In Bid to Leave Ozone Pact, NH Says Ending Emissions Tests Won’t Worsen Pollution. Some Have Doubts.

Earlier this year, the commissioner of the New Hampshire Department of Environmental Services cautioned lawmakers about potential air quality challenges and federal consequences the state could face if it moved to end mandatory vehicle emissions testing without approval from the U.S. Environmental Protection Agency. 

Those consequences would take the form of sanctions up to the possible withholding of federal highway funding from New Hampshire, EPA Region One Administrator Mark Sanborn told the state in an April letter.

Lawmakers passed legislation ending the program anyway, approving an early 2026 sunset on emissions testing and vehicle inspections along with the biennial budget in June. 

Now, to enact that while sidestepping federal penalties, New Hampshire is preparing to file with the EPA to amend the state’s air quality protection plan and to take the historic step of becoming the first state to fully withdraw from the Ozone Transport Commission, a regional air pollution control organization. 

NHDES argues in draft versions of those applications that the state’s air quality will not worsen when emissions testing ends. But the data that case is built on, some experts said, might not paint a full picture of the ways ending the program could affect long-term air quality across the state.

Emissions concerns and testing

During a New Hampshire emissions test, a technician scans a vehicle’s computer, which is designed to track the performance of key pollution-reducing systems and flag malfunctions. This is less involved and, according to NHDES, less costly than the physical “tailpipe” testing required in some other states, where the gas released from a car’s exhaust system is analyzed for excessive pollution. 

Both varieties of emissions tests are intended to limit the release of harmful byproducts formed when fuel burns. As vehicles age and components like the exhaust-scrubbing catalytic converter begin to fail, they emit more and more of these byproducts, including compounds like nitrogen oxides, volatile organic compounds, and particulate matter. These chemicals have a wide range of harmful effects, said vehicle emissions expert Maria Cecilia Pinto de Moura, who works with the Union of Concerned Scientists in Washington, D.C.

In addition to contributing to acid rain formation, nitrogen oxides and volatile organic compounds react with sunlight in the atmosphere to form ozone, a respiratory irritant that can make it harder to breathe and aggravates lung conditions like asthma and emphysema. Ozone can be dangerous even at low levels, according to the EPA; when it becomes concentrated near the ground, it is the main component of the hazy pollution often referred to as smog.

Nitrogen oxides can also be converted into particulate matter in the atmosphere, Pinto de Moura said. Particulate matter is linked to a broad range of illnesses from lung cancer and cardiovascular disease to diabetes, obesity, and neurological conditions, including Alzheimer’s and Parkinson’s diseases.

“It’s really damning,” Pinto de Moura said.

Vehicle emissions are the most significant source of human exposure to those chemicals, according to John Dunlap, former chair of the California Air Resources Board. 

“Fifty-five percent or so of human exposure to air pollution comes from cars and the burning of fuel,” he said. “That’s consequential.”

Clean Air Act says states must play a role in reducing smog

Enacted in response to nationwide concerns around air quality, acid rain, and human health, the Clean Air Act of 1963 was the first federal legislation aimed at controlling smog and other forms of air pollution. The act and subsequent amendments established air quality targets and called for states to have “State Implementation Plans” in place to continue meeting federal air quality goals.

In many states, including New Hampshire, vehicle emissions testing has been a central component of those plans. And a 1990 amendment to the Clean Air Act united New Hampshire, 11 other Northeast and Mid-Atlantic states, and the District of Columbia into the Ozone Transport Commission in response to interstate air quality concerns across the region. Members of the commission are required to conduct emissions testing.

Because New Hampshire has previously listed emissions testing among the programs it employs to remain in compliance with the Clean Air Act, in addition to putting New Hampshire out of compliance with requirements of Ozone Transport Commission membership, eliminating the program could put the state in violation of the Clean Air Act if the State Implementation Plan is not updated and approved by the EPA.

“If this legislation passes, New Hampshire will not meet the (Clean Air Act) requirements for (inspection and maintenance) programs,” wrote Sanborn, the EPA Region One administrator, in an April 25, 2025, letter to NHDES Commissioner Robert Scott.

To remedy this, according to NHDES, New Hampshire is taking steps to leave the Ozone Transport Commission and amend its State Implementation Plan. That includes describing how the state will continue to meet air quality targets even without emissions tests.

“With legislation eliminating a part, an important part, of the state implementation plan, the state has to demonstrate to EPA that it can and will comply with the Clean Air Act without the program,” said Tom Irwin, vice president of the Conservation Law Foundation’s New Hampshire chapter.

Projections and doubts

In a draft version of the amended implementation plan, NHDES cites air quality monitoring data that shows New Hampshire with lower-than-required levels of several emissions-related pollutants in recent years.

This is consistent with overall trends, which show pollution on the decline in New Hampshire as a whole, said NHDES spokesperson Jim Martin in an email on Nov. 3. “Current emission levels in New Hampshire are well below the levels that NH was experiencing when the state first came into attainment with National Ambient Air Quality Standards,” he wrote.

The proposed revision also cites a projection of air quality measurements for New Hampshire into 2026 as evidence that air quality will not decline when mandatory emissions testing ends.

Compared to 2022, the 2026 projection shows a 2.7% reduction in volatile organic compounds, a 16% reduction in nitrogen oxides, and a 1.9% reduction in carbon monoxide. According to attachments included with the draft implementation plan revision, more of this decline is driven by continued reduction in emissions from vehicles than by reductions from other emissions sources, like power plants, which show less of a decrease.

This is because even without emissions testing in place, modern vehicles are continually becoming more fuel-efficient, Martin said.

The state’s draft petition to leave the Ozone Transport Commission includes modeling that stretches further into the future, to 2038, and shows a continued decrease in nitrogen oxide and volatile organic compound emissions.

NHDES generated this data with an EPA emissions modeling platform that uses a broad assortment of data, including the miles and type of roadways in a state, the number and age of cars on the road, and weather data, to estimate the amount of emissions released into the environment in a given time period, said Pinto de Moura.

This modeling method is widely used and generally reliable, she said. However, she added, the reliability of the platform’s output depends on the breadth of information it is fed.

The effects of ending emissions testing would likely grow over time, said Jack Dibb, emeritus research associate professor in earth sciences at the University of New Hampshire who has studied vehicle emissions. Dibb said he worried the effects of ending the program would compound as more cars’ emissions control systems fail without a testing program in place to make drivers aware or to compel repairs.

“Right away, nothing major’s going to happen, but over time, air quality could seriously degrade,” he said.

In a letter dated Feb. 10, 2025, NHDES Commissioner Scott addressed Rep. Thomas Walsh, a Hooksett Republican and chairman of the House Transportation Committee, about the then-proposed elimination of the emissions testing program.

NHDES did not take a position on the proposal, according to Scott’s letter. But later in the letter, Scott outlined the impact of vehicle emissions on New Hampshire’s air quality,

“Motor vehicles in New Hampshire account for half of the air pollution that causes the formation of ground level ozone,” he wrote.

While the model shows air pollution on the decline overall in New Hampshire, it was not clear how much of an effect on the overall trend the elimination of emissions testing would have on the total pollutants still being released into the atmosphere. NHDES did not run a model to compare air pollution totals if emissions testing were to continue, Martin said.

“If NHDES had modeled a scenario in which (the inspection and maintenance) program was not discontinued, 2026 emission estimates would have been even lower,” he said.

Balancing the burdens of air quality and cost

Last session, proponents of the inspection and maintenance program repeal said it will benefit consumers, particularly those for whom the cost of an inspection and repairs could be a significant burden.

Studies have indeed shown that “the older cars, which are the highest emitters, are predominantly owned by lower income groups,” said Pinto de Moura.

Irwin echoed those concerns.

“There are some legitimate equity concerns associated with inspection and maintenance programs,” he said. “The impacts, the costs and inconveniences of the program, can often fall more heavily on low-income communities.”

But the burden and the harms of air pollution also fall unevenly across society, Pinto de Moura said. 

Air pollution can vary widely within a relatively small area according to factors like traffic patterns and housing proximity to highways. In a 2019 study, Pinto de Moura and fellow Union of Concerned Scientists researcher David Reichmuth demonstrated how that variability is borne unequally by people of color, with Asian American, Black, and Latino residents of the Northeast and Mid-Atlantic regions breathing air that is, on average, 40% more polluted than the air breathed by the typical white resident.

“Exposure itself is inequitable,” Pinto de Moura said. “… Controlling these substances has an incredible impact on communities.”

In New Hampshire specifically, she said, the regional variability of air pollution also means that the state’s overall low average pollution totals could obscure more severe pockets of concentrated pollutants.

“Averages hide reality,” Pinto de Moura said. 

What happens now

The public comment period on draft versions of the implementation plan revision and the state’s application to leave the OTC will end Friday, Nov. 21. A public hearing on the revision will be held at 1:30 p.m. Monday, Nov. 17, at NHDES headquarters in Concord. 

“NHDES will review all comments from interested parties, including EPA,” Martin said.

NHDES will then file those proposals with the EPA — but, as Scott noted in his Feb. 10 letter, it is not clear how long it will take the EPA to review the applications. Generally, it can take the EPA 18 to 24 months to review implementation plan revisions, he wrote; as the emissions testing program is set to end Jan. 31, this would put the state’s policy out of compliance in the interim.

This could put the state at risk to face federal sanctions, Scott wrote, “up to and including loss of federal highway funds.” Sanborn said in his April 25 letter the EPA would consider imposing sanctions on New Hampshire if the state violates its Clean Air Act commitments.

If the EPA determines a state has violated the Clean Air Act, a two-year clock begins to tick, after which the agency can restrict available highway funds as a penalty. Road improvement projects like new interchanges, repaving and resurfacing, lane additions, and more would no longer be eligible for federal grants until the state comes back into compliance, according to the website of the Federal Highway Administration. However, the restriction does not affect problems that would improve safety or emissions, like road improvements designed to reduce crashes and ease polluting traffic.

Even after the review period, it’s not clear whether the EPA will approve the implementation plan revision at all. If the EPA does not approve of the revision as proposed, New Hampshire will have to find another way to meet Clean Air Act requirements, Irwin said.

“Citizen lawsuits,” when individuals or organizations sue those they believe are violating the Clean Air Act, are another possible outcome of the program’s ending ahead of EPA approval. Scott cited the risk of such cases in his Feb. 10 letter, saying they “could pose unquantifiable, but potentially significant, costs for NHDES and NHDOJ to defend.”

However, in his Nov. 3 email, Martin said the draft proposals were “technically and legally sound” and addressed “all the requirements of the Clean Air Act.”

“The fact that New Hampshire is proposing to leave the (Ozone Transport Commission) because of improvements in air quality shows that the CAA is working as Congress intended,” Martin said.