"Photochemical smog" is another environmental problem caused by photochemical reactions. Light causes automobile exhaust to chemically react and form irritating materials that produce smog. This occurs around high-traffic roadways on hot, windless days. Automobile engines and other machinery emit nitrogen monoxide (NO). Immediately after entering the atmosphere, NO reacts with oxygen molecules, transforming into nitrogen dioxide, a brownish gas that readily absorbs visible light, causing a photochemical reaction that breaks it down into NO and oxygen atoms. However, this NO once again immediately reacts with oxygen molecules, transforming back into nitrogen dioxide.