Carbon monoxide is a toxic gas. It is colourless and odourless, so it has no colour and no smell to give a warning, and people cannot easily detect it. It is dangerous because it is breathed in without warning, 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 breathing difficulties and irritated lungs. They also dissolve in rain water and cause acid rain, which damages buildings and harms plants and living things in lakes.
Particulates are tiny solid particles, such as soot, in the air. They cause health problems for humans when breathed in. They also cause global dimming, which means less sunlight reaches the Earth's surface because particles in the air reflect and block some of the light.
Increased amounts of these pollutants in the air therefore make the air less safe to breathe and harm the environment. The more fuel that is burned, the more of these pollutants are released.