In Chicago, people protected themselves from icicles falling off the 100-story John Hancock Center with their backpacks and purses. In New York, one man was struck in the face by a chunk of ice as he was trying to warn others to stay away; his cuts required 80 stitches.

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And as a thaw finally sets in, experts expect the problem of ice falling off skyscrapers to get worse, The Associated Press reports.

“The snow starts to melt and the liquid drips off and makes bigger and bigger icicles, or chunks of ice that break off skyscrapers,” Joey Picca, a meteorologist at the National Weather Service in New York, told The AP. “If you see ice hanging from a building, find another route. Don’t walk under hanging ice.”

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Energy efficient buildings and skyscrapers can make the conditions riper for more, and larger, icicles.

“They keep more heat inside, which means the outside is getting colder and that allows more snow and ice to form,” engineer Roman Stangl, founder of the consulting firm Northern Microclimate in Cambridge, Ontario, told The AP.

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Some architects are designing new buildings with icicles in mind, using darker colors to discourage icicles from forming, for example. In New York, a street close to 1 World Trade Center was closed to pedestrians and crews have been removing the ice.

“This happens all over the country, all over the world, in cold climates,” architect Chris Benedict said.