CHICAGO/NEW YORK (Reuters) - As the biting cold put Chicago into partial hibernation this week and the U.S. Postal Service kept mail carriers off the streets, Daly O’Brien was on the job, braving extreme conditions to deliver food to customers cocooned inside their toasty homes.

A worker cleans up after a large fire extinguished by the New York Fire Department (NYFD) as they worked in frigid conditions in the Brooklyn Borough of New York, U.S., January 31, 2019. REUTERS/Lloyd Mitchell

O’Brien, 25, who works for grocery delivery company Instacart, was out on his rounds as usual on Thursday morning, bundled up in a woolly cap, thermal leggings and parka as he made a stop in the Boystown neighborhood on the city’s North Side.

The temperature was hovering at minus 11 Fahrenheit (minus 23 Celsius), and it felt like minus 27F with the wind, thanks to the reservoir of arctic air known as the polar vortex that has sagged over the region this week.

“This is definitely the coldest it’s ever been in the four years I’ve had this job,” Daly said with a laugh, pointing out that his father, a mail carrier, had the day off. “As an independent contractor, you just have to do the grind.”

Daly was hardly alone. For first responders, tow truck drivers, package deliverers and people doing a variety of other jobs, it was business as usual. Many had no choice but to brave the brutal weather, which pushed to the U.S. Northeast on Wednesday night and early Thursday morning.

In Elmwood Park, New Jersey, 17 miles northwest of Midtown Manhattan, hundreds of firefighters and other emergency workers endured single-digit temperatures for hours as they battled a spectacular blaze that eventually destroyed a historic paper factory.

Fire Chief Jacob Mamo from the neighboring town of Fair Lawn said his company’s job was to pump water out of the adjacent Passaic River. The night was “horrible,” he said, despite occasional breaks in warming tents.

Slideshow ( 2 images )

“Once we started flowing all the water, the mist from all the trucks, all those hoses and stuff just started icing up and made for very miserable conditions - walls of ice,” he said.

Mamo, 36, said his feet quickly froze as he walked through puddles of water and his hands stung from having to take off his gloves so many times during the operation.

“The cold starts to set in pretty quick,” he said. “That’s when you get a real feel of reality.”

Back in Chicago, Jesse Medina, a 49-year-old tow truck driver, said he planned on working all day on Thursday after taking a few hours off the day before because of the weather.

“I’m cold as can be,” Medina said as he dropped off a car at a garage on the city’s North Side. “But it’s a way to make a living - you have to.”

Parts of Chicago were among the ZIP codes in 10 states where the U.S. Postal Service shut down mail delivery, after dangerously low temperatures forced Midwesterners, who pride themselves on their winter toughness, to huddle indoors

Oscar Ramos, a 26-year-old paramedic who lives in the Chicago suburb of Skokie, was not one of them. He worked on both Wednesday and Thursday, though he admitted the weather made it difficult to do the job.

“You gotta just adjust, do what’s best for the patient. So we try to keep it warm in the back,” Ramos said, standing outside a Chicago hospital where he had just dropped off a patient.

Chris Poon, a New York Police Department officer who was working an eight-hour shift in Manhattan’s Times Square on Thursday, invested in a battery-powered jacket to keep warm.

“I’ve got to tell you, the heated jacket works wonders,” he said. “You buy one, it has batteries and whatnot, and it heats up, it’s almost like a heated glove.”