Sitting in a field in the Chinese port city of Tianjin, the 23-foot-high Smog Free Tower looks a little like a giant fan. It’s actually a park-scale air purifier, designed to pull in the city’s pollution, filter it, and create a bubble of breathable air for people passing by. In new tests, researchers found that the device can help–though only in a small area.

When the tower–which was designed by artist Daan Roosegaarde in 2015, and temporarily installed in Beijing in 2016–sucks in surrounding air in an open field, the test found that it can capture 70% of PM10, tiny particles of pollution that can lodge in the lungs. When the filtered air is released, mixing with the dirty air around it, the result is air with an up to a 45% reduction in PM10 pollution within 20 meters of the tower. The tower also reduces another form of pollution, PM2.5, by as much as 25%. The results were contrary to a 2016 test by the China Forum of Environmental Journalists that saw little benefit to the tower when it was installed in Beijing (the team behind the tower says that those tests were not based on validated measurements; the new research was led by a scientist from Eindhoven University of Technology).

For Tianjin–the sixth most populated city in the world, where heavy smog shut down the airport in December 2016–or for other polluted cities, a network of smog-sucking towers every 20 meters would likely be impractical. But Roosegaarde, who designed the tower as part of his larger Smog Free Project, believes that the technology could help provide some relief as longer-term solutions (like China’s $361 billion investment in renewable energy) scale up. His design for a smog-eating bicycle uses similar technology. The projects, along with rings and other jewelry made from the collected pollution, also serve to deliver a message about the need for change.

“This is stuff we were sucking up from the urban sky,” Roosegaarde said in a recent video about the project as he held up a plastic bag filled with black pollution. “It’s the same as 17 cigarettes a day. That is insane. When did we say yes to that? This is not the bright future you and I imagined.”