One person was taken to the hospital after high levels of carbon monoxide were found at two La Mesa restaurants on Sunday.
Heartland Fire, La Mesa Police Department, San Diego County Hazmat and San Diego Fire-Rescue all responded to the scene after more than one carbon monoxide alarm went off around 8 a.m.
The Starbucks, Golden Bagel, and other businesses located on Fletcher Parkway were briefly evacuated.
One Starbucks employee was taken to UC Medical Center after complaining about having difficulty breathing.
"(The employee) who received medical attention is doing well and the store has since reopened," Starbucks said in a statement to NBC 7.
An exhaust fan above the boiler at Golden Bagel was turned off, causing carbon monoxide to accumulate and spread to the nearby businesses, Heartland Fire said.
The CDC said thousands of emergency rooms and hospitalizations each year are connected to carbon monoxide poisoning.
The most common symptoms are dizziness, upset stomach, vomiting, chest pain or a headache.