Fourth Precinct officers and fire officials responded to the Best Yet Market at 696 Dogwood Ave. at about 10:45 a.m. because of a foul odor inside the building, police said.
Police found several people -- both employees and customers -- complaining of headaches, dizziness and nausea, according to a news release.
Franklin Square Fire Department detected elevated levels of carbon monoxide inside the store, police said.
Andrew Schmitt, a supervisor with the Nassau County Fire Marshal's Office, said the incident is still under investigation and the cause of the carbon monoxide had yet to be determined.
The store was evacuated, and 19 people were taken to the East Meadow medical facility with carbon monoxide poisoning, hospital spokeswoman Shelley Lotenberg said. Five were treated in a hyperbaric chamber, she said.
Twelve people were treated at the scene, police said, as several other fire departments also responded.
Calls to the Best Yet main office in Bethpage were not immediately returned.
A person who answered the phone at the Franklin Square store Friday afternoon said the store had been reopened and "things were back to normal."