Increasing human activity results in various forms of pollution which obstruct or distort the receipt of light by telescopes or of electromagnetic radiation by radio-telescopes.
Aside from normal cloud cover, observations may be obstructed by smoke, dust and other atmospheric pollutants of industrial origin. Observations at night are severely hindered by reflections from urban lighting. Both from ground level and in space, observations may be obstructed by radiation leakages from orbiting nuclear reactors, as in the case of the military satellites of the USSR which have effectively blinded a number of orbiting telescopes during the 1980s.