The apartment complex manager noticed the chlorine smell this morning when she went into the pool room, and contacted authorities around 11 a.m., Alameda County fire spokeswoman Aisha Knowles said.
The fire department and a hazardous materials team responded, evacuated the 30 some residents from the 42-unit complex at 20499 Santa Maria Ave., and ordered people at nearby businesses to shelter in place, Knowles said.
The manager was transported to a local hospital after breathing in the fumes. No one else required a hospital visit, Knowles said.
Moisture was found to have "somehow gotten into the container, which caused it to have an exothermic reaction, or self-heat and emit gas," Knowles said.
The danger was eliminated shortly after noon when the container was removed from the pool room and poured into the swimming pool, after a hazardous materials team spent more than an hour evaluating the situation, Knowles said.
The pool will be monitored by maintenance personnel until its chlorine levels return to normal, Knowles said.
Nearby schools were not affected by the incident, Knowles said.