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From what it means for America’s climate goals to how it might make American cars smaller again

The Biden administration just kicked off the next phase of the electric-vehicle revolution.
The Environmental Protection Agency unveiled Wednesday some of the world’s most aggressive climate rules on the transportation sector, a sweeping effort that aims to ensure that two-thirds of new cars, SUVs, and pickups — and one-quarter of new heavy-duty trucks — sold in the United States in 2032 will be all electric.
The rules, which are the most ambitious attempt to regulate greenhouse-gas pollution in American history, would put the country at the forefront of the global transition to electric vehicles. If adopted and enforced as proposed, the new standards could eventually prevent 10 billion tons of carbon pollution, roughly double America’s total annual emissions last year, the EPA says.
The rules would roughly halve carbon pollution from America’s massive car and truck fleet, the world’s third largest, within a decade. Such a cut is in line with Biden’s Paris Agreement goal of cutting carbon pollution from across the economy in half by 2030.
Transportation generates more carbon pollution than any other part of the U.S. economy. America’s hundreds of millions of cars, SUVs, pickups, 18-wheelers, and other vehicles generated roughly 25% of total U.S. carbon emissions last year, a figure roughly equal to the entire power sector’s.
In short, the proposal is a big deal with many implications. Here are seven of them.

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Every country around the world must cut its emissions in half by 2030 in order for the world to avoid 1.5 degrees Celsius of temperature rise, according to the Intergovernmental Panel on Climate Change. That goal, enshrined in the Paris Agreement, is a widely used benchmark for the arrival of climate change’s worst impacts — deadly heat waves, stronger storms, and a near total die-off of coral reefs.
The new proposal would bring America’s cars and trucks roughly in line with that requirement. According to an EPA estimate, the vehicle fleet’s net carbon emissions would be 46% lower in 2032 than they stand today.
That means that rules of this ambition and stringency are a necessary part of meeting America’s goals under the Paris Agreement. The United States has pledged to halve its carbon emissions, as compared to its all-time high, by 2020. The country is not on track to meet that goal today, but robust federal, state, and corporate action — including strict vehicle rules — could help it get there, a recent report from the Rhodium Group, an energy-research firm, found.

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Until this week, California and the European Union had been leading the world’s transition to electric vehicles. Both jurisdictions have pledged to ban sales of new fossil-fuel-powered cars after 2035 and set aggressive targets to meet that goal — although Europe recently watered down its commitment by allowing some cars to burn synthetic fuels.
The United States hasn’t issued a similar ban. But under the new rules, its timeline for adopting EVs will come close to both jurisdictions — although it may slightly lag California’s. By 2030, EVs will make up about 58% of new vehicles sold in Europe, according to the think tank Transportation & Environment; that is roughly in line with the EPA’s goals.
California, meanwhile, expects two-thirds of new car sales to be EVs by the same year, putting it ahead of the EPA’s proposal. The difference between California’s targets and the EPA’s may come down to technical accounting differences, however. The Washington Post has reported that the new EPA rules are meant to harmonize the national standards with California’s.

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With or without the rules, the United States was already likely to see far more EVs in the future. Ford has said that it would aim for half of its global sales to be electric by 2030, and Stellantis, which owns Chrysler and Jeep, announced that half of its American sales and all its European sales must be all-electric by that same date. General Motors has pledged to sell only EVs after 2035. In fact, the EPA expects that automakers are collectively on track for 44% of vehicle sales to be electric by 2030 without any changes to emissions rules.
But every manufacturer is on a different timeline, and some weren’t planning to move quite this quickly. John Bozella, the president of Alliance for Automotive Innovation, has struck a skeptical note about the proposal. “Remember this: A lot has to go right for this massive — and unprecedented — change in our automotive market and industrial base to succeed,” he told The New York Times.
The proposed rules would unify the industry and push it a bit further than current plans suggest.

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The EPA’s proposal would see sales of all-electric heavy trucks grow beginning with model year 2027. The agency estimates that by 2032, some 50% of “vocational” vehicles sold — like delivery trucks, garbage trucks, and cement mixers — will be zero-emissions, as well as 35% of short-haul tractors and 25% of long-haul tractor trailers. This would save about 1.8 billion tons of CO2 through 2055 — roughly equivalent to one year’s worth of emissions from the transportation sector.
But the proposal falls short of where the market is already headed, some environmental groups pointed out. “It’s not driving manufacturers to do anything,” said Paul Cort, director of Earthjustice’s Right to Zero campaign. “It’s following what’s happening in the market in a very conservative way.”
Last year, California passed rules requiring 60% of vocational truck sales and 40% of tractors to be zero-emissions by 2032. Daimler, the world’s largest truck manufacturer, has said that zero emissions trucks would make up 60% of its truck sales by 2030 and 100% by 2039. Volvo Trucks, another major player, said it aims for 50% of its vehicle deliveries to be electric by 2030.

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One of the more interesting aspects of the new rules is that they pick up on a controversy that has been running on and off for the past 13 years.
In 2010, the Obama administration issued the first-ever greenhouse-gas regulations for light-duty cars, SUVs, and trucks. In order to avoid a Supreme Court challenge to the rules, the White House did something unprecedented: It got every automaker to agree to meet the standards even before they became law.
This was a milestone in the history of American environmental law. Because the automakers agreed to the rules, they were in effect conceding that the EPA had the legal authority to regulate their greenhouse-gas pollution in the first place. That shored up the EPA’s legal authority to limit greenhouse gases from any part of the economy, allowing the agency to move on to limiting carbon pollution from power plants and factories.
But that acquiescence came at a cost. The Obama administration agreed to what are called “vehicle footprint” provisions, which put its rules on a sliding scale based on vehicle size. Essentially, these footprint provisions said that a larger vehicle — such as a three-row SUV or full-sized pickup — did not have to meet the same standards as a compact sedan. What’s more, an automaker only had to meet the standards that matched the footprint of the cars it actually sold. In other words, a company that sold only SUVs and pickups would face lower overall requirements than one that also sold sedans, coupes, and station wagons.
Some of this decision was out of Obama’s hands: Congress had required that the Department of Transportation, which issues a similar set of rules, consider vehicle footprint in laws that passed in 2007 and 1975. Those same laws also created the regulatory divide between cars and trucks.
But over the past decade, SUV and truck sales have boomed in the United States, while the market for old-fashioned cars has withered. In 2019, SUVs outsold cars two to one; big SUVs and trucks of every type now make up nearly half the new car market. In the past decade, too, the crossover — a new type of car-like vehicle that resembles a light-duty truck — has come to dominate the American road. This has had repercussions not just for emissions, but pedestrian fatalities as well.
Researchers have argued that the footprint rules may be at least partially to blame for this trend. In 2018, economists at the University of Chicago and UC Berkeley argued Japan’s tailpipe rules, which also include a footprint mechanism, pushed automakers to super-size their cars. Modeling studies have reached the same conclusion about the American rules.
For the first time, the EPA’s proposal seems to recognize this criticism and tries to address it. The new rules make the greenhouse-gas requirements for cars and trucks more similar than they have been in the past, so as to not “inadvertently provide an incentive for manufacturers to change the size or regulatory class of vehicles as a compliance strategy,” the EPA says in a regulatory filing.
The new rules also tighten requirements on big cars and trucks so that automakers can’t simply meet the rules by enlarging their vehicles.
These changes may not reverse the trend toward larger cars. It might even reveal how much cars’ recent growth is driven by consumer taste: SUVs’ share of the new car market has been growing almost without exception since the Ford Explorer debuted in 1991. But it marks the first admission by the agency that in trying to secure a climate win, it may have accidentally created a monster.

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The EPA is trumpeting the energy security benefits of the proposal, in addition to its climate benefits.
While the U.S. is a net exporter of crude — and that’s not expected to change in the coming decades — U.S. refineries still rely on “significant imports of heavy crude which could be subject to supply disruptions,” the agency notes. This reliance ties the U.S. to authoritarian regimes around the world and also exposes American consumers to wilder swings in gas prices.
But the new greenhouse gas rules are expected to severely diminish the country’s dependence on foreign oil. Between cars and trucks, the rules would cut crude oil imports by 124 million barrels per year by 2030, and 1 billion barrels in 2050. For context, the United States imported about 2.2 billion barrels of crude oil in 2021.
This would also be a turning point for gas stations. Americans consumed about 135 billion gallons of gasoline in 2022. The rules would cut into gas sales by about 6.5 billion gallons by 2030, and by more than 50 billion gallons by 2050. Gas stations are going to have to adapt or fade away.

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Although it may seem like these new electric vehicles could tax our aging, stressed electricity grid, the EPA claims these rules won’t change the status quo very much. The agency estimates the rules would require a small, 0.4% increase in electricity generation to meet new EV demand by 2030 compared to business as usual, with generation needs increasing by 4% by 2050. “The expected increase in electric power demand attributable to vehicle electrification is not expected to adversely affect grid reliability,” the EPA wrote.
Still, that’s compared to the trajectory we’re already on. With or without these rules, we’ll need a lot of investment in new power generation and reliability improvements in the coming years to handle an electrifying economy. “Standards or no standards, we have to have grid operators preparing for EVs,” said Samantha Houston, a senior vehicles analyst at the Union of Concerned Scientists.
The reduction in greenhouse gas emissions from replacing gas cars will also far outweigh any emissions related to increased power demands. The EPA estimates that between now and 2055, the rules could drive up power plant pollution by 710 million metric tons, but will cut emissions from cars by 8 billion tons.
This article was last updated on April 13 at 12:37 PM ET.
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On NRC moves, Blue Energy, and China’s solar and methanol breakthroughs
Current conditions: The World Cup’s final match between Argentina and Spain is set to take place Sunday in New Jersey, where the thick orange haze of Canadian wildfire smoke is still hovering • Temperatures are soaring to 110 degrees Fahrenheit in Ethiopia’s northeast Afar province • Researchers just categorized the first major dust storm of Arizona’s monsoon season, which struck Phoenix earlier in the week, as a Category 3.

On Tuesday, I told you about the United Arab Emirates’ plan to build a new port to bypass the Strait of Hormuz. Iraq and its oil partners are looking westward. The Financial Times reported yesterday that Chevron and Baghdad are in advanced discussions to form a consortium to build and restore a pipeline network through Syria as an alternate route to export oil. The U.S. oil giant is working with the Los Angeles-based TI Capital and an investment group owned by the Syrian-Qatari billionaire Al-Khayyat brothers, who own a major construction company in the Gulf nation and are, according to Bloomberg, “betting big” on Syria’s post-war reconstruction.
It’s yet another sign that, as my colleague Matthew Zeitlin wrote, it’ll be a long time before the Strait of Hormuz returns to normal operations — especially now that the war is back on.
Just two weeks ago, I told you that the Nuclear Regulatory Commission had proposed both overhauling how it measures the risks from radiation exposure and giving more flexibility to developers to prove their reactors operate safely. Now the agency is continuing its regulatory blitz with another rule, posted Thursday to the Federal Register, to smooth the way for license renewals, speed up approvals to begin construction on certain components and structures at new nuclear plants, and provide more guidance for technologies that use coolants other than water.
In Spain, meanwhile, the country’s Nuclear Safety Council gave the country’s oldest nuclear station, the Almaraz plant two hours west of Madrid, the greenlight to continue operating until 2030, according to NucNet. Currently, the Spanish government is pursuing the world’s only active nuclear phaseout policy. Virtually every country that has phased out atomic energy now regrets it. Switzerland and Belgium already reversed course. German politicians complain constantly about what a mistake it was to quit nuclear power. Taiwan, which shut down its last reactor last year, now wants to reopen at least one. Even Italy, the first country to abandon nuclear energy, is now looking to revive the industry.
Constellation Energy knows a thing or two about what works with nuclear power. So it’s quite notable that the largest operator of civilian reactors in the nation is making a bet on one of the more unique startups hoping to shape the next generation of atomic power stations. Constellation’s venture arm announced a strategic equity investment into Blue Energy, a developer that is pitching itself as a project manager to get small modular reactors built on time and on budget. Unlike most other players in the nuclear game at the moment, Blue Energy isn’t designing its own reactor. The company calls itself “reactor agnostic.” Rather, Constellation said the company would focus instead on building GE Vernova Hitachi Nuclear Energy’s BWRX-300, a 300-megawatt boiling water reactor that is currently one of the leading designs in the U.S. “With demand for near-term power rising, Constellation’s investment will help Blue Energy meet America’s need by making new nuclear development predictable, rapidly scalable, and project financeable for the first time in history,” Blue Energy CEO Jake Jurewicz said in a statement. “This relationship helps us leverage an established operator, proven technology, and innovative, project-financeable deployment models to expand access to nuclear energy.”
Meanwhile, one of the most attention-grabbing startups in the next-generation reactor race is looking at an eye-popping valuation. Led by its 27-year-old CEO Isaiah Taylor, Valar Atomics made waves when it worked with the U.S. military to transport the components for its gas-cooled microreactor by plane. The company is now eyeing a $6 billion valuation, The Information reported last night.
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New York City’s brand-new power line connecting the five boroughs to Quebec’s hydroelectric system is down for repairs in the midst of the summer heat. Hydro-Quebec, the French-speaking province’s state-owned utility, said its teams had “identified a fault with the terrestrial cable” at a location on the U.S. stretch of the route. Governor Kathy Hochul’s office called the outage “unacceptable” in a statement to Gothamist.
Over in Hawaii, Governor Josh Green, a fellow Democrat, signed legislation to adopt a clean fuel standard, making the island state the fifth in the nation to adopt such a policy. The program will come into full effect at the start of 2029, and will use market incentives to reduce the carbon intensity of fuel over time. Texas, meanwhile, is serving as the model for the new bipartisan permitting reform bill my colleague Robinson Meyer broke news of last night.
Chinese panel manufacturer LONGi’s newest solar cell has made a breakthrough in increasing the power conversion efficiency of its panels to 35.5%. That figure was confirmed this week by a European certification test. The cell design is called a crystalline silicon-perovskite tandem cell, which PV Tech described as “widely regarded as a leading technology pathway for next generation” solar panels. A perovskite top cell with a crystalline silicon bottom cell allows the solar panel to tap into both technologies’ efficiencies. By contrast, the efficiency by percentage of energy converted to electricity in thin-film solar cells like those the U.S. manufacturer First Solar sells tap out somewhere in the teens. The more popular crystalline silicon cells that China has dominated have efficiency rates of up to 24%. So LONGi’s announcement represents a significant improvement.
Meanwhile, China’s state-owned pipeline company, PipeChina, successfully shipped two batches of methanol about 125 miles through existing oil pipelines in northwest China. Hydrogen Insight hailed the test as “a record-breaking trial that could transform” a sector long plagued by questions about how to transport fuel. It’s the latest sign, as I told you last month, that Beijing is doubling down on green hydrogen.
Like a Mesopotamian metal merchant of yore, I like to train a keen eye on copper prices in this newsletter. And with good reason: It’s the basic building block of the electrical system, and it’s subject to some wild geopolitical price pressures. Just look at why the price is sliding now. Per Mining.com, the major storms in Chile and the flareup of hostilities in Iran are depressing the market for the metal, which had hit an all-time high earlier this year.
Senator Martin Heinrich’s bill, which makes it easier to connect new power plants to the grid, is an encouraging sign for bipartisan permitting reform.
An important part of a bipartisan permitting reform deal may be falling into place.
Senator Martin Heinrich of New Mexico introduced a bill on Thursday that would make it easier for new power plants to hook up to electricity markets across the country.
The legislation, which applies to all types of generation, would allow new power plants to connect to the grid without waiting for the arduous technical studies — and without paying the exorbitant equipment upgrade fees — now required in much of the country.
Instead, the bill would let power plants opt into a much faster safety study and offer what it cheekily dubs “basic access service for energy-only delivery” — that is, BASED service — to the local electricity market.
A similar approach is already used in Texas, which has added more new generation than any other U.S. power market in recent years. In effect, Heinrich hopes to bring that cheaper, faster, and more laissez-faire method to the rest of the country.
“As electricity demand grows, we need to find better, faster ways to add more affordable, reliable power to the grid,” Heinrich said in a statement. “Right now, unnecessary delays are slowing projects that could help lower energy costs and deliver the low-cost energy we need.”
Outside experts have pushed for wider adoption of Texas’s approach, which is dubbed “connect and manage,” for some time. Although Heinrich’s proposal would apply to all kinds of power plants, Texas has been particularly successful at bringing new solar, battery, and natural gas power plants online in recent years — and it has done so while keeping connection costs lower than other markets.
“We’re seeing the success of the free market in Texas,” Sarah Toth Kotwiss, an electricity researcher at the energy and climate think tank RMI, told me, noting the state has added far more generation in recent years than much bigger and more populous U.S. grid zones. “They’re leading the way, and replicating that free market attitude could go a long way in the rest of the U.S.”
More broadly, the proposal is the kind of legislation that would slot into the bipartisan permitting package expected later this year — and as soon as next month. Heinrich’s proposal may be a sign that the senator, the ranking Democratic member of the natural resources committee, takes the prospect of reaching a deal seriously.
Across much of the country, a new power plant can only connect to the power grid after the local grid operator completes what’s called an “interconnection study” — an intensive technical account of how that new plant will affect the overall system.
These studies examine a slew of worst-case scenarios, simulating how the plant would behave at full capacity under extremely congested grid conditions, such as during a heat wave. The new plant’s developer is then required to pay for the grid and transmission line upgrades that would allow their project to run at full blast at those moments of maximum stress.
In theory, that approach maximizes the amount of money a developer can make on a new power plant. But because the grid is a big, interconnected system, that method can cause long delays and rippling costs in practice. In one case, a new 300-megawatt plant in North Dakota near the Canadian border could not start operating until it paid nearly $3 million to upgrade power lines and transformers in Missouri — more than 1,000 miles away.
And because interconnection studies try to model a proposed power plant’s influence on the power grid for years into the future, a single cancellation can have a cascading effect. When a power plant pulls out of the interconnection queue, every project in line behind it sometimes needs to be studied again, causing delays and costs to spiral even further.
In one famous example, a solar and battery plant in Maryland was initially told that it needed to pay for $1.25 million to connect to the local grid. But after a series of cancellations and new rounds of study, that figure was revised — to nearly $72 million. The solar project got shelved.
While interconnection queues used to be relatively quick, the process of hooking up a new power plant to the grid can now regularly take eight years, Kotwiss said.
As I discussed with the electricity researchers Tyler Norris and Claire Waymer on Heatmap’s podcast Shift Key in 2024, these lengthening wait times have changed how power plant developers behave. Many developers now “spam the queue,” filing study requests for any project that they could ever conceivably want to build. That has led delays to spiral even further.
The end result of all this spamming is that the total capacity of power plants asking to connect to the grid now exceeds the size of the U.S. grid itself. At the end of 2025, more than 2,000 gigawatts of new generation or storage projects were waiting in interconnection queues, according to the Lawrence Berkeley National Lab. The country’s operating power plant fleet is only about 1,400 gigawatts.
To be clear, most of those proposed projects will never be built — they are hypothetical queries submitted by developers who are trying to claim a place in line. Yet even switching on a small set of plants could transform the power grid.
These long wait times aren’t the norm in Texas. In the Lone Star state, it takes less than four years to bring a new plant online.
That’s because new power plants in Texas can hook up to the grid — and start generating power — as soon as the local grid operator completes a more rudimentary engineering and safety study. Then during moments of peak grid congestion, power plants must curtail their own generation, reducing their electricity production to the level that the overall grid can support. While this means that a given solar farm or natural gas plant might not run at full bore all the time, the overall approach gets that plant up and running much sooner, allowing it to sell energy into the grid during most of the year.
Heinrich’s law would order electricity markets and grid operators to make this faster option available to new power plants across the country. It would let power plants opt into receiving a much simpler and faster study, one that checks only that adding the new power plant will be safe for the immediate grid.
A power plant that opts into the new BASED service would still have the option of entering the traditional interconnection queue. Doing so would let it eventually increase its operation over time, paying for grid upgrades so that it can participate in capacity markets and other auctions.
Expanding Texas’s approach to other states could help cut costs for electricity consumers by bringing more energy onto the market faster, Kotwiss said. Even in complicated power markets that include additional auctions — for capacity, for instance, or reliability — energy still makes up most wholesale costs.
It could also help ease the strains on the grid — especially in congested regions like the Mid-Atlantic — caused by artificial intelligence data centers and new factories.
The Texas-inspired technique could help the solar and battery industries, because it keeps a given project’s upfront expenses low and allows those technologies’ low costs to dominate their economics. Solar has boomed in Texas in recent years, and the state now has more utility-scale solar installed than California does.
But the BASED approach would likely help natural gas plants and other forms of newer, cheaper generation, too, because it strengthens new entrants as compared to incumbents. Jacob Mays, a Cornell engineering professor, has studied how slow and wonky interconnection queues can prevent electricity markets from functioning well. The existing interconnection approach used in most of the country “amounts to a significant barrier on new entry to new generation,” he told me, and it has “some anticompetitive impacts.”
Heinrich has said that he plans on introducing more electricity system reforms soon, including a bill to push utilities to adopt technologies that get more capacity out of their existing equipment.
I think it’s an encouraging sign for permitting reform advocates that ranking Senate Democrats are advancing these kinds of technology-neutral power market bills. An eventual deal will likely ultimately rest on Democrats’ willingness to support policy like this — and whether they can strike a deal with Republicans to rewrite parts of long-standing environmental or permitting laws, including the National Historic Preservation Act and Clean Water Act.
But just as importantly, it will depend on Republicans — and the White House — reining in President Trump’s powers to kill energy projects by fiat. With this bill, Democrats are suggesting they’re willing to be, well, a little BASED. Whether the president will join them remains to be seen.
Your mileage may vary — but you’ll probably want to keep the outdoor runs to a minimum.
I became a runner in the spring of 2020. My run streak was my sourdough starter. Those were the Wild West days of respiratory spray warnings, when I’d get dirty looks from strangers even if I passed them while wearing my Under Armour running mask. But I wasn’t about to let a deadly pandemic — much less the wildfire smoke that descended on New York that fall — get in the way of logging my miles.
These days, I am at least a little bit older and wiser. I’ve also learned a lot about wildfire smoke in the interim — how it kills more than 20,000 people in the U.S. every year, how there’s a lot of freaky stuff in it that you don’t want in your body, and how there’s no safe threshold for exposure. But while it’s clearly a bad idea to go for a run right now if you live in Milwaukee, where the air is literally yellow due to the fires in Minnesota and Ontario, it’s maybe less clear if you’re somewhere where the AQI is still only moderate or “unhealthy for sensitive groups.” Do you really, actually need to skip your run in those conditions? Can you just go to the gym instead?
At the end of the day, everyone should make decisions based on their own risk tolerance. But John C. Quindry — a professor of integrative physiology and athletic training at the University of Montana, who described his team as “first to the party” when it comes to understanding the risks of exercising in the smoke — said his research shows that not only is exercising in the smoke hazardous to otherwise healthy individuals, there’s also a class of people for whom it might be extra dangerous, and they might not even know it.
Our conversation has been edited and condensed for clarity.
Why is exercising in the smoke worse than, say, commuting in the smoke?
If you exercise, you take more breaths per minute, and you take deeper breaths, so the total volume of air you breathe at a given time is greater.
Inhaling wildfire smoke is sometimes compared to smoking cigarettes. Some back-of-the-envelope math puts a run in Chicago this morning at the equivalent of smoking a couple of cigarettes, which sounds pretty minimal. Why take this seriously?
We don’t know for sure that when you exercise outside, it’s equivalent to, in this case, smoking a couple of cigarettes or a pack. We’ve been working for years trying to actually figure out how bad it is, and we just don’t know.
One of the things that complicates this is that not all smoke is created equal. Wildfire smoke in the West is different from wildfire smoke east of the Mississippi, where there are many different types of vegetation. And separately, if a wildfire consumes a house and all the plastics, organic solvents, and things in the roof, that’s certainly worse than just biomass burning — and that is all separate from cigarette smoke. So the chemicals that you inhale are part of it; it’s not just the particulate.
I don’t think it’s inappropriate to say we know how much particulate a filtered cigarette is going to deliver on a puff-by-puff basis and try to equate it to inhaling wood smoke or being downwind of a fire event. Those back-of-the-envelope calculations can make it one-to-one. But what is the impact on health? We don’t know.
Going back to something you said, is smoke in the West or East worse? Why?
We don’t really know, and one reason is that smoke that starts in the West goes east. There are some really good studies that look at the rates of emergency room visits and deaths downwind of fires. You can apply some pretty fancy math, and it’s clearly demonstrated by multiple research groups that whatever the source of the smoke is, there are people who are extra sensitive to it, and they show up in the emergency room more frequently. Tragically, to a smaller degree, they also occasionally die more frequently.
But if we put those data aside and say, “What does the smoke look like from Western biomass versus Eastern biomass?” We know the Western biomass, at least this time of year, is much drier. And when it’s dry, it tends to burn a little “cleaner,” which is to say, if you were to take a certain number of grams of pine wood — which burns fairly cleanly — how much PM 2.5 do you get? You’re going to get a higher PM 2.5 from wet wood. That’s the effect when you start to move to the middle or eastern part of the country. Deciduous trees, even when they’re dry, put out more PM 2.5 unit by unit.
Now, is that worse for the body in the short or long term? We really don’t know. Once you breathe in the smoke, it’s pretty easy to measure what’s in the blood: You just take a blood sample before and after the exposure. You can try to gauge how bad the smoke is by looking at what appears in the blood. Do you get oxidative stress? Do you get inflammation? Is there something else there? Has it changed metabolism?
We also look at exhaled breath condensate. It’s the immune cells, which are the first line of defense when you’re exposed to particulates, that are really sounding the alarm. In some people, that alarm gets sounded more than in others. So we’ve been trying to figure out what are these subtle but important biochemical and physiologic signals? How do they go together? They tell us how bad the smoke is, but it’s taken us years to unfold this story.
I saw in your 2025 study that half of your participants had a heightened response to physiological stress, and that you selected them for that reason. I was hoping you could tell me what a “heightened response to physiological stress” means and why it was important to include those candidates.
This is critical. Let me give you the backstory. When we would do these studies, we would put participants in the lab and burn Western locally sourced pine dried out to 15% humidity. We’d burn it carefully, measure the dose of PM 2.5, and have people breathe it in. We’d conduct studies before and after exercise, whether on a bike or a treadmill, and we varied the intensity and duration. And we didn’t find very much when we looked at the blood and exhaled breath condensate for how well the blood vessels constrict or how reactive the body’s autonomic nervous system was in controlling cardiovascular function. We’d find subtle changes, but statistically speaking, we could never really draw firm conclusions.
But we would notice that, on a person-by-person basis, there would be notable spikes in what we were looking at. We’d see, “Oh, these three or four people really seem to have this notable response, and everybody else really didn’t.” We’d publish our studies and essentially say, “Yeah, we didn’t find much.” It was honest data, and people believe it when you publish negative data that says, “We went through that much trouble, we spent $100,000, and we found nothing.” But we found these people who were a little bit different than the others.
So we’d sit around and spitball. These were normal people, by the way; we were not taking asthmatics or people with [chronic obstructive pulmonary disease]. They were not diabetic. There are people we know full well will have an exaggerated response, and we didn’t look at any of them. We looked at what are called “apparently healthy people” — the people who go to the physician and are told nothing’s wrong with them.
My doctorate was not in exercise science; it was in biomedical science, from a medical school environment. Somewhere along the line, I was exposed to something called the cold pressor test. It has been around longer than both of us. Physicians 80 years ago would have somebody come into their lab or clinic, and they’d say, “Hey, we’re going to put your hand in a bucket of ice water for two minutes and see what that does to your blood pressure.” It almost sounds like a fourth-grade sleepover prank for the first kid to fall asleep.
But we started doing the test, and we took a pretty conservative approach, meaning that if we put someone’s hand in the ice water and their systolic blood pressure — that’s the top number, 120 over 80 is sort of the high end for good blood pressure — and if their systolic number went to at least 20 millimeters of mercury, then we call them cold pressure test positive, or CPT positive.
None of these were people who were hypertensive. They had normal blood pressure. Nothing was wrong with them as far as their medical team could observe. But how many of these [CPT positive] people are sitting around? If you take the average population, 10% to 15% will be cold pressor test positive. We don’t necessarily know if this is a bad thing. It may be contextual, but these are people we can verifiably measure with something as simple as a blood pressure cuff and a bucket of ice water. You can verify their physiology is more reactive within a couple of minutes than someone else’s.
We suspected that if we took these people and then exercised them in the smoke, they would have an exaggerated response compared with people who weren’t CPT positive. And so that’s what we did, and that’s what we found.
That’s fascinating. So the practical takeaway is, unless you’ve done this test yourself, you don’t know if you’re one of these people who’s particularly reactive to wildfire smoke?
You have to always be careful that you don’t make big, broad, blanket applications from one study. We would need to confirm or refute it with additional studies, and that’s a matter of getting grant money and conducting those next-generation investigations.
But here’s something we can glean: We know that people who have an exaggerated response to a cold pressor test are more likely to have hypertension. And they’re more likely to have it earlier — instead of getting high blood pressure at 50, 60, or 70, they’re going to get it at 30 or 40. We know they’re more likely to eventually suffer from heart failure; it seems to develop more in people who have this sort of reactive response. While the cold pressor test response is not predictive in and of itself of anything — future diabetes, heart failure, heart attacks, hypertension, none of that — people with all of those conditions are more likely to be cold pressor test positive.
The way I would take it is, if you are somebody who is not in great health, isn’t fit, or if you have creeping blood pressure or a family history of heart disease, blood pressure, or metabolic derangement like diabetes, you are the kind of person who needs to be a little bit more careful [in the smoke].
Is there an air quality level at which you would tell a young, healthy person “Don’t go for your morning run?”
An educated guess is the best we can do right now. Every study is a brick in the bigger metaphorical wall of understanding this. What we can say is: If you are young and apparently healthy, the threshold would be maybe even up into the “unhealthy for sensitive groups” number. They’re probably okay. But then again, how hard are they working? How long are they outside? If you’re working very hard, your ventilatory rate is much, much higher. And how long are you staying out? What is the long-term impact year after year?
There are epidemiologic studies that can demonstrate that if you’ve lived downwind from major wildfire events for decades, it’s going to impact your health. That’s not debated. But what is the short-term impact? We don’t know.
Indoor air pollution is a major issue during these events, too. Would you advise someone to skip the gym on a particularly smoky day?
When it’s smoky outside, it makes sense to say, “Let’s just stay indoors.” But then the question is, is some of that smoke getting indoors? Sure enough, it is. Then the question is, how much? And the answer is, it depends on the air handling systems in the building; how many people are coming in and out; whether the windows open; how many doors are there; are they left open; what are they using to heat and cool the place; do they have high-volume HEPA filtration; and are they changing that filtration?
But let’s dial back. We have two ends of a spectrum. You have a leaky building. Doors are open. Windows are open. Lots of people are coming in and out. In that scenario, the air inside is only about 20% or 30% better than the air outside. And that’s not very good.
But then let’s go to the other end of the spectrum, to a building that is being kept shut. They’re controlling who can come in and out, and there are two sets of doors to help buffer that. Which is better in a smoky situation? The places with the HEPA filtration, high-volume air turnover, and where they change the filters. But in that case, you’re still going to have 25% of that particulate matter inside, even if you can’t perceive that it’s there. That’s not good news.