The Louisa coal plant sits outside of the rural community of Fruitland, Iowa and looks over the Mississippi River. Built over 40 years ago, it burns coal shipped in from Wyoming’s Powder River basin - and in 2025 emitted over 4,200 tons of oxides of nitrogen (NOx), a byproduct of fossil combustion and a key ingredient for ground-level ozone, or what most people know as smog. It is estimated that the NOx and particulate emissions from Louisa ultimately increase premature mortality rates by about 40 statistical lives each year — in other words, the population exposed to its downwind pollution can expect about 40 shortened lives, in aggregate, from those toxic emissions. Louisa is also in a very rural part of the country with a spread out, but still vulnerable, population.
A new Sierra Club survey of air permits in Virginia, shows that Amazon, the largest data center provider in the region (and the world), has obtained air permits allowing its backup generators to emit 4,400 tons of NOx in Northern Virginia, about the same as the Louisa coal plant. Unlike the four-decade old coal plant however, most of these permits have been issued over the last three years, and almost entirely in secret. And unlike the rural coal plant, the backup generators for these data centers will be emitting pollution right in the heart of the seventh largest metropolitan area in the country, feasibly during the worst air quality days imaginable.
And while Amazon holds permits for the greatest amount of air pollution in Virginia, it’s not alone. Microsoft is permitted to emit over a thousand tons of NOx annually, while QTS, CyrusOne, Digital Realty, and Stack - all major data center providers, hold permits for over 500 tons each. Collectively, backup generators in the state are allowed to emit nearly 13,000 tons of NOx, or as much pollution as all of Virginia’s front-of-meter power plants emitted in 2025.
So how did we get to a place where data centers might end up with emissions comparable to that of a coal plant on the outskirts of a major city and nobody knows about it? The answer lies in what are called “minor source” air permits, which are fairly easy to obtain. They get that name because the source - in this case, a cluster of backup diesel generators at a data center - are not able (or allowed) to emit over 100 tons of any criteria pollutant, or pollution that’s been deemed harmful to human health. When a source promises to emit under that threshold, it can go through a pro-forma permitting process. In most states, minor source air permit applications are not open for public review or comment, may not even be readily viewable, and often don’t even require that the source report to regulators.
Since 2000, Virginia has issued over 180 “minor source” air permits for backup generators at data centers. Nearly half of those have been updated or issued since 2023, and almost every single recent permit is just under the 100 ton limit. So while any given data center has a “minor” contribution to pollution, in aggregate the impact is enormous. Virginia does not require its air agencies to account for cumulative impacts, so a dozen minor sources can sit right next to each other — and they do. For example, the unincorporated area of Sterling Virginia, just north of Dulles Airport, hosts three dozen data center emissions sources which are permitted to emit over 3,000 tons of NOx. That’s sort of like having Arkansas’ White Bluff coal plant sitting in your backyard.
The vast majority of the permits show that data centers are operating dozens of uncontrolled diesel generators for emergency backup generation. These aren’t your backyard diesel generators either. The average generator is 2,500 kilowatts, about the same as seven 18-wheeler semi-trucks. The average data center hosts around 20 of these generators, but two dozen data center campuses host over 100 of them.
The pattern across Virginia's data center permits is difficult to explain as coincidence. Of the permits issued since 2023, nearly every single one caps annual NOx emissions within a few tons of the 100-ton minor source threshold — the level at which a permit would require public notice, invite community comment, and face meaningful regulatory scrutiny. Below that line, permits are processed quietly, without public input, and in most cases without any requirement to report emissions to regulators at all. The data center industry, it appears, has learned exactly where the line is drawn.
Two recent permits for Amazon facilities in Sterling illustrate just how deliberately that line is being managed. In August and September of 2025, Virginia DEQ issued two separate permits to Amazon Data Services for data centers about five miles apart. The August permit covers three facilities with 69 uncontrolled backup generators, while the September permit covers a single facility with just 29 diesel generators. And yet their NOx caps are identical: 96.03 tons per year — four tons below the threshold that would require public notice. Increasingly, aiming for just below the threshold is the rule, rather than the exception. Four out of every five permits issued since 2023 allow data center facilities to emit between 94 and 100 tons of NOx, a very finely tuned threshold.
Last year, the Virginia Department of Environmental Quality began encouraging data centers to install scrubbers on their backup generators — devices that can cut NOx emissions by 90 percent or more. The industry's response was not what regulators appear to have intended. A permit for five buildings at Digital Realty's Efficiency Drive facility in Sterling, issued in October 2025, authorizes 260 diesel generators — roughly the generating capacity of a small power plant. Unlike many other permits, the revision does not designate that the diesel generators are for emergency use only, and removes any operating hours. And like every other permit, it is allowed to produce just enough NOx emissions to slide under the 100 ton limit.
The outcome is predictable, but maybe not what regulators had intended. Because while the uncontrolled emergency generators were limited to 500 hours per year, they were also limited to emergency conditions, like line faults, outages, and critical shortfalls on the grid. But the controlled generators bear no such limits, and are likely to run simply when electricity prices are high. In other words, they’re far more likely to actually hit their emissions limits. The controls aren't a concession to cleaner air. They're a tool for maximizing how much these generators can operate — all while remaining invisible to the public.
Across the industry, pressure is mounting — from data center operators, grid planners, and federal policy — to allow these generators to serve as primary power sources during periods of grid stress. The Trump administration's pledge to make data centers "pay their own way" on energy has accelerated that push, and the logic has a certain blunt appeal: the grid is increasingly strained by data centers, and thousands of diesel generators are already sitting on-site. Why not use them?
The permits answer that question. These generators were never reviewed as power plants, and the permitting process never anticipated that they might all run at the same time, blanketing the DC area with noxious emissions. The permits were never subject to public comment or scrutiny in the way required of a major source. They were permitted, one quiet approval at a time, as minor sources — each one individually too small to matter, but collectively the equivalent of all of Virginia’s coal and gas plants.