Carbon monoxide is a toxic gas. It is colourless and odourless, so people cannot see it or smell it and it is not easily detected. It is dangerous because it stops the blood carrying oxygen properly around the body. Breathing in too much can cause fainting and even death, for example from a faulty gas boiler in a poorly ventilated room.
Sulfur dioxide and oxides of nitrogen cause respiratory problems in humans, such as irritation of the lungs and breathing difficulties, which are worse for people with asthma. They also dissolve in water droplets in clouds to form acidic solutions, so they cause acid rain. Acid rain can damage buildings and statues, kill trees and make lakes too acidic for fish to survive.
Particulates are tiny solid particles, such as soot, in the air. They cause global dimming, because they reflect some sunlight back into space so less reaches the Earth's surface. They also cause health problems for humans when they are breathed in, because they can damage the lungs.