Sheboygan, WI- The Sheboygan Press – Three Sheboygan firefighters and a police officer were treated for exposure to high levels of carbon monoxide after responding to a call about a possible heart attack about 8:15 p.m. Tuesday.

As firefighters were assisting Orange Cross Ambulance personnel with the patient, the patient’s brother said the furnace had been worked on that day, according to a press release from the fire department.

After a pet parakeet was found dead, firefighters found carbon monoxide readings above 500 parts per million, according to the release. “It might have been higher than that,” Commander Joel Daum said.

The fire department’s meter peaks at 500 ppm, Daum said.

“This certainly was a serious call,” he said. “You can’t see it. You can’t taste it.”

The furnace was shut down, and the house in the 500 block of North Sixth Street was ventilated, the release said.

The four rescue workers were taken to St. Nicholas Hospital for treatment and released, according to the release. The original patient also was taken to St. Nicholas, but no condition report was available.