The artists at 543 Union Street knew all about the famously toxic canal that runs next to their building, a former box factory in Gowanus. But in the decades since they converted the factory to broad, timber-beamed lofts, the artists’ most pressing concerns were typical for a century-old building in Brooklyn — broken pipes, hallways in need of a fresh coat of paint, a perpetually damp basement.

That was before a letter arrived from the New York State Department of Health just before New Year’s Day 2023. It asked an unexpected and alarming question. Could the Health Department test the artists’ lofts for a chemical called trichloroethylene?

No one in the building had ever heard of trichloroethylene, or TCE, but they began to wonder whether their health was in danger. And there was another worry. Would they have to leave the building they had worked so hard to make hospitable?

This is the paradox of Gowanus. In 2010, the Gowanus Canal was listed as a national Superfund site, a designation the Environmental Protection Agency uses to prioritize cleaning up the most toxic places in the country. Yet since then, Gowanus has emerged from its industrial past to become one of Brooklyn’s hottest real estate markets and is in the midst of a construction boom. Fifty-two new buildings are either under construction or are expected to be built over the next five years.

The pollutants in the canal are well documented (benzene, mercury, chloroform, vinyl chloride, coal tar), and efforts to clean the waterway are proceeding, though years behind schedule. But the discovery of TCE, a chemical that has been linked to cancer, birth defects and Parkinson’s disease, revealed just how polluted the rest of Gowanus might be.

Since the spring of 2023, more than 100 buildings have been tested for TCE; 21 were found to have levels of chemicals in their indoor air that required immediate cleanup. At the box factory, TCE levels were 450 times the state’s allowable guidelines.