HCl can be produced by several methods. The majority (90%) of the HCl produced in the United States is a by-product of various chlorination processes (Hisham and Bommaraju 1995). Lesser amounts (8%) are produced directly from hydrogen (H₂) and chlorine (Cl₂). Combustion of chlorine-containing organic compounds results in the formation of HCl. Average HCl concentrations in combustion flue gas have been reported as high as 3,030 ppm (Sebacher et al. 1980). HCl is also found naturally in volcanic gases particularly in Mexico and South America and might have been one of the gases in the original atmosphere of the earth (Hisham and Bommaraju 1995). HCl from sea salt is the main source of tropospheric HCl from natural sources (Symonds et al. 1988). Combustion of fossil fuels (especially coal) is the most common anthropogenic source of ambient HCl concentrations, which have been measured in the range of 0.5-7.6 ppb (Kamrin 1992). In areas near sources of HCl from combustion, Kamrin (1992) estimated the maximum HCl concentrations to be in the range of 20-30 ppb.

HCl is formed during the combustion of rocket propellants containing ammonium perchlorate (NH₄Cl0₄). The major combustion reaction producing HCl is

C_nH_m + NH₄Cl0₄ → CO + CO₂ + HCl + N₂ + H₂ + H₂O.

The HCl concentration in the exit plane of a solid propellant rocket using ammonium perchlorate as an oxidizer was calculated to be 18.3119.41 grams (g)/100 g of propellant burned for three types of rockets (Bennett 1996). Pellett et al. (1983) reported that the exhaust from a space-shuttle launch using a solid rocket fuel contained 60 tons of HCl. The total range of peak HCl concentrations measured in eight Titan III