Hydrochloric acid (acid aerosols including mists, vapors, gas, fog, and other airborne forms of any particle size)

Hydrochloric acid (HCl) has been known since the 10th century, when the Persian alchemist Rhazes described its preparation from common salt and vitriol. [1] Industrial production began on a large scale in the 19th century as a byproduct of the Leblanc soda ash process, and today it is primarily produced as a byproduct of organic chlorination reactions (e.g., chlorination of benzene, methane, or ethylene) and by burning hydrogen in chlorine gas. Global production exceeds 20 million tons per year. Its industrial uses are extraordinarily diverse: steel pickling to remove rust and scale, semiconductor etching and cleaning, production of polyvinyl chloride (PVC) precursors, pH control in water treatment, food processing (for glucose production from corn starch), and regeneration of ion exchange resins. In the home, dilute HCl is the active ingredient in many toilet bowl cleaners and tile grout cleaners. [2] The EPA lists acid aerosols (all airborne forms of HCl) as a Hazardous Air Pollutant because they cause respiratory tract damage at low concentrations, and emissions from industrial facilities (steel mills, chemical plants, PVC manufacturing) represent a significant air quality concern. Accidental releases from tank cars and industrial processes have caused community health emergencies — HCl is listed as an extremely hazardous substance under EPCRA for emergency planning. [3]

Industrial workers in steel pickling, PVC manufacturing, semiconductor fabrication, and chemical synthesis face occupational inhalation of HCl gas and aerosols. Workers in swimming pool maintenance, water treatment, and food processing encounter dilute HCl solutions. The general public can encounter HCl from consumer cleaning products (toilet bowl cleaners, grout cleaners) in vapor form in enclosed bathrooms. Communities near HCl-releasing industrial facilities, and those near transportation accidents involving HCl tank cars, face emergency exposure risks.

Hydrogen chloride gas and acid aerosols cause immediate respiratory tract irritation, with the extent and severity depending on concentration and duration. [2] At low concentrations (1–5 ppm), eye and nose irritation occurs. At higher concentrations (10–50 ppm), severe coughing, choking, and bronchospasm occur. Extremely high concentrations (>50 ppm) cause pulmonary edema, laryngeal spasm, and potential asphyxiation. Chronic low-level occupational exposure has been associated with erosion of tooth enamel, chronic bronchitis, and increased risk of respiratory tract cancer. The WHO classifies sufficient evidence of HCl causing laryngeal cancer in occupationally exposed workers.

Steel mill workers in pickling operations, semiconductor fab workers, PVC manufacturing workers, and workers handling concentrated HCl solutions face the highest occupational risks. Emergency responders at HCl spill or rail incidents face acute severe exposure risk. Household consumers mixing HCl-based toilet bowl cleaners with bleach (which generates chlorine gas as well as HCl) face acute exposure.

1. Never mix toilet bowl cleaners or other acidic cleaners with bleach or ammonia — the resulting gas releases can cause severe respiratory damage. 2. Use HCl-based cleaners in well-ventilated areas and avoid breathing fumes. 3. Industrial workers must use continuous air monitoring, local exhaust ventilation, and supplied-air respirators for high-concentration operations. 4. Chemical-resistant gloves and face shield or goggles required for all HCl liquid handling. 5. Know your community's EPCRA emergency notification contacts for industrial facilities using large quantities of HCl.