Most power systems operate with alternating current (AC). But there are some direct current (DC) power lines, which produce DC or static EMFs.
Not many studies have been done of the health of people living near DC power lines. This is probably because there has not been any strong driver to do this sort of study. There has not been the sort of epidemiological findings that have driven the AC EMF issue, for instance, and given that we all live and have evolved in the earth's static magnetic field, it would be surprising if static fields of a similar magnitude from power lines had any effect on us.
What studies have been done?
The only relevant studies we are aware of are (none of these are published or publicly available):
Minnesota Landowner Health Perceptions Survey
This was a questionnaire sent by post and distributed by campaign groups in 1980 to people living near a 400 kV DC overhead power line in Minnesota, followed up by telephone calls. 35% of the respondents said they had suffered adverse health effects that they attributed to the power line. But the ad hoc nature of the survey and the reliance on the participants' own opinions about their symptoms makes it very difficult to interpret.
UPA sick leave study
A study in 1982 looked at the sick leave records of workers at a power company, United Power Association (UPA). A small group of these had worked on DC equipment, and these were sick slightly less often than comparable groups working on AC equipment. But there were no data on what exposures the staff actually received, and sick leave is not necessarilly the same thing as sickness.
North Dakota Landowner Survey
This was another telephone survey, in 1981, of residents along a different stretch of the same DC line as the Minnesota survey. It found much lower levels of reported ill health, and was better conducted, but again, this methodology is inherently unsatisfactory.
Saugus, California Health Perceptions Survey
Saugus is a town in California on the route of a DC overhead line. A survey of residents was conducted in 1981. It compared residents close to the line with those more distant, using a standard health questionnaire. There were no systematic or significant differences between the two results. This was a well-designed study, but not conclusive, partly because the operating conditions of the line varied a lot during the survey period.
What is the conclusion?
None of these studies is very strong. But the only one to suggest health effects - the first one, in Minnesota - is undoubtedly the weakest, and the subsequent studies, all of which were better, did not find health effects of DC power lines.
Because all these studies are unpublished, we have relied on summaries of them in a paper by Banks and Williams from 1983.