Volkswagen’s ruse to circumvent U.S. auto emissions standards has left many wondering about the precise environmental impact of its cars, which emitted more pollutants than regulations allow. Although the extra pollution is impossible to quantify so soon, experts agree that although the amount is globally insignificant, it might add to Europe’s regional health concerns.

On September 18 the U.S. Environmental Protection Agency discovered that four Volkswagen vehicles from model years 2009 to 2015 had been rigged with illegal software. They used a sophisticated algorithm that would make the cars run cleanly during emissions tests but then stop so the cars would get better fuel economy and driving ability. As such, the unrestricted vehicles released higher-than-acceptable emissions in everyday driving situations. The German automaker quickly recalled 482,000 VW and Audi brand cars in the U.S. alone, and later admitted that the software might have been fitted to 11 million vehicles worldwide.

EPA now suspects that these cars emitted 10 to 40 times more nitrogen oxide—a pollutant that can harm human health—than standards allow. Many news organizations were quick to jump on this number. The Guardian ran its own analysis, claiming that the scandal may have caused nearly one million extra metric tons of pollution yearly. But experts remain skeptical.

John Heywood, a mechanical engineer at Massachusetts Institute of Technology who focuses on internal combustion engines and air pollution is hesitant to agree with such high numbers. He has identified a key change in how the engine operates (by delaying the start of combustion) that would improve the snappiness of the driving, but it would only increase nitrogen oxide emissions by three to five times.

Travis Bradford, director of Energy and Environment Concentration at Columbia University, agrees. He argues that a number as high as 40 likely represents a spike while the car is accelerating. It cannot be anywhere near the average. “Fuels these days are not that dirty and emissions control systems are not that clean,” Bradford says. “So the idea that it would on average be 40 times the amount of emissions is pretty incredulous.”

Still, experts agree that nitrogen oxide (pdf) is a nasty pollutant. Once released into the air it quickly converts into nitrogen dioxide—a reddish-brown gas with a pungent odor—and then absorbs sunlight to transform into the yellow-brown haze that blankets cities. It is this smog that can exacerbate dozens of health problems, including asthma, bronchitis and emphysema. Alternatively, it can be washed into the ground in the form of acid rain, which can kill plants and animals. Once the damage is done “there is no antidote,” says Yiannis Levendis, an engineering professor at Northeastern University who focuses on diesel emissions.

The news is not tragic for those living in the U.S., where the portion of diesel-powered cars is small (roughly 1 percent). But in Europe that number is much higher, clocking in at roughly 50 percent. In some European cities there is already so much nitrogen dioxide that it is “toxic in its own right,” Heywood says. But that was prior to the scandal. VW just upped the dosage.

All experts agree that on a local scale, the extra pollution can only make matters worse; on a global scale, however, it is insignificant. According to the EPA, small cars released roughly one billion metric tons (pdf) of greenhouse gases in 2011 alone. The Guardian’s estimate, which experts agree is likely too high, is that the rigged cars account for only 0.1 percent of that. “Unfortunately, in the grand scheme of things, this is a drop in the bucket in terms of our aggregate pollution,” Bradford says. He says “unfortunately” mostly because he thinks it’s a shame that pollution is already so high, and partially because he is flabbergasted that a company of VW’s stature could stoop so low. “They literally stole public property,” he says. “They took air that could have been cleaner and available to all the people in the U.S. because they wanted to sell cars.”

Heywood will keep crunching the numbers. But he’s waiting for Volkswagen and EPA to release more concrete information. “We've got to let the dust settle on the numbers,” he says, before we jump to any radical conclusions.