Nine people — including four people listed in serious condition — were removed from a Flatiron construction site Tuesday evening and hospitalized after they were exposed to carbon monoxide, the city fire department said.

It happened around 4:30 p.m. at a building at 30 East 29th Street near Madison Avenue in Midtown.

The FDNY said that is when a construction worker began to feel dizzy. Firefighters responded as more construction workers began to feel dizzy and nauseous in a confined space underground where the workers were pouring concrete.

A few people were able to get up to the street on their own, the FDNY said, but two had to be lifted up by a rope as paramedics began treating them.

All of the people removed from the site are expected to survive, the fire department said. Everyone was removed from the site by 6 p.m., authorities said.

Sustained carbon monoxide exposure levels above 150 to 200 ppm can cause symptoms that include disorientation, unconsciousness, and even death; the FDNY said the reading where the construction workers were working was over 750 ppm.

The fire department said the source of the carbon monoxide appears to have been a gasoline-powered generator, which is against code.