This action finalizes national emission standards for hazardous air pollutants (NESHAP) for hydrochloric acid (HCl) production facilities, including HCl production at fume silica facilities. The EPA has identified hydrochloric acid production facilities as major sources of hazardous air pollutant (HAP) emissions. These standards will implement section 112(d) of the Clean Air Act (CAA) by requiring all such major sources to meet HAP emission standards and implement work practice standards that reflect the application of maximum achievable control technology (MACT). The primary HAP that will be controlled with this action is hydrochloric acid. This HAP is associated with a variety of adverse health effects including chronic health disorders (for example, effects on the central nervous system, blood, and heart) and acute health disorders (for example, irritation of eyes, throat, and mucous membranes and damage to the liver and kidneys).