40 CFR §63.5285

1. (a)You are subject to this subpart if you own or operate a leather finishing operation that is a major source of hazardous air pollutants (HAP) emissions or that is located at, or is part of, a major source of HAP emissions. A leather finishing operation is defined in § 63.5460. In general, a leather finishing operation is a single process or group of processes used to adjust and improve the physical and aesthetic characteristics of the leather surface through multistage application of a coating comprised of dyes, pigments, film-forming materials, and performance modifiers dissolved or suspended in liquid carriers.
2. (b)You are a major source of HAP emissions if you own or operate a plant site that emits or has the potential to emit any single HAP at a rate of 10 tons (9.07 megagrams) or more per year or any combination of HAP at a rate of 25 tons (22.68 megagrams) or more per year.
3. (c)You are not subject to this subpart if your source finishes leather solely for the purpose of research and development.