You are subject to this subpart if you own or operate a primary magnesium refinery that is (or is part of) a major source of hazardous air pollutant (HAP) emissions. Your primary magnesium refinery is a major source of HAP if it emits or has the potential to emit any single HAP at a rate of 10 tons or more per year or any combination of HAP at a rate of 25 tons or more per year.