Fog vs Smog: Key Differences, Causes, And Health Risks Explained

Fog and smog may look similar at first glance, but they are very different in origin, composition, and impact on health. While fog is a natural weather phenomenon, smog is a harmful, human-made pollutant. Understanding the differences between the two can help you protect your health and the environment.

What is Fog?

Fog forms naturally when air contains tiny water droplets or ice crystals suspended near the ground, reducing visibility. It occurs when warm, moist air cools to its dew point, causing water vapour to condense into fine droplets.

Fog is usually seen on cool, humid mornings and disappears as the day warms up. It is completely harmless and a normal part of weather patterns.

What is Smog?

Smog is a man-made combination of pollutants including sulfur dioxide, nitrogen oxides, volatile organic compounds (VOCs), and particulate matter (PM2.5 and PM10). These pollutants mostly come from vehicles, industries, and fossil fuel combustion.

The word “smog” was coined in early 20th-century London as a combination of “smoke” and “fog,” describing the thick, polluted air caused by industrial coal burning. Unlike fog, smog is dangerous to humans, plants, and the environment.

Fog vs Smog : Major Differences

Feature	Fog	Smog
Origin	Natural (condensed water vapour)	Human-made (pollutants + moisture/sunlight)
Colour	White or light grey	Yellowish or brownish
Smell	Odorless	Acrid or chemical odour
Health Impact	Harmless	Harmful – respiratory issues and long-term diseases
Formation Conditions	Cool, humid air; early mornings	Warm, stagnant air; sunlight; urban areas
Composition	Water droplets	Pollutants like NOx, SO₂, VOCs, PM2.5, PM10

Fog forms naturally and is harmless, while smog forms from trapped pollutants in the lower atmosphere. If your eyes or throat feel irritated, it’s likely smog, not fog.

Why Smog is Dangerous

Smog is not just “dirty air”; it contains toxic chemicals that can seriously affect human health and the environment.

Health Effects of Smog Exposure:

• Respiratory disorders such as asthma and bronchitis
• Eye, nose, and throat irritation
• Fatigue, nausea, and reduced concentration
• Circulatory problems and heart disease

Smog also damages plants, soil, and aquatic life, lowering agricultural productivity and contributing to global warming.

Although fog and smog may appear similar, the differences are vast. Fog is natural and harmless, while smog is a toxic byproduct of human activity. By monitoring air quality, using clean energy alternatives, and adopting greener practices, we can protect our health and work toward a cleaner, healthier environment.