Federal investigators estimate hundreds of thousands of Americans have illegally rigged their diesel pickups to collectively emit 16 times the level of pollution they otherwise would have over the last 10 years, according to a report by the Environmental Protection Agency.

Clean air advocates say the findings uncovered by the EPA are massive in scope, dramatically outweighing the extra emissions discovered during Volkswagen AG's "Dieselgate" scandal. But solving the issue will be challenging, as regulators grapple with how to catch tampering done by individual vehicle owners and garages rather than a single corporation.

Addressing the problem, they say, will likely take help from underfunded state and local agencies. For example, Michigan — the epicenter of the U.S. auto industry — is one of 18 states that don't have an emissions testing program, which experts say is the first line of defense against illegal tampering, or any state laws that would allow local enforcement.

An emissions testing program would not be guaranteed to catch those tampering with the emissions controls on their vehicles, according to the EPA. But experts say it would be a start to solving a larger problem driven by a cultural interest in high-powered trucks.

"State-based testing programs would be helpful — indeed the only way — to address the problem," John DeCicco, a professor emeritus at the University of Michigan whose research focuses on vehicle emissions, said via email.

But "what will be at least as important is a broad-based and long-term public education effort. Otherwise, testing and attempts at enforcement likely will be stymied by backlash and resultant political obstacles."

Excess pollution

Over the last five years, the EPA's Air Enforcement Division has been investigating the use of aftermarket tampering devices installed in medium-weight diesel pickups since 2010.

Often known as defeat devices, the tools are hardware or software installed in trucks to override the manufacturer's emissions controls which help reduce the amount of harmful NOx and particulate matter the vehicles produce — air pollution that can have adverse health effects and contribute to climate change.

Tampering with a vehicle's emissions controls is illegal under federal law, but people do it to improve the mileage, power or look of their vehicle, among other reasons. And even though selling the devices is illegal, many retailers openly market the devices online and often avoid penalties in part due to limited resources from enforcement agencies like the EPA.

The EPA aggregated data from its investigations, including device sales records, on-site inspections, interviews and more, to estimate a base level of how many Class 2b and 3 diesel pickups (such as Ford F-250s or GMC Sierra 3500s) had installed a device that would do a "full delete" of emissions controls, the report says.

They estimated that emissions controls have been removed from at least 550,000 vehicles over the last decade — around 15% of all diesel trucks sold with emissions controls in the U.S. during the same timeframe.

The reality is likely even more severe. EPA investigators were careful not to double-count sales and focused narrowly on one type of vehicle that defeat devices are used on. "This is the floor," said Evan Belser, deputy director of the EPA's Air Enforcement Division.

Investigators used emissions testing and modeling on three trucks with defeat devices to extrapolate the potential impact for emissions. What they concluded stunned many environmental advocates and local regulators: Just over a half million trucks are polluting at the level of 9 million trucks, releasing more than 575,000 tons of excess pollutants into the air.

"Aftermarket tampering we know is a huge deal, but we didn't know how huge a deal it was," said Miles Keogh, executive director of the National Association of Clean Air Agencies, which represents state and local air pollution control agencies across the country.

He estimated the excess emissions caused by the tampered diesel pickups studied by the EPA are 10-40 times greater than the excess emissions caused by Volkswagen's emissions cheating scandal in 2015: "Forty times larger than Dieselgate is definitely attention-grabbing."

David Pettit, an environmental attorney with the National Resources Defense Council, also said he was surprised by the extent of the findings. "The magnitude of the excess emissions that are caused by these devices are really staggering," he said.

However, Gary Bishop, a leading scientist studying vehicle emissions at the University of Denver, said he was skeptical that the number of tampered trucks the EPA found actually translates to the level of extra emissions they estimated.

"As the trucks get older their mean emissions would start to go up," Bishop said, which isn't reflected in the data he's collected in localized studies.

A decentralized problem

Compared with many other high-profile incidents of emissions tampering, the EPA's recent findings implicate thousands of individuals, garages and independent retailers.

That kind of decentralized problem is tougher to tamp down, experts say. Adding to the challenge is that the law barring tampering is not delegated to the states for enforcement.

EPA has "been getting starved for money for decades," Keogh said, and it's already spread thin working to enforce other environmental hazards. The agency can go after retailers and already has: Earlier this year, the EPA and Department of Justice got one retailer to pay $850,000 in civil penalties for making and selling defeat devices.

State and local agencies are usually working with even fewer resources, but EPA investigators say they have played a crucial role for decades in controlling air pollution from vehicles.

"There are a number of things that some states already have on the books that could be activated and be used as a source of authority if states chose to take enforcement," said Besler of EPA. That might include additional state laws making emissions control tampering illegal — which would allow aggressive state attorneys general to pursue cases on the issue — or programs requiring regular emissions testing, which might identify device installations.

Yet some experts argue that defeat devices are designed to evade emissions testing, so any testing program that would identify most tampered vehicles would have to replicate actual driving conditions.

The problem with emissions tests, Bishop of the University of Denver said, is that truck owners know when they're coming.

"For these types of trucks, the only way you can catch stuff like this is to do what you do with drunk drivers: find them, pull them over, give them a test and if they flunk it they're removed," he said.

Most experts agreed that beginning an inspection program, state and federal enforcement of retailers and improving technology to catch emissions cheating are good places to start. But what may be most effective, they said, is a campaign to change the way people think about their trucks.

Gratuitous pollution for the sake of a "bad boy" truck image, performance, fun or freedom "is fundamentally not a technical problem, and so a purely technical solution may not get very far," DeCicco of UM said via email. "A social sanction will need to be developed, so that the behavior comes to be frowned upon enough to support legal sanction."

Most people who choose to override emissions controls on their trucks likely aren't aware that there are hundreds of thousands of others out there like them, collectively having "a significant public health impact. They think they're just one dude, driving around in their dirty truck," said Keogh.

"It's way too early to say what folks will do about it," he added. "But I do not think it's too early to say folks are going to do something about this."

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