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.
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.