(CNN) When an Indian engineering student visited a traditional brick kiln, he was appalled at what he saw.

"My moment of reckoning came during that field trip," says Abhishek Banerjee, recalling the 2016 visit. "We saw that the workers there were being treated very inhumanely. And the working conditions of that brick kiln were very improper -- people were digging clay with their bare hands."

Working conditions in brick kilns can be harsh and bonded labor is widespread -- a type of modern slavery where employees are made to work to pay off loans at usurious interest rates.

But India's 140,000 brick kilns also have an environmental cost. As well as creating dust and sulfur dioxide, which can cause respiratory diseases and put stress on local crops and wildlife, one study estimated that India's brick kilns burn 15 - 20 million tons of coal each year. This releases over 40 million tons of climate-warming carbon dioxide into the atmosphere.

'Like Lego bricks'