Fog is caused by water droplets in the air. But, Smog - Smog is Smoke + Fog.
It is a sign of severe air pollution.
Look at the image below of a city in China, notice the effect of smog in the atmosphere.
Let's discuss the differences between Fog and Smog
|Fog is caused by water droplets in the air||Smog is made up of Smoke and Fog|
|It reduces visibility, but breathing in fog causes no harm.||Breathing in Smog can cause Eye Irritation, Asthma and decreases Lung's Breathing Capacity|
|Color of fog is white||Color of Smog is usually blackish, but may change depending on other pollutants in the atmosphere|