Companies in Ontario who wish to build new facilities, make modifications to existing operations that may result in a change in atmospheric emissions, or to operate existing facilities built after 1971, are required to obtain a Certificate of Approval (Air) (C of A) from the Ontario Ministry of the Environment (MOE) to demonstrate that they will not be adversely affecting air quality.

Facilities requiring applications for air licences and permits may range in complexity from small commercial facilities (i.e. parking garage, paint spray booth, etc.) to large industrial complexes. All aspects of air emissions from the processes involved may have to be included in the application. An inventory of emissions from the facility is a requirement for a successful application, and the use of regulatory or other air dispersion models, to demonstrate the effects of the facility on air quality, is required.