a lead shield will block a magnetic field. The fact is that lead has relatively little effect on a magnetic field compared with nickel, iron or cobalt. To verify this try moving apiece of lead near a compass and the compass needle will not move. I believe that Mr Best is confusing magnetic fields with ionizing radiations, like X-rays, which are best shielded against by lead. Robert Owen, Electrial engineer, Victoria, Australia.

Simon Best replies,

Regarding the 200-300 nT risk level, this range has been corroborated in at least a dozen studies. These figures relate to artificial alternating magnetic fields, whereas the earth geomagnetic field (GMF) is a static field. However, some research seems also to implicate the GMF, and a number of researchers believe this should be measured in future studies.

As for shielding, the electric components of EMFs can be relatively easily shielded by trees and walls. The magnetic field component, however, is not easily shielded. Lead is used for ionizing radiation, as Robert Owen correctly says, and should have been corrected.