The indoor ice rink's ventilation system failed, causing it to recirculate the polluted air, Gunnison fire marshal Dennis Spritzer said.
The people were checked at Gunnison Valley Hospital Sunday, said Randy Phelps, the chief executive. All but one of the 15 kept for observation were released by Sunday night and the remaining person was discharged Monday.
Two people were taken to a Denver hospital, Phelps said.
It wasn't immediately known if the two were still in the hospital.
Firefighters were trying to determine why the system's damper failed to open, and they contacted the company that made it.The city-owned hockey rink didn't have carbon monoxide detectors, but Spritzer said the devices will be installed before it reopens.
Spritzer said there haven't been any previous problems at the rink, which opened three years ago.
In 2009, more than 30 people at a youth hockey tournament in Greeley were treated for exposure to carbon monoxide. The fire department in the city said the source was the same as in Gunnison - a buildup of exhaust fumes from ice-cleaning equipment.