What is known as smog? What harm is done by smog especially in winter?

Smog is a combination of smoke and fog.

During the winter months, due to severe cold, the use of coal and fossil fuels to heat homes and for other purposes increases. These lead to the formation of smog. Smog is harmful for health as it causes asthma, bronchitis, shortness of breath, eye and nose irritation, etc. It also reduces visibility and leads to problems in traffic movement.