The FEM How to Higg Guide provides guidance on the basic emission sources or pollutant types you might need to cover.
A key pollutant is generally defined by its source. For example, the key pollutants from combustion sources are CO2, carbon monoxide, particulate matter, sulfur dioxide, or nitrous oxide.
It is recommended that you follow the legal requirements for more information on the key pollutants. You may check the FEM How to Higg Guide here.