Badly designed and implemented outdoor lighting such as street lights, house lights, business lights and lit signs all contribute to light pollution. This negatively effects not only our night sky but our nocturnal environment, the ecology of it, which in-turn effects people, wildlife and even air quality.
The effects of light pollution are not confined to the city or town itself that is causing it. This photo I took shows the light dome (skyglow) from Kitchener and Waterloo in Ontario, rising into the night sky and washing out the stars. It was taken a 30 minute drive outside of the cities, near Linwood, Ontario.
Light pollution created by cities and towns is adversely effecting rural areas around them even. The spread of light pollution has become much like a cancer of the night environment.
Everyone needs to help reduce the spread of it. Shielding outdoor lights, pointing the light down and not up and turning off lights over night when not needed are just a few small things you can do to stop this cancer of the night. Not using LED lights that are bright white which contains a lot of blue wavelength which has been shown to increase skyglow light pollution is also good. Warmer whites with colour temperatures 3000K or less (2200-2700K) are better.
For more information on light pollution and how you can help reduce it visit the International Darksky Association.