Teachers in Baltimore are calling for city schools to be closed until heating problems are resolved, saying many students have been forced to attend classes bundled up in coats, hats and gloves during the recent frigid weather.

The Baltimore Teachers Union sent a letter on Wednesday to Sonja Brookins Santelises, the chief executive officer of Baltimore City Public Schools, saying that this week students and teachers have endured dangerously low temperatures in buildings that are struggling to operate with bursting boilers and drafty windows.

“Trying to provide a stable learning environment in these extreme conditions is unfair and inhumane, to say the least,” said Marietta English, the president of the union, in the letter, which was published in The Baltimore Sun on Wednesday.

That day, four schools were closed and three let out their students early because of the heating issues and cold in their buildings. As blizzard conditions raged along the East Coast on Thursday, the closings extended to all Baltimore city schools, as well as those in other major cities including New York City, Boston, Philadelphia and Washington.