If you wanted to make air pure or free from dust,smog and so on what type of filters would you need?

Answer 1

A HEPA filter is designed to remove dust and is effective at removing some organic contaminates (ie pollen and bacteria). Smog is not just particulates, it has gasses mixed in such as ozone, hydrocarbons, carbon monoxide and oxides of nitrogen. No filter can separate the various types of molecules in air although activated carbon has some ability to absorb hydrocarbons on its surface. What can be done is to chill air to become a liquid. Each gas has a distinct boiling point allowing capturing Oxigen and Nitrogen components for example at high purity which could be recombined to give you a "purified" air.

Answer 2

a filter that would go around the entire earth