By Jim Wedeking, a counsel with Sidley Austin LLP in the firm’s Washington, DC office. He is the WLF Legal Pulse’s Featured Expert Contributor on environmental-law matters.

Although the U.S. Environmental Protection Agency’s hot pursuit of mobile source Clean Air Act violations is a few years old, citizen suits for automotive “defeat devices” are still relatively new. One company can sell or install defeat devices for thousands of vehicles all over the country. The federal government can pursue those violations anywhere, but as a recent U.S. Court of Appeals for the Tenth Circuit decision confirmed, citizen suit plaintiffs face challenges. To establish causation for purposes of Article III standing, they must demonstrate a geographic nexus with the area where the affected vehicles may be degrading air quality. The presumptions used in establishing that nexus, however, are unique to mobile sources.

The Clean Air Act is best known for its complex regulation of “stationary source” industrial facilities, like power plants and oil refineries. Title II of the Clean Air Act, however, is devoted to pollutant emissions from cars, trucks, and motorcycles, known as “mobile sources.” Where the regulation of stationary sources involves cooperative federalism programs that often see state environmental agencies as the primary permit writers, inspectors, and enforcers, the Title II mobile source program is, with one exception, an exclusively federal affair.1   “[T]he difficulty of subjecting motor vehicles, which readily move across state boundaries, to control by individual states” and “the possibility of 50 different state regulatory regimes,” raises “the spectre of an anarchic patchwork of federal and state regulatory programs, a prospect which threatened to create nightmares for the manufacturers.”[/note] Engine Manufacturers Association v. EPA, 88 F.3d 1075, 1079 (D.C. Cir. 1996) (internal quotations omitted).[/note]  

The U.S. Environmental Protection Agency (“EPA”) mobile source program issues a suite of vehicle emission standards based on model year, fuel type (gasoline or diesel), and vehicle type (e.g., light-duty trucks, medium-duty passenger vehicles, etc.).2 To lawfully sell a vehicle in the United States, manufacturers put vehicles through a series of complex tests to demonstrate that they will meet EPA’s emission standards for their entire useful life.3  These test results are part of an application package, submitted by every vehicle manufacturer every year for every model they want to sell the following year, to obtain an EPA certificate of conformity.4

Emission standards are a big reason why cars have swapped carburetors for computers. Everything from air-to-fuel ratio to ignition timing has to be maintained within certain parameters to work with emission control devices, such as the catalytic converter, diesel particulate filter, or the exhaust gas recirculation valve. Highly computerized cars may obviously be the bane of the shade-tree mechanic tasked with their repair, but the environmentally vital computer systems have other potential enemies.

The first potential enemy may be an automobile’s manufacturer. The Volkswagen emissions scandal showed that, as emission standards get tighter, manufacturers have opportunities to cheat. Volkswagen used a software program that recognized when vehicles were being tested for emissions. In “dyno mode,” the computer added high volumes of fluid into the exhaust system that reduced nitrogen oxide emissions enough to meet EPA standards.5  When back on the street, the software injected far less fluid in order to preserve performance.6 Nitrogen oxide emissions ballooned to 35 or 40 times the EPA standard.7 This type of software program is one example of an illegal “defeat device,” designed to turn off emission controls or reduce their effectiveness.8

The second potential enemy of highly computerized cars is the gearhead. A motivated gearhead will strip an engine and exhaust system down to the last gasket, swapping out cylinder heads, air intake systems, transmissions, exhaust headers, and even re-tuning the computer system in order wrench out every last bit of horsepower and torque. Although EPA will look the other way if these cars are towed to the local track on Saturday night, they cannot be driven on public roads. To meet emission standards, manufacturers take great pains to finely tune vehicles for mediocre performance. Even minor changes to a vehicle’s engine or exhaust system can increase emissions beyond EPA standards.

Enforcement actions against companies that sell aftermarket parts considered to be defeat devices became an EPA National Compliance Initiative. The aftermarket-part industry has been around for decades, making the recent enforcement initiative a shock to many businesses as products they had been selling to gearheads for years were now considered illegal. EPA racked up hundreds of defeat-device settlements. A handful involve household names, but most are with small companies.9 Some of the defendants who were entirely unaware that selling performance parts did anything other than improve performance, openly and unknowingly advertising illegal parts. Others were not so naïve, selling parts specifically designed to disable  or replace emission controls. The Diesel Brothers seemed to fall into both groups, building enormous uncontrolled (and hence, illegal) diesel trucks, with names like “Truck Norris” and the “BroDozer,” that belched out black smoke for a televised audience.

David Sparks, Josh Stuart, Keaton Hoskins, and David Kiley, through their collection of companies, modified diesel trucks and resold them.10 Adept social media marketing netted them a series on the Discovery Channel, where they adopted the nom de télévision of “The Diesel Brothers.”11 A group called Utah Physicians for a Healthy Environment sued the Diesel Brothers, alleging that, through their television show and various businesses, the quartet removed emission control devices from diesel vehicles, installed aftermarket defeat devices, sold aftermarket defeat devices, and sold vehicles that used defeat devices, a total of 25 violations.12  After summary judgment proceedings and a bench trial, Utah Physicians for a Healthy Environment prevailed and the trial court imposed more than $765,000 in civil penalties on The Diesel Brothers and their affiliated companies.13 They appealed, raising several arguments.

Although the Tenth Circuit largely affirmed the judgment, it did require the district court to re-visit an aspect of Article III standing. Before the district court, Utah Physicians for a Healthy Environment successfully argued that the Diesel Brothers’ trucks emitted excess pollutants that exacerbated respiratory problems experienced by its members and reduced visibility in scenic areas in the Wasatch Front area of Utah. The Diesel Brothers contested standing on appeal.

Their primary standing argument was that the modified diesel trucks contributed “a negligible fraction of the total pollution that comes from a myriad of sources.”14 Although the argument failed, it does demonstrate how, for standing purposes, mobile sources distinguish themselves from stationary sources. Where a plaintiff alleges harm from stationary source emissions, they need to show (1) their physical injuries are consistent with inhaling the plant’s industrial pollutants, and (2) that they live close enough to the plant to show actual exposure. But mobile sources are mobile and, unlike industrial plants, they operate on an intermittent basis. This means that parties may not know where they were driven or the quantity of excess pollutants emitted. Here, the parties estimated that the trucks drove less than 4,285 miles within Utah, which would result in 0.02 tons of excess nitrogen oxides (about 40 pounds) and 0.0004 tons of excess particulate matter (i.e., less than a pound). 15  The Diesel Brothers argued that this could not meaningfully contribute to the injuries of the group’s members.

The Tenth Circuit disagreed, finding that Congress intended the Clean Air Act citizen suit provision to authorize enforcement actions for even minute amounts of unlawful emissions. To do otherwise, the court held, would be to allow “only for claims against the largest polluters,” amounting “to major surgery on the CAA’s citizen-suit provision.” 16  Allowing suits against small emitters was consistent with the citizen suit provision’s purpose, permitting private enforcement when government officials lacked interest. 17  Government is unlikely to expend limited resources targeting small players with small emissions.18 But this sidestepped the real question of causation for purposes of Article III standing, a question that congressional intent cannot answer.

The Tenth Circuit believed it found the answer in tort law. In a lengthy footnote, the court explained that “when, as here, there are multiple sufficient sets of causes of an injury, the injured person has a claim against any negligent actor responsible for one of the causes unless that actor’s ‘negligent conduct constitutes only a trivial contribution to a causal set.’”19  But this appears to beg the question: are 40 pounds of nitrogen oxides and one pound of particulate matter emitted somewhere within the 100-mile Wasatch Front “sufficient” to injure the plaintiff’s members, or are they “only a trivial contribution?”

Nevertheless, the Tenth Circuit considered this question resolved, moving on to compare the minute contributions of nomadic emission sources to a case where the defendant unsuccessfully argued that its coal-fired power plant contributed only a portion of the smog reducing visibility at nearby national parks.20 This is somewhat troubling as the question of whether the power plant’s emissions reached the parks and reduced visibility was not in doubt.21 Further, the Tenth Circuit opinion touted in support other cases holding that there was “no need for specific evidence of air pollution’s ‘geographic range when plaintiffs sit squarely in the discharge zone of a polluting facility such that their proximity speaks for itself.’”22 With a handful of diesel trucks driven somewhere within the Wasatch Front, the question of whether plaintiff’s members were exposed to those emissions, and if so, whether approximately 41 pounds of excess emissions really contributed to their respiratory problems, seems entirely different.

The Tenth Circuit, however, did not lose all sight of the differences between mobile and stationary sources. While it was happy to assume exposure for small amounts of pollution within the Wasatch Front,23 “if the vehicle was never driven in Utah, or the defeat [device] part was sold to someone out-of-state, [plaintiff] has not established standing” as “the emission of pollutants outside of Utah” could not cause their injuries.24 The court created a rebuttable presumption that emissions caused a plaintiff’s injury if the pollutants were emitted in the same EPA-designated nonattainment area where the plaintiff resides.25 The court explained that Utah Physicians for a Healthy Environment members could have established standing for vehicles driven in southern Wyoming if pollutants from “southern Wyoming were contributing to the failure of the Salt Lake City area to meet air-quality standards and the EPA designated that part of Wyoming as included in the Salt Lake City nonattainment area.”26

EPA’s regional nonattainment air districts are a curious shorthand for presuming that a defendant’s emissions cause a plaintiff’s injury for purposes of Article III standing. It does serve the purpose of establishing a clear standard and honoring Tenth Circuit precedent requiring “a geographic nexus” between the plaintiff and the location suffering from the defendant’s pollution.27 Usually, this involves the plaintiff providing an affidavit stating that they visit a polluted park or recreate on a contaminated stream. But there are still question with respect to the Tenth Circuit’s use of nonattainment areas to create boundaries for standing purposes.

For instance, it is not clear what the court would hold if the plaintiff and the pollutant’s emissions were in the same attainment area, meaning that the area complies with EPA’s ambient air quality standards. Would there be a rebuttable presumption against causation as the area’s ambient air quality is deemed by EPA to be healthy? Further, it is not clear that there is any scientific or legal basis for presuming causation simply because both the plaintiff and the emissions are co-located in a regional area created by EPA for a different purpose. Using the Washington, D.C. nonattainment area as an example, it may not be credible to presume that diesel trucks emitting excess emissions in Lusby, Maryland cause ill health effects in Leesburg, Virginia, roughly 80 miles to the northwest, simply because EPA collected them into the same air district.

Nevertheless, the Tenth Circuit’s Utah Physicians for a Healthy Environment decision is likely to have a significant impact on the potential for future Clean Air Act citizen suits targeting mobile source emission violations. The geographic limitations mean that national environmental groups are more likely to target dense metropolitan areas where many defeat devices could be on the road. Utah Physicians for a Healthy Environment filed suit in 2017, endured motion practice and a bench trial, litigated an appeal, and will now return to the district court and re-litigate the civil penalty amount based on the Tenth Circuit’s decision to exclude certain violations outside of the nonattainment area. This is a lot of time and effort for 25 alleged violations. Future plaintiffs could cast a wider net in the Los Angeles-South Coast Air Basin nonattainment area, but they cannot drag their net nationwide.


  1. Under 42 U.S.C. § 7543(a), “[n]o state or any political subdivision thereof shall adopt or attempt to enforce any standard relating to the control of emissions from new motor vehicles or new motor vehicle engines.” A state may apply to EPA for a “waiver” allowing its own emission standards for new motor vehicles or new motor vehicle engines, but only if it first “adopted standards …. Prior to March 30, 1966.” Id. § 7543(b)(1). The only state meeting that description is California and the California Air Resources Board has been very active in both issuing its own mobile source standards and enforcing them.
  2. See generally, 40 C.F.R. Part 86.
  3. See generally, 40 C.F.R., Part 86, Subpart S. The appendices to Part 86, governing everything from how to operate the vehicle while on a dynamometer to how to accelerate and decelerate the vehicle on a road course to how to simulate catalyst aging over time gives a flavor for the technical complexity of EPA’s testing regime.
  4. 40 C.F.R. § 86.148-01.
  5. See In re Volkswagen “Clean Diesel” Litig., 959 F.3d 1201, 1207-08 (9th Cir. 2020).
  6. Id.
  7. Id.
  8. EPA defines “defeat device” very broadly: any “auxiliary emission control device (AECD) that reduces the effectiveness of the emission control system under conditions which may reasonably be expected to be encountered in normal vehicle operation and use.” 40 C.F.R. § 86.094-2. “AECD” is even broader in that it captures “any element of design which senses temperature, vehicle speed, engine [rotations per minute], transmission gear, manifold vacuum, or any other parameter for the purpose of activating, modulating, delaying, or deactivating the operation of any part of the emission control system.” Id. § 86.1803-01 (emphasis added). Thus, an unlawful AECD can be a software change, a different location for the oxygen sensor, or anything in between.
  9. For instance, EPA settled with Boost Diesel Repair LLC, a shop in Houston, for the installation of defeat devices on 15 vehicles. The company paid $100. EPA’s mobile source settlement database can be found here.
  10. Utah Physicians for a Health Environment v. Diesel Power Gear, LLC, Case No. 20-4043 (10th Cir. Dec. 28, 2021) Slip Opinion (“Slip Op.”) at 10-11.
  11. Id. at 11; see also Discovery, Diesel Brothers.
  12. Slip Op. at 12.
  13. Id. at 13-14.
  14. Id. at 18.
  15. Id.
  16. Id. at 20.
  17. Id.
  18. Id. (“Uncommon would be the case in which mobile sources of air pollution, particularly a few motor vehicles, would be the target.”).
  19. Id. n. 10 (quoting Restatement (Third) of Torts: Phys. & Emot. Harm § 36 (2010) (emphasis deleted). The court also compared the case to one of aggregate nuisance, where one person may contribute only a slight amount to stream pollution but, in conjunction with the similar acts of others, the stream becomes so polluted that it threatens the rights of water users. Id. (citing Restatement (Second) of Torts § 840E cmt. b (1979)).
  20. Id. at 21 (citing Sierra Club v. EPA, 964 F.3d 882, 887-88 (10th Cir. 2020)).
  21. Sierra Club, 964 F.3d at 889-90 (citing 81 Fed. Reg. 2,004, 2013 (Jan. 15, 2016)).
  22. Slip Op. at 22 (quoting Env’t Tex. Citizen Lobby, Inc. v. ExxonMobil Corp., 968 F.3d 357, 370 (5th Cir. 2020)).
  23. The court seemed to cement its bright-line rule on this issue later in the opinion, stating that “[i]f the vehicle was driven, however little, in the Salt Lake City area, [plaintiff] has established that its members’ injuries from excessive pollution can be fairly traced to the CAA violation; so standing can be predicated on pollution from that vehicle.” Id. at 30. The notion of a court assuming that even briefly starting up and test driving a vehicle with a defeat device causes injuries to a person that could be dozens of miles away seems unsatisfying, especially since some defeat devices do not operate until certain driving conditions are present, such as heavy acceleration.
  24. Id. at 30-31.
  25. A nonattainment area is any place that is not in compliance with EPA’s various National Ambient Air Quality Standards. See generally, 42 U.S.C. § 7407(d). An “area” for purposes of attainment or nonattainment is typically a metropolitan area consisting of several counties and towns. The Washington, DC nonattainment area, for example, consists of the District of Columbia, five counties in Maryland, and nine cities and counties in Virginia.
  26. Slip op. at 31.
  27. See, e.g., DINÉ Citizens Against Ruining the Env’t v. Bernhardt, 923 F.3d 831, 841 (10th Cir. 2019).