Human activity strayed far from the norm this year as the coronavirus pandemic forced many countries to effectively shut down. Now, many months after the first countries went into lockdown, scientists from NASA have gaged just how much pollution levels have dropped due to Covid-related declines in human activity.

What the scientists discovered is that since February, pandemic restrictions have slashed global nitrogen dioxide concentrations by 20 percent. To come to this conclusion, scientists used computer models to generate a Covid-free 2020, which was then compared with the actual statistics gathered by 46 countries at 5,756 different observation sites.

Nitrogen dioxide is an air pollutant produced through the combustion of fossil fuels. The pollutant can aggravate respiratory diseases in humans such as asthma and causes the atmosphere to form acid rain. On a city level, NASA said 50 of the 61 analyzed cities showed nitrogen dioxide reductions between 20 and 50 percent.

“In some ways, I was surprised by how much it dropped,” said Christoph Keller, lead author of the study. “Many countries have already done a very good job in lowering their nitrogen dioxide concentrations over the last decades due to clean air regulations, but what our results clearly show is that there is still a significant human behavior-driven contribution.”