A Multi-Celled organism has many cells (more than one). Examples would be plants, humans and animals/mammals.
A Single-Celled organism only has one cell that makes up it's entire self. Examples would be bacteria and archaea.
If you can connect air pollution leads to global warming and thus climate change.
So in order to decrease pollution levels
1. Lessen the usage of vehicles (motorised)
2. Industrial waste should be treated properly before releasing into the atmosphere