The incident was reported at 6:37 a.m. at West Coast Recycling, a facility at the pier located near the Hunters Point neighborhood in the city, fire Lt. Mindy Talmadge said.
A box was going down a conveyor belt in the facility when it broke open and a white powder was released into the air, according to Talmadge.
Twenty people at the facility reported irritation of the eyes due to the release, and 12 people required treatment at the scene. One of those 12 victims required additional treatment and was transported to a local hospital, Talmadge said.
The building was evacuated and secured shortly before 8 a.m., and a hazardous materials team went inside to find the box and determine what was inside, she said.
The box was eventually found and the chemical was determined to be ammonium phosphate, which is frequently used in fertilizers, according to Talmadge. The only cleanup required is to wet down the substance, she said.
Authorities planned to reopen the building and allow employees back inside later this morning, Talmadge said.
Officials from the U.S. Environmental Protection Agency, San Francisco Department of Public Health, and the city's fire and police departments responded to the incident, she said.