Sulphuric acid mist (H2SO4 mist)

Sulphuric acid mist is a fine airborne aerosol of sulphuric acid (H₂SO₄) droplets, formed when sulphuric acid vapour or SO₃ condenses, or when the acid is sprayed, heated or violently reacted. It is highly corrosive to the respiratory tract and eyes — far more damaging at a given mass than SO₂ gas because the liquid acid attacks tissue directly. The Indian stack emission limit for sulphuric acid mist is 50 mg/Nm³.

The mist arises in sulphuric acid manufacturing plants, metal pickling and surface-treatment lines, lead-acid battery formation and charging, and some fertiliser processes. In recycling, the most direct relevance is to lead-acid battery recycling (the spent electrolyte is dilute sulphuric acid, and battery breaking, draining and any acid handling can generate mist) and to hydrometallurgical metal recovery in e-waste and lithium-ion battery processing, where strong acids are used for leaching and the leaching/dissolution steps can throw off acid mist.

The hazard is acute respiratory and eye injury to workers near acid handling, plus corrosion of building structures and equipment. Because the droplets are fine, they travel and persist, so an uncontrolled acid-handling area exposes a wider zone than just the immediate operator.

Control is containment and scrubbing: enclose acid-handling operations, provide local exhaust ventilation drawing to a mist eliminator / alkaline scrubber, neutralise spent acid rather than letting it off-gas, and protect workers with acid-resistant PPE and eye protection. Spent battery acid must be neutralised and managed as hazardous waste under the Battery Waste Management Rules 2022. Stack acid mist is tested against the 50 mg/Nm³ limit where acid processes are present.