The Walmart on Plaza Parkway was evacuated Friday afternoon when the smell of sulfur filled the store, sickening nine people.
The apparent source, according to the Modesto Fire Department, was an old car battery returned to the store earlier in the day.
“The battery gets old, and as the water drains out of the battery and the acid becomes exposed, it becomes real pungent and will cause that smell, especially if it gets overheated,” said Battalion Chief Alan Ernst.
Nine people were treated for symptoms including headaches, nausea and weakness, but all declined to be taken by ambulance to the hospital.
The store was evacuated about 4:30 p.m. but Wal-Mart employee Catrina Johnson said the “rotten egg smell” started about two hours earlier.
Johnson said she complained to management three times in two hours but repeatedly was told the issue was likely a stink bomb and nothing more serious.
She said the McDonald’s inside the store was having issues with its sewer system. Ernst confirmed that the health department was at the scene to investigate potential sewer issues, though the outcome of the investigation was not known Friday night.
Wal-Mart officials could not be reached after office hours to comment about the response to the sickening smell.
Johnson said it was she and customers who had to call Pacific Gas and Electric Co. and 911 to get a response to the store.
Hours after leaving the store, Johnson said, she still felt a tightness in her throat and chest.
The Fire Department cleared the scene about 7 p.m. but managers at the Walmart kept the store closed to have it cleaned.