Dirty air is choking India’s quest for clean energy.

High levels of environmental pollutants and dust in northern India are expected to reduce the region’s solar power generation capacity by around 17%, a new study published in the Environmental Science & Technology Letters journal says. India’s northern plains are dotted with a number of heavily polluted cities, including New Delhi, which has worse air quality than almost anywhere else in the world.

At current levels, particulate matter (PM)—either airborne or deposited on solar panels—could wipe out nearly 776 megawatt (MW) of solar power, which is more than the capacity of India’s largest solar plant.

The study was initiated after Michael Bergin, a professor at Duke University, noticed that some rooftop solar installations in India were incredibly dirty. ”I thought the dirt had to affect their efficiencies, but there weren’t any studies out there estimating the losses,” he told a university publication. “So we put together a comprehensive model to do just that.”

Along with researchers from the Indian Institute of Technology (IIT), Gandhinagar, and the University of Wisconsin-Madison, Bergin began collecting samples from multiple solar panels located at IIT-Gandhinagar. They found that the efficiency of the panels would improve by nearly 50% when they were cleaned after certain periods of time.

Screenshot/Environmental Science & Technology Letters

“The partially cleaned solar panels clearly show that PM covers the panel surfaces and suggests that the coating may be influencing solar energy production,” the study noted.

When the researchers examined the deposits on the panels, they discovered a small but significant percentage of man-made pollutants, aside from dust.

These particles were responsible for roughly half of the 17% fall in solar energy production in northern India, the study estimated, with dust making up for the rest.

“The manmade particles are also small and sticky, making them much more difficult to clean off,” said Bergin, also the lead author of the study. “You might think you could just clean the solar panels more often, but the more you clean them, the higher your risk of damaging them.”

Bergin and his team also used data from NASA’s Goddard Institute for Space Studies to understand the effect of airborne and deposited particles, eventually working out a way to estimate the loss of solar energy production worldwide. “In northwestern India there are areas that experience a decrease in the annual amount of available shortwave energy for solar production by as much as ~50%,” the study reported, partly due to the presence of man-made pollutants and dust in the air.

Aside from India, the study revealed, China is the other major market where unclear air is sapping the solar energy industry. In eastern-central China, for instance, non-dust airborne particles are mostly responsible for a 17% reduction (7,400MW) in solar energy production. “China is already looking at tens of billions of dollars being lost each year, with more than 80% of that coming from losses due to pollution,” Bergin added.

For India, with its ambition of generating 100 gigawatt of solar power by 2022, the lesson is straightforward: It’s time to clear the air.