"For the past year local air regulators have been tracking the chemicals companies use in oil and gas exploration around Southern California. In a report released Wednesday, four environmental groups crunched the tracking reports to find that more than 40 toxic chemicals have been used in dozens of drilling operations, often near homes, schools, and hospitals.
The South Coast Air Quality Management District has sought information about so-called "well stimulation" activities. This includes hydraulic fracturing or “fracking” and also processes known as "acidization" and "gravel packing." The methods differ slightly, but all seek to inject material into fissures in the earth in order to extract hard-to-reach oil and gas.
The top chemicals used during the past year include crystalline silica, methanol, hydrofluoric acid, and formaldehyde."