If an addition is made to or a modification is made in a device whose primary purpose is operational, but the addition or modification is a "facility, device or method for the control of air or water pollution" as defined in NRS 361.077, the value of the addition or modification, but not the value of the entire device, is exempt from taxation.
Nev. Admin. Code § 361.048
NRS 360.090, 361.077