Visual range has been considered a useful measure of urban air properties. Atmospheric particles reduce visibility by scattering and absorbing light. In Europe, this issue gets lower priority than many other environmental topics because there is no biological toxicity or clearly definable cost associated with it; however, it results in a pervasive and apparent reduction in environmental quality. In the US, impairment of visibility has been a much more significant issue because of the large areas of pristine national park, and legislation has been designed specifically to protect and improve visual range. The original Clean Air Act made it a goal to remedy any existing impairment of visual range due to air pollution, but recognised that understanding of cause and effect was too limited at that time to bring in legislation.