MEMORANDUM *

Nancy and James Oliver appeal the district court’s grant of partial summary judgment in favor of the United States and conclusion that they committed over 7,000 violations of the Clean Air Act (“CAA”). The Olivers also appeal the district court’s imposition of a $75,000 civil penalty and issuance of a permanent injunction preventing them from continuing to incinerate medical waste.

The Olivers operated an incinerator that disposes of medical and industrial waste, but contend they did not have to comply with CAA emissions requirements for such incinerators because they qualified for the “co-fired combustor” exemption. 40 C.F.R. § 62.14400(a), (b)(2). A co-fired combustor is an incinerator for which medical waste comprises less than ten-percent of the weight of materials incinerated, including fuels. See 40 C.F.R. § 62.14490.

The district court correctly held that the Olivers did not operate a co-fired combu-stor. The Environmental Protection Agency determined that the air that is mixed with natural gas as it enters the combustion chamber is not a fuel. This determination is not contrary to or inconsistent with the regulation defining a co-fired combustor. See United States v. Alisal Water Corp., 431 F.3d 643, 651-52 (9th Cir. 2005). Indeed, in common English us *377 age, air is not considered to be a “fuel.” The Olivers’ proposed construction of the co-fíred combuster exemption would allow incinerators that dispose of exclusively medical waste to avoid CAA emissions requirements. See id.

The district court did not abuse its discretion in imposing a $75,000 civil penalty. The court weighed the factors required by the CAA, and imposed a much smaller penalty than that sought by the government due to the small size of the Olivers’ business. See 42 U.S.C. § 7413(e)(1). The district court also did not abuse its discretion in issuing a permanent injunction preventing the Olivers from incinerating medical waste until they demonstrate they can do so while complying with the CAA. See Natural Res. Def. Council v. Sw. Marine, Inc., 236 F.3d 985, 1000 (9th Cir. 2000).

AFFIRMED.