Smog looks like a mixture of smoke and fog. It is made up of particulates, nitrogen oxides, ozone, and unreacted hydrocarbons, etc. A brownish haze and painful eyes are often indicators of photochemical smog. Nitrogen dioxide is responsible for the brownish color of the haze. The reactions that lead to the formation of photochemical smog are initiated by sunlight and involve hydrocarbons and nitrogen oxides emitted from automobiles. The combination of sunlight, the catalysis by particulates and the abundant pollutants present in modern cities provide favorable conditions for smog formation.