Nine construction workers had to be hospitalized early Tuesday evening after being exposed to carbon monoxide at a site in Midtown, fire officials said.

The hardhats were working at 30 East 29th St. when one of them began feeling “ill” around 4:30 p.m., according to FDNY Chief of Special Operations John Esposito.

Firefighters and paramedics arrived on the scene minutes later and found several others who felt “dizzy and nauseous,” Esposito said.

Four workers wound up being hospitalized in serious condition and five were treated for minor injuries.

Investigators believe gasoline-powered generators — which were running in a small, “confined” space — are ultimately to blame. Of the nine workers who got exposed, two of them had to be lifted up “on rope” in a high-angle rescue, according to Esposito.

“Readings of [carbon monoxide] were upwards of 700 parts per million,” Esposito said. “A normal reading in your house is zero. If we have 10 parts per million of CO, that’s a dangerous condition. So we had over 750 parts per million where they were working, above where they were working.”

Esposito noted how workers are not supposed to be using gas-powered generators indoors.

“They’re supposed to be outside in a well-ventilated area,” he said, adding that the Department of Buildings will be investigating the incident.