What if we just made sure the air was clean in schools?

It’s common knowledge that air pollution is harmful to our physical health, but more and more evidence is mounting for how it might be negatively affecting kids’ academic performance, as well. Studies have linked long-term exposure to air pollution, increased car pollution specifically, and exposure to pollutant-heavy dust storms with declines in test scores. Wouldn’t it make sense, then, that installing affordable air filters in school buildings could be an easy, quick fix?

That’s the thinking of Michael Gilraine, a researcher at New York University who recently wrote about a natural experiment that seems to link air filters with higher test scores. In a working paper (meaning it hasn’t yet been published in a peer-reviewed or academic journal) titled “Air Filters, Pollution and Student Achievement,” Gilraine lays out an interesting thing that happened in Los Angeles in 2015: The Aliso Canyon gas leak prompted 18 nearby schools to install air filters as a safety measure, setting up the opportunity to compare before-and-after data.

Air testing conducted around the time of the filter installation showed that these schools didn’t actually have above-normal levels of natural gas pollutants, and that’s due to the fact that natural gas is lighter than air, so once the filters were installed, it had already dissipated into the atmosphere. The filters weren’t really necessary to protect against the leak, then, but what they did do was clean other pollutants present in the indoor air. Gilraine looked at test scores from the prior year to the end of the school year that the leak occurred: test scores went up substantially once the filters were installed.

Though experts have been monitoring outdoor pollution for decades, the quality of our indoor air hasn’t been as scrutinized. We spend most of our time inside, though, and researchers are realizing that the air in our homes, offices, and schools may be more dangerous than we thought.

After these air filters cleaned up the indoor air full of everyday, background pollution, Gilraine found that math scores went up by 0.20 standard deviations (a measure of how many test scores are spread out around the average) and English scores by 0.18 standard deviations. That improvement is comparable, Gilraine notes, to the benefits of smaller class sizes on test scores, according to studies like one from economist Alan Krugeuer. His 2011 study found that when class sizes were cut by a third, standardized test performance increased by 0.22 standard deviations.

But making classes smaller, which means hiring more teachers, is a costly change. In 2011, the national student-to-teacher ratio was 15.3 to 1. Decreasing that to just 14.3 students for every teacher would require, per the Brookings Institute, “hiring 226,000 additional teachers, which at $55,000 per teacher would cost $12.4 billion/year in salary costs alone.” The filters installed in these schools cost $700 each, and electricity and filter replacement costs add another $600 per year, Gilraine says. “Assuming a five-year life span for the unit and the need to install 1.5 units for each class to cover common area,” he writes, we end up with an approximate $1,000 per class-year cost—way less than the annual cost of another teacher’s salary.

Gilraine admits that his paper deals with a small sample size, but it still shows promise. “This is kind of the first natural experiment to take a look at this,” he says. It needs to be replicated at a larger scale, but, he says, “this provides evidence that this could actually be quite a fruitful avenue for further research and a fruitful avenue that could raise student’s achievements, as well.” To Gilraine, it’s clear we need to look into this more, and he’s in the process of applying for grants so he can do just that through randomized control trials.