Alkaline Aerosols
Acids & Corrosives · Acids & Corrosives overview
Alkaline aerosols are the airborne mists of strong bases — most commonly sodium hydroxide (caustic soda), potassium hydroxide and ammonia solutions — released wherever heated, electrolysed or jetted alkaline baths sit open to workshop air. They are typical of caustic cleaning and degreasing, aluminium etching, electrolytic cleaning, bottle washing in food and brewing, and parts washing in heavy industry. Although the public attention often focuses on acids, caustic mists are equally aggressive to the upper airway, eyes and skin and are controlled under the COSHH Regulations 2002.
Where alkaline aerosols are generated
Alkaline aerosols form by the same routes as acid mists — bursting bubbles at heated bath surfaces, electrolytic gassing in cathodic cleaning, and atomisation in high-pressure or spray-wash systems. The droplets are typically respirable and travel into the breathing zone of operators loading parts, sampling baths or working at the rim of wash tanks.
In UK industry the dominant sources are caustic etching of aluminium ahead of anodising, hot caustic cleaning of mild steel, electrolytic alkaline cleaning before plating, bottle and crate washing in food and beverage, and parts washing on heavy equipment overhaul.
- Caustic etch tanks ahead of aluminium anodising.
- Hot alkaline soak and electrolytic cleaning lines in metal finishing.
- High-pressure caustic spray washing for engine, bearing and aerospace parts.
- Bottle washing, crate washing and CIP plant in food, brewing and dairy.
- Paint stripping using hot caustic and methylene-chloride-free strippers.
- Pulp, paper and detergent manufacturing.
Why alkaline aerosol exposure matters
Sodium and potassium hydroxide aerosols carry low UK Workplace Exposure Limits because they are intensely corrosive to mucous membranes. Even brief exposure can cause sharp irritation of the nose and throat, with the potential for chemical burns of the eye and skin where the aerosol settles on bare skin. Chronic low-level exposure is linked to upper-airway irritation, chronic rhinitis and laryngeal symptoms in long-tenure operators.
Caustic aerosols are particularly insidious because they are usually clear, do not smell at low concentrations, and feel soapy rather than sharp on the skin — operators routinely under-report exposures that quantitative sampling shows to be at or above the WEL.
Monitoring caustic and alkaline aerosols
Sodium and potassium hydroxide aerosols are sampled by drawing breathing-zone air through a treated filter at a low flow rate, with laboratory analysis for sodium or potassium ion by ion chromatography or AAS against MDHS-equivalent methods. Ammonia is sampled separately on a sorbent tube and read against its own EH40 limit. The strategy follows BS EN 689 — focus on the most exposed operators across a full shift, and add short-term samples where peak releases are likely.
Process-area or static samples can support LEV diagnostics and incident investigation; they do not replace personal sampling for the COSHH judgement.
Controlling alkaline aerosols
Control follows the COSHH hierarchy. Substitution to lower-concentration or non-caustic chemistries — neutral degreasers, aqueous parts washers, citric or organic-acid descalers — should be the first step where the process allows. Where caustic remains essential, tank covers and reduced free surface, lip or push-pull LEV designed for the actual bath area, splash guards on spray-wash cabinets, and acid- and alkali-resistant ducting and scrubbed discharge form the engineering package.
Procedural controls — limit bath temperature, schedule loading to minimise splash, use full face protection during sampling and dosing, and provide RPE for tank cleaning — complete the control package.
When to review alkaline aerosol exposure
Review the COSHH assessment on a change of caustic concentration, temperature, throughput or chemistry; modification of LEV or spray cabinet; new caustic supplier or product; reports of cough, rhinitis or eye irritation; an enforcement or insurer visit; and at a defined two- to three-year refresh. LEV Thorough Examination and Test under COSHH Reg 9 is required at least every 14 months.
Frequently asked questions
Is caustic mist as hazardous as acid mist?
Yes. Sodium and potassium hydroxide aerosols carry low EH40 limits and cause sharp upper-airway and ocular irritation. The risk is often under-perceived because caustic does not smell sharp and feels soapy on the skin.
How is caustic aerosol sampled?
By personal pump and filter, with laboratory analysis for sodium or potassium ion against an MDHS-equivalent method. Ammonia is sampled on a separate sorbent tube against its own EH40 limit.
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