Acid Mists
Acid Mist · Acid Mist overview
Acid mists are the airborne aerosols of strong inorganic acids — sulphuric, hydrochloric, nitric, phosphoric and chromic — released from open tanks, plating cells, pickling lines, battery formation rooms and chemical reactors. In UK workplaces they are managed under the Control of Substances Hazardous to Health (COSHH) Regulations 2002 and compared against the Workplace Exposure Limits published in HSE EH40.
What an acid mist is
An acid mist is the suspension of fine acid droplets in workplace air, formed when bubbles burst at the surface of a heated or electrolysed acid bath, when high-pressure jets atomise an acid solution, or when a gaseous acid such as HCl re-condenses on cooler workshop surfaces. The droplets are typically respirable in size and carry the acid into the operator's breathing zone, where they deposit on the upper airways, eyes and skin.
From a COSHH perspective, an acid mist is treated as both a corrosive and an inhalation hazard. Even at concentrations well below the level needed to damage building fabric, acid mists can cause irritation of the nose, throat and bronchi, dental erosion in chronically exposed operators, and — for chromic acid mist in particular — recognised long-term respiratory effects.
Where acid mists are generated
Acid mists are encountered across UK metal finishing, chemical processing and battery manufacturing. The most common sources are open or partially enclosed process tanks where heat, gassing electrolysis or agitation lift droplets into the air above the bath.
- Electroplating: chromic acid plating, sulphuric acid anodising, nickel and copper plating baths.
- Metal pickling and descaling using sulphuric, hydrochloric or mixed acid.
- Stainless steel and aluminium passivation using nitric or nitric/HF mixtures.
- Battery manufacturing: lead-acid formation rooms and electrolyte filling.
- Chemical processing: acid reactors, scrubbers, tanker offloading and bulk transfer.
- Laboratory acid digestion, fume cupboards and analytical sample preparation.
Why acid mist exposure matters
COSHH places a duty on UK employers to prevent or adequately control exposure to substances hazardous to health. For acid mists this duty applies whether or not a visible plume is present — many of the most significant exposures occur from clear or only faintly visible aerosols that the operator does not perceive.
The relevant EH40 WELs depend on the acid: sulphuric acid mist, hydrochloric acid, nitric acid, nitrogen dioxide and chromic acid (as chromium VI) each have their own limit, and several are short-term (15-minute) limits set specifically to control acid mist peaks at tank-side. Treating acid mist as a single generic hazard usually leads to under-controlled exposure.
How acid mists are assessed
Acid mist assessment combines a structured COSHH review of each process with personal air sampling of the operators most exposed. Sampling typically uses low-flow personal pumps drawing air through impregnated filters or sorbent tubes selected for the target acid, with laboratory analysis by ion chromatography or other validated methods.
Results are read against the relevant EH40 WEL (8-hour TWA and short-term where applicable) and against the BS EN 689 strategy for comparison with limit values. The output is an exposure judgement — adequately controlled, borderline or not adequately controlled — with a defined next step in each case.
How acid mists are controlled
Effective acid mist control follows the COSHH hierarchy. Substitution to a lower-strength or less volatile process chemistry is considered first, followed by tank covers, push-pull or lip extraction LEV, fume suppressants on plating baths, scrubbed extraction discharge, and only then RPE for residual exposure.
Acid LEV must be designed for corrosive duty — typically polypropylene or FRP ducting, acid-resistant fans, and packed-tower or venturi scrubbing — and is subject to Thorough Examination and Test at least every 14 months under COSHH Reg 9.
Frequently asked questions
Are acid mists always visible?
No. Many acid mists are clear or only faintly visible, particularly at concentrations close to the EH40 WEL. Visible plume is a useful warning sign but not a reliable measure of operator exposure; air sampling is needed to confirm control.
Which acids have UK Workplace Exposure Limits?
Sulphuric acid mist, hydrochloric acid, nitric acid, nitrogen dioxide, chromium VI (as chromic acid mist), hydrofluoric acid and several others have EH40 WELs, with both 8-hour TWA and short-term limits in many cases. The appropriate limit depends on the process chemistry.
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