Two hundred people had been exposed to the gas, and the dozens that were sickened mostly were experiencing dizziness, city spokeswoman Alexis Newell told the Los Angeles Times.
Smoke was reported emerging from a structure in the 100 block of Railroad Avenue at about 11 a.m., Newell said.
"Employees within the business and the business next door then began to complain of feeling dizzy," she said.
The Monrovia Fire Department issued a shelter-in-place order that applies to all residents and businesses in the area.
Interpol investigating stolen passports on missing flight
Teacher apologizes for showing sexual image of herself in class