The incident was reported at about 2:30 p.m. at the Glacier Ice Company, located at 43960 Fremont Blvd.
According to Battalion Chief Mark Meveau, a hazardous materials team is getting ready to enter the building and adjacent streets have been closed down.
Meveau said the reported leak of anhydrous ammonia caused some workers at adjacent businesses, including REI and Home Depot, mild respiratory problems, but no one was hospitalized.
The Fremont Fire Department has received mutual aid from Alameda County fire teams, according to Meveau.
