The transmission of electricity impacts on the environment. Transmission lines are generally built above ground, their length being related to the type of power station and its location. Hydropower schemes, usually being remote from the consumer, require especially long lines.
The main impact of such lines comes from their visual intrusion, but they also restrict agriculture on bands of land 30-120 m wide and may cause some interference with nearby radio and television reception. It has been postulated that the lines may affect bird behaviour (especially water fowl), perhaps because they sway or hum in the wind; other hypotheses involve the effects of electrical fields. Several studies indicate exposure to the electrical and magnetic fields surrounding power lines may result in cancer, lethargy and loss of sex drive; due to the difficulty of actually measuring the amount of exposure to these fields exact relationships have not been proven. The presence of transmission lines often lowers house prices in neighbouring areas. Because it is expensive to place transmission lines underground (where access for maintenance is also more difficult), this is usually only done in areas of particularly outstanding landscape quality.
Studies have shown a higher incidence of leukaemias and brain tumours in children living near overhead power lines. In 1994 the UK government was subject to a court case by 50 children due to the installation of high voltage underground power lines in a residential street. A report of the UK National Radiological Protection Board in 1992 indicated that no evidence had been found establishing a link between power lines and cancer; in 1994 the person responsible for the study indicated that since then evidence had emerged suggesting that there could be an important link.
In 1996 the US Academy of Sciences evaluated hundreds of scientific papers on the subject of the effect of electricity transmission lines and found no evidence that exposure to these electrical fields presented a hazard to human health. The report pointed out that housing near power lines tended to be poor, densely populated, and subject to heavier traffic emissions and industrial pollution; these, the report says, are the likely cause of the higher rate of cancers in children living near power lines.
In 1997 the US National Cancer Institute concluded after an 8 year study involving 1250 subjects that children who live near high-voltage power lines do not have a greater risk of developing cancer than other young people.
Where lines traverse forests they produce useful fire-breaks and also provide permanently maintained clearings bearing herbaceous or scrub vegetation, which increases ecological diversity and provides grazing for herbivores.