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The finding by Stanford researchers confirms the danger — and lack of precedent — for the East Coast’s ongoing wildfire smoke crisis.
Wednesday, June 7, was by far the worst day for wildfire smoke exposure in American history, subjecting more people to greater amounts of soot and ash than any previous day on record, according to a new and rapid analysis conducted by Stanford researchers.
The finding confirms the danger — and lack of precedent — for the ongoing wildfire smoke crisis, which has smothered the northern and eastern United States in an acrid haze. Since Monday, a strong low-pressure system has fanned dozens of wildfires in northern Quebec, then blown the resulting smoke into North America’s most densely populated corridor, covering New York City, Philadelphia, Washington, D.C., and Toronto.
Even as the smoke moved in, it made history. Tuesday became the country’s third-worst day for wildfire smoke on record, according to research from the same Stanford team. Only two days in September 2020 — when vast wildfires sheathed California, Idaho, and the Pacific Northwest in smoke — had exposed more Americans to higher intensities of toxic particulate matter.
Wednesday smashed those records. By the late afternoon, New York and New Jersey’s sky had turned a pungent orange, and their air quality fell to “very unhealthy” or even “hazardous” levels for all groups, the worst category. The rest of the Northeast and Mid-Atlantic faced similar or only slightly better conditions, and wildfire smoke at least somewhat afflicted virtually everywhere between Minneapolis, Atlanta, and Boston.
Stanford.
The vast geographic sweep of the smoke — and the fact that it was so intense in the country’s biggest city — obliterated the old record. The country’s previous worst day for smoke exposure was September 13, 2020, when nearly 35 million Americans were exposed to more than 50 micrograms per cubic meter of microscopic soot and ash. That is five times worse than what the World Health Organization considers safe.
But on Wednesday, roughly 62 million Americans — nearly one in five — were exposed to such high levels of particulate matter.
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It was “just a massive, awful event, with highly populated areas getting hit with unprecedented levels of pollution,” Marshall Burke, an economist and professor at the Stanford Doerr School of Sustainability, said on Twitter. He leads the school’s Environmental Change and Human Outcomes Lab, which conducted the research.
The study encompassed data from 2006 to 2023. The researchers studied which days were worst on a population- and exposure-adjusted basis. In other words, on which days in history were the most Americans subjected to the most intense amounts of wildfire smoke?
Because the U.S. population has grown over time, and because wildfires have enlarged over the past few decades, it is unlikely that any days prior to 2006 saw so many Americans exposed to so much smoke. Most of the record days in the data set have happened since 2018.
With the new record in place, Tuesday falls to the fourth-worst day for wildfire smoke on record.
Speaking earlier this week, Burke said that what made this event so distinct from previous ones is that the wildfire smoke hit the country’s largest and most populated cities. “It’s mainly due to the East Coast having so many people. It’s New York, Boston, D.C., Detroit. Out West, our cities are just smaller,” he told me.
Read more about the wildfire smoke engulfing the East Coast:
Wednesday Was the Worst Day for Wildfire Pollution in U.S. History
The 5 Big Questions About the 2023 Wildfire Smoke Crisis
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In a special episode of Shift Key, Rob interviews Representative Sean Casten about his new energy price bill, plus Emerald AI’s Arushi Sharma Frank.
Artificial intelligence is helping to drive up electricity demand in America. Energy costs are rising, and utilities are struggling to adjust. How should policymakers — and companies — respond to this moment?
On this special episode of Shift Key, recorded live at Heatmap House during New York Climate Week, Rob leads a conversation about some potential paths forward. He’s joined first by Representative Sean Casten, the coauthor of a new Democratic bill seeking to lower electricity costs for consumers. How should the grid change for this new moment, and what can Democrats do to become the party of cheap energy?
Then he’s joined by Arushi Sharma Frank, an adviser to Emerald AI, an Nvidia-seeded startup that helps data centers flexibly adjust their power consumption to better serve the grid. Sharma Frank has worked for utilities and tech companies — she helped stand up Tesla’s energy business in Texas — and she discusses what utilities, tech companies, and startups can learn from each other?
Congressman Casten represents Illinois’s 6th congressional district in the U.S. House of Representatives. He is a former clean energy entrepreneur and CEO, and he sits on the House Financial Services Committee and the Joint Economic Committee. He is also vice chair of the House Sustainable Energy and Environment Coalition.
Arushi Sharma Frank is an adviser to She has previously worked in roles at Tesla, Exelon Constellation, the Electric Power Supply Association, and the American Gas Association. She is a non-resident expert at the Center for Strategic and International Studies, a nonpartisan think tank in Washington, D.C.
Shift Key is hosted by Robinson Meyer, the founding executive editor of Heatmap, and Jesse Jenkins, a professor of energy systems engineering at Princeton University. Jesse is off this week.
Subscribe to “Shift Key” and find this episode on Apple Podcasts, Spotify, Amazon, YouTube, or wherever you get your podcasts.
You can also add the show’s RSS feed to your podcast app to follow us directly.
Here is an excerpt from our conversation:
Robinson Meyer: Earlier you said something that I want to go back to, which was that our energy system doesn’t reward cheap energy, and it hasn’t been set up to reward cheap energy. What did you mean by that?
Representative Sean Casten: So at a high level, no market, left to its own devices, will reward cheap things. Because if I’m a buyer, I want to buy things for cheap. If you’re a seller, you want to sell things for a lot of money. I remember my dad, when I was a kid, had a little paperweight on his desk. It was an oil barrel, and on one side it said, “Relax, the price will go down,” and on the other side it said, “Relax, the price will go up.” And depending on which side of a negotiation you were on, that was how you pointed the oil barrel.
What’s happened in the energy sector that has made that hard is that, because it is such a highly regulated sector, we’ve vastly over-advantaged the producers in what would otherwise be an even negotiation. So, for example, if you as a consumer want to put a solar panel on the roof of your house, you have to get permission from your local utility, who’s going to lose the revenue, who can raise all sorts of technical objections and do that.
If you have a solar panel and you say, boy, there’s hours when I’m making more power than I want, or than I need, maybe my neighbor would like to have some of my excess — well, you’re not a regular utility. You’re not allowed to do that. Your neighbor can’t buy it from you. These are because of laws we’ve set up that says only that utility has the right to do it.
Outside of the electric space, there’s a law that’s been on the book since 1935, the Natural Gas Act, that says that you cannot build a gas export facilities in the United States unless it is in the national interest. Is it in the national interest to raise people’s price of gas? That was never specified in the act. And so when the Trump administration went through and approved all those assets — which by the way, the Biden administration had shut down in part because they said it’s in the national interest — they said, well, we think it’s in the national interest to look out for our gas producers.
Somewhat more recently than that, when the price of oil collapsed during COVID in April of 2020, Trump called the Saudis and said, we are going to withhold military aid from Saudi Arabia unless you raise the price of oil. The Saudis flinched and the price of oil went up, and he was praised on the cover of all the business magazines as saving our oil industry.
Why didn’t we do the same thing two years later when everybody was complaining about the price of oil being so high and we had a Democrat in the White House? We’ve always had this feeling, like, I need to look out for producers, because the producers have had more political clout. We’ve connected those things together, and you can be angry about that. You can be embarrassed about that. Or you can see it as an unbelievable opportunity to generate a tremendous amount of wealth to lower energy costs — and oh, by the way, cut a bunch of CO2 emissions.
Mentioned:
Democrats Bid to Become the Party of Cheap Energy
Heatmap’s Katie Brigham on Emerald AI, a.k.a. The Software That Could Save the Grid
This episode of Shift Key is sponsored by ...
Salesforce, presenting sponsor of Heatmap House at New York Climate Week 2025.
The failure of the once-promising sodium-ion manufacturer caused a chill among industry observers. But its problems may have been more its own.
When the promising and well funded sodium-ion battery company Natron Energy announced that it was shutting down operations a few weeks ago, early post-mortems pinned its failure on the challenge of finding a viable market for this alternate battery chemistry. Some went so far as to foreclose on the possibility of manufacturing batteries in the U.S. for the time being.
But that’s not the takeaway for many industry insiders — including some who are skeptical of sodium-ion’s market potential. Adrian Yao, for instance, is the founder of the lithium-ion battery company EnPower and current PhD student in materials science and engineering at Stanford. He authored a paper earlier this year outlining the many unresolved hurdles these batteries must clear to compete with lithium-iron-phosphate batteries, also known as LFP. A cheaper, more efficient variant on the standard lithium-ion chemistry, LFP has started to overtake the dominant lithium-ion chemistry in the electric vehicle sector, and is now the dominant technology for energy storage systems.
But, he told me, “Don’t let this headline conclude that battery manufacturing in the United States will never work, or that sodium-ion itself is uncompetitive. I think both those statements are naive and lack technological nuance.”
Opinions differ on the primary advantages of sodium-ion compared to lithium-ion, but one frequently cited benefit is the potential to build a U.S.-based supply chain. Sodium is cheaper and more abundant than lithium, and China hasn’t yet secured dominance in this emerging market, though it has taken an early lead. Sodium-ion batteries also perform better at lower temperatures, have the potential to be less flammable, and — under the right market conditions — could eventually become more cost-effective than lithium-ion, which is subject to more price volatility because it’s expensive to extract and concentrated in just a few places.
Yao’s paper didn’t examine Natron’s specific technology, which relied on a cathode material known as “Prussian Blue Analogue,” as the material’s chemical structure resembles that of the pigment Prussian Blue. This formula enabled the company’s batteries to discharge large bursts of power extremely quickly while maintaining a long cycle life, making it promising for a niche — but crucial — domestic market: data center backup power.
Natron’s batteries were designed to bridge the brief gap between a power outage and a generator coming online. Today, that role is often served by lead-acid batteries, which are cheap but bulky, with a lower energy density and shorter cycle life than sodium-ion. Thus, Yao saw this market — though far smaller than that of grid-scale energy storage — as a “technologically pragmatic” opportunity for the company.
“It’s almost like a supercapacitor, not a battery,” one executive in the sodium-ion battery space who wished to remain anonymous told me of Natron’s battery. Supercapacitors are energy storage devices that — like Natron’s tech — can release large amounts of power practically immediately, but store far less total energy than batteries.
“The thing that has been disappointing about the whole story is that people talk about Natron and their products and their journey as if it’s relevant at all to the sodium-ion grid scale storage space,” the executive told me. The grid-scale market, they said, is where most companies are looking to deploy sodium-ion batteries today. “What happened to Natron, I think, is very specific to Natron.”
But what exactly did happen to the once-promising startup, which raised over $363 million in private investment from big name backers such as Khosla Ventures and Prelude Ventures? What we know for sure is that it ran out of money, canceling plans to build a $1.4 billion battery manufacturing facility in North Carolina. The company was waiting on certification from an independent safety body, which would have unleashed $25 million in booked orders, but was forced to fold before that approval came through.
Perhaps seeing the writing on the wall, Natron’s founder, Colin Wessells, stepped down as CEO last December and left the company altogether in June.
“I got bored,” Wessels told The Information of his initial decision to relinquish the CEO role. “I found as I was spending all my time on fundraising and stockholder and board management that it wasn’t all that much fun.”
It’s also worth noting, however, that according to publicly available data, the investor makeup of Natron appears to have changed significantly between the company’s $35 million funding round in 2020 and its subsequent $58 million raise in 2021, which could indicate qualms among early backers about the direction of the company going back years. That said, not all information about who invested and when is publicly known. I reached out to both Wessels and Natron’s PR team for comment but did not receive a reply.
The company submitted a WARN notice — a requirement from employers prior to mass layoffs or plant closures — to the Michigan Department of Labor and Economic Opportunity on August 28. It explained that while Natron had explored various funding avenues including follow-on investment from existing shareholders, a Series B equity round, and debt financing, none of these materialized, leaving the company unable “to cover the required additional working capital and operational expenses of the business.”
Yao told me that the startup could have simply been a victim of bad timing. “While in some ways I think the AI boom was perfect timing for Natron, I also think it might have been a couple years too early — not because it’s not needed, but because of bandwidth,” he explained. “My guess is that the biggest thing on hyperscalers’ minds are currently still just getting connected to the grid, keeping up with continuous improvements to power efficiency, and how to actually operate in an energy efficient manner.” Perhaps in this environment, hyperscalers simply viewed deploying new battery tech for a niche application as too risky, Yao hypothesized, though he doesn’t have personal knowledge of the company’s partnerships or commercial activity.
The sodium-ion executive also thought timing might have been part of the problem. “He had a good team, and the circumstances were just really tough because he was so early,” they said. Wessells founded Natron in 2012, based on his PhD research at Stanford. “Maybe they were too early, and five years from now would have been a better fit,” the executive said. “But, you know, who’s to say?”
The executive also considers it telling that Natron only had $25 million in contracts, calling this “a drop in the bucket” relative to the potential they see for sodium-ion technology in the grid-scale market. While Natron wasn’t chasing the big bucks associated with this larger market opportunity, other domestic sodium-based battery companies such as Inlyte Energy and Peak Energy are looking to deploy grid-scale systems, as are Chinese battery companies such as BYD and HiNa Battery.
But it’s certainly true that manufacturing this tech in the U.S. won’t be easy. While Chinese companies benefit from state support that can prop up the emergent sodium-ion storage industry whether it’s cost-competitive or not, sodium-ion storage companies in the U.S. will need to go head-to-head with LFP batteries on price if they want to gain significant market share. And while a few years ago experts were predicting a lithium shortage, these days, the price of lithium is about 90% off its record high, making it a struggle for sodium-ion systems to match the cost of lithium-ion.
Sodium-ion chemistry still offers certain advantages that could make it a good option in particular geographies, however. It performs better in low-temperature conditions, where lithium-ion suffers notable performance degradation. And — at least in Natron’s case — it offers superior thermal stability, meaning it’s less likely to catch fire.
Some even argue that sodium-ion can still be a cost-effective option once manufacturing ramps up due to the ubiquity of sodium, plus additional savings throughout the batteries’ useful life. Peak Energy, for example, expects its battery systems to be more expensive upfront but cheaper over their entire lifetime, having designed a passive cooling system that eliminates the need for traditional temperature control components such as pumps and fans.
Ultimately, though, Yao thinks U.S. companies should be considering sodium-ion as a “low-temperature, high-power counterpart” — not a replacement — for LFP batteries. That’s how the Chinese battery giants are approaching it, he said, whereas he thinks the U.S. market remains fixated on framing the two technologies as competitors.
“I think the safe assumption is that China will come to dominate sodium-ion battery production,” Yao told me. “They already are far ahead of us.” But that doesn’t mean it’s impossible to build out a domestic supply chain — or at least that it’s not worth trying. “We need to execute with technologically pragmatic solutions and target beachhead markets capable of tolerating cost premiums before we can play in the big leagues of EVs or [battery energy storage systems],” he said.
And that, he affirmed, is exactly what Natron was trying to do. RIP.
They may not refuel as quickly as gas cars, but it’s getting faster all the time to recharge an electric car.
A family of four pulls their Hyundai Ioniq 5 into a roadside stop, plugs in, and sits down to order some food. By the time it arrives, they realize their EV has added enough charge that they can continue their journey. Instead of eating a leisurely meal, they get their grub to go and jump back in the car.
The message of this ad, which ran incessantly on some of my streaming services this summer, is a telling evolution in how EVs are marketed. The game-changing feature is not power or range, but rather charging speed, which gets the EV driver back on the road quickly rather than forcing them to find new and creative ways to kill time until the battery is ready. Marketing now frequently highlights an electric car’s ability to add a whole lot of miles in just 15 to 20 minutes of charge time.
Charging speed might be a particularly effective selling point for convincing a wary public. EVs are superior to gasoline vehicles in a host of ways, from instantaneous torque to lower fuel costs to energy efficiency. The one thing they can’t match is the pump-and-go pace of petroleum — the way combustion cars can add enough fuel in a minute or two to carry them for hundreds of miles. But as more EVs on the market can charge at faster speeds, even this distinction is beginning to disappear.
In the first years of the EV race, the focus tended to fall on battery range, and for good reason. A decade ago, many models could travel just 125 or 150 miles on a charge. Between the sparseness of early charging infrastructure and the way some EVs underperform their stated range numbers at highway speeds, those models were not useful for anything other than short hauls.
By the time I got my Tesla in 2019, things were better, but still not ideal. My Model 3’s 240 miles of max range, along with the expansion of the brand’s Supercharger network, made it possible to road-trip in the EV. Still, I pushed the battery to its limits as we crossed worryingly long gaps between charging stations in the wide open expanses of the American West. Close calls burned into my mind a hyper-awareness of range, which is why I encourage EV shoppers to pay extra for a bigger battery with additional range if they can afford it. You just had to make it there; how fast the car charged once you arrived was a secondary concern. But these days, we may be reaching a point at which how fast your EV charges is more important than how far it goes on a charge.
For one thing, the charging map is filling up. Even with an anti-EV American government, more chargers are being built all the time. This growth is beginning to eliminate charging deserts in urban areas and cut the number of very long gaps between stations out on the highway. The more of them come online, the less range anxiety EV drivers have about reaching the next plug.
Super-fast charging is a huge lifestyle convenience for people who cannot charge at home, a group that could represent the next big segment of Americans to electrify. Speed was no big deal for the prototypical early adopter who charged in their driveway or garage; the battery recharged slowly overnight to be ready to go in the morning. But for apartment-dwellers who rely on public infrastructure, speed can be the difference between getting a week’s worth of miles in 15 to 20 minutes and sitting around a charging station for the better part of an hour.
Crucially, an improvement in charging speed makes a long EV journey feel more like the driving rhythm of old. No, battery-powered vehicles still can’t get back on the road in five minutes or less. But many of the newer models can travel, say, three hours before needing to charge for a reasonable amount of time — which is about as long as most people would want to drive without a break, anyway.
An impressive burst of technological improvement is making all this possible. Early EVs like the original Chevy Bolt could accept a maximum of around 50 kilowatts of charge, and so that was how much many of the early DC fast charging stations would dispense. By comparison, Tesla in the past few years pushed Supercharger speed to 250 kilowatts, then 325. Third-party charging companies like Electrify America and EVgo have reached 350 kilowatts with some plugs. The result is that lots of current EVs can take on 10 or more miles of driving range per minute under ideal conditions.
It helps, too, that the ranges of EVs have been steadily improving. What those car commercials don’t mention is that the charging rate falls off dramatically after the battery is half full; you might add miles at lightning speed up to 50% of charge, but as it approaches capacity it begins to crawl. If you have a car with 350 miles of range, then, you probably can put on 175 miles in a heartbeat. (Efficiency counts for a lot, too. The more miles per kilowatt-hour your car can get, the farther it can go on 15 minutes of charge.)
Yet here again is an area where the West is falling behind China’s disruptive EV industry. That country has rolled out “megawatt” charging that would fill up half the battery in just four minutes, a pace that would make the difference between a gasoline pit stop and a charging stop feel negligible. This level of innovation isn’t coming to America anytime soon. But with automakers and charging companies focused on getting faster, the gap between electric and gas will continue to close.