A study by American-based scientists has shown that lighting at night can contribute to air pollution in urban areas.

Research by the National Oceanic and Atmospheric Administration and the Cooperative Institute for Research in Environmental Sciences at the University of Colorado shows that outdoor lighting that contributes to 'sky glow' over cities interferes with chemical reactions that clean the air.

The findings, presented at the American Geophysical Union in San Francisco, said that every night chemicals from vehicle exhaust and other human created sources are broken down and prevented from becoming smog, ozone and other irritants by a form of nitrogen oxide called the nitrate radical.

Sunlight destroys the radical so this process occurs at night. However, measurements taken over Los Angeles showed that light pollution from cities suppresses the radical.

The research revealed that city lights can slow down the nighttime cleansing by up to 7% and increase the starting chemicals for ozone pollution the next day by up to 5%.

The findings have been welcomed by the International Dark-Sky Association, which is raising awareness about light pollution.

Executive Director Bob Parks said he hoped the research would encourage cities to make changes in outdoor lighting.

He said: ''The impending transition to LED outdoor lighting will allow cities to take advantage of new technologies like adaptive lighting controls. This technology will allow them to reduce lighting levels after midnight or turn off lights that are not needed. Not only will this vastly reduce energy consumption, based on this new research, it could also improve air quality.''