It happened around 4:30 p.m. at East 29th Street and Madison Avenue in the Flatiron District.
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The FDNY received a call about a worker feeling dizzy, and crews who responded then found multiple construction workers feeling dizzy and nauseous, fire officials said.
Authorities say there were carbon monoxide readings of 750 parts per million at the underground site. 10 parts per million is considered dangerous.
Workers were pouring foundation in an area about 60 square feet.
"It was very difficult to ventilate because there's only a very small area to enter into the confined space," said FDNY Deputy Chief John Sarrocco.
All of the workers were hospitalized in stable condition with non-life-threatening injuries.
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The FDNY and EMS were evaluating several patients, and two people who suffered more severe injuries could be seen on stretchers.
"We were able to begin IV treatment, cardiac monitoring while our peers on the fire side did all the heavy lifting of pulling them out," said EMS Deputy Chief Kathleen Knuth.
A residential building is being constructed at the scene, and investigators were eyeing an underground gas-powered generator as a possible source for carbon monoxide.
The Department of Buildings has issued a Full Stop Work Order for the site, and a violation was issued to the general contractor for a failure to safeguard the site.
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