Explosions engulfed the Copper State Chemical Co. at 748 E. 16th St., Tucson, in fire on July 2, 1970. The assistant fire chief arrived on scene and immediately ordered a second alarm. Two large explosions scattered debris and started fires at Home Gas Co., O'Malley's Lumber Co., and Tucson Warehouse Co. (now Barrio Brewing) adjacent to Copper State.

Soon after, a third alarm was dispatched and a "full alert" was issued, recalling all off-duty firefighters to the scene. One third-alarm unit, Engine 7, collided with a car driven by a teenager, but was able to make it to the scene.

Fire commanders ordered the evacuation of people and homes in the area due to the toxic cloud of smoke, which included pesticides and ammonium nitrate. Injured firefighters totaled 56, mostly for smoke inhalation and eye and skin irritation.

Though the building was not terribly large, the contents within created a massive fire that required more than 2 million gallons of water pumped through 11,750 feet of fire hose. Nearly all of the department's staff and equipment responded to the fire. Fire trucks from Hughes Aircraft, Davis-Monthan AFB and S. Tucson Fire staffed empty stations while TFD units fought the fire.

The toxic stew created with the millions of gallons of water was routed into a temporary dam in the Santa Cruz River. Nearby drinking water wells were shut down until water could be tested for contaminants.

Exact cause of the fire is not known, but spontaneous combustion was suspected. Employees saw containers in a shipment of Nu-Lawn Weeder exploding in flames, which set off a series of explosions in other chemicals.

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