Ventura County fire Capt. Mike Lindbery says only a handful of smouldering hot spots remain Wednesday at the site in an agricultural area near Santa Paula, northwest of Los Angeles.
A vacuum truck exploded early Tuesday at Santa Clara Waste Water Co., spreading a chemical mixture that contained sulfuric acid and a highly combustible organic peroxide.
Lindbery says the material is highly toxic and combustible.
No burn injuries were reported, but 37 people were washed down or treated for complaints such as breathing problems, red eyes and skin rashes.