Twenty-nine people were sickened Friday by an unknown substance at the UW Health West Clinic on Junction Road.

Twenty-nine people were taken from the UW Health West Clinic to area hospitals Friday morning after people inside the building reported feeling light-headed and nauseated due to an unknown gas.

The hazardous materials unit of the Madison Fire Department and a crew from Madison Gas and Electric went to the building at 451 Junction Road to see what the cause of the illness was, but a cause had not been determined Friday evening.

One possible cause is sewer gas, which is a mix of toxic and non-toxic gases found in sewer systems.

“One patient in an ambulance smelled sewer gas while on Mineral Point Road and said it smelled just like the smell inside the building,” said Madison Fire Department spokeswoman Cynthia Schuster.

UW Health spokeswoman Toni Morrissey said the West Clinic was evacuated when people started getting sick.

The building was closed for the day by 1:30 p.m., with employees allowed back inside to get their personal belongings before heading home.

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Schuster said 15 people initially had been seen in a triage area set up by paramedics outside the facility, with eight taken by ambulance to the hospital, and others possibly going there in their own vehicles. That number expanded to 29 later in the day.

“It’s all a low level of illness at this point,” Schuster said.

Everyone treated reported headaches and nausea, with no major illnesses seen, authorities said.

All of the 15 patients sent to UW Hospital had been discharged by Friday evening, said UW Health spokesman Gian Galassi said.

The clinic remained closed throughout the day, but Galassi said it will reopen Monday.

“The hazardous materials team checked the air quality, and all levels indicated normal amounts of oxygen and no carbon monoxide,” Schuster said.

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