Introductory Guide to Fog, Haze, Smog and PM2.5

• Fog is a collection of liquid water droplets or ice crystals suspended in the air at or near the Earth's surface.

• Haze is traditionally an atmospheric phenomenon where dust, smoke and other dry particles obscure the clarity of the sky.

• Smog is a type of air pollution; the word "smog" was coined in the early 20th century as a portmanteau of the words smoke and fog to refer to smoky fog.

• PM2.5 particles are air pollutants with a diameter of 2.5 micrometers or less, small enough to invade even the smallest airways.