Acid Mist Monitoring
Acid Mist · Acid Mist overview
Acid mist monitoring measures the airborne concentration of sulphuric, hydrochloric, nitric, chromic or phosphoric acid mist that an operator actually breathes around plating tanks, pickling lines, battery formation rooms and acid reactors. Results are compared with the Workplace Exposure Limits in HSE EH40 to demonstrate adequate control under COSHH.
What acid mist monitoring checks
Acid mist monitoring turns a COSHH acid risk assessment into a measured exposure figure. A low-flow personal pump worn by the operator draws air through a sampling medium selected for the target acid — typically an impregnated filter, sorbent tube or denuder — for a representative portion of the shift.
Samples are analysed in a UKAS-accredited laboratory, most often by ion chromatography for sulphate, chloride, nitrate or fluoride, and reported against the appropriate EH40 8-hour TWA and short-term WELs. The judgement that follows — adequately controlled, borderline or not — is the basis for any change to LEV, tank suppression or RPE.
When acid mist monitoring is needed
Monitoring is most commonly arranged when the COSHH assessment cannot reasonably conclude that exposure is adequately controlled — for example after a change in process chemistry, a new plating or pickling line, modification of LEV, a complaint of nose or throat irritation, or a gap in the LEV Thorough Examination and Test record.
- After commissioning a new plating, anodising, pickling or battery line.
- When tank temperature, current density or acid concentration is changed.
- Following modification, repair or removal of acid LEV.
- Where operators report nose, throat, eye or dental symptoms.
- To support periodic review of the COSHH acid risk assessment.
How acid mist monitoring is planned
A defensible programme starts with similar exposure groups — operators who work the same tanks, shifts and tasks. Sample numbers per group are set against the BS EN 689 framework so that the result can be read as representative of the group rather than a single worker on a single shift.
Each sample is recorded with the tank, acid chemistry, concentration, temperature, current density, LEV mode, RPE worn and task pattern, so the laboratory result is tied directly to the operating condition that produced it.
What the report tells you
A useful acid mist monitoring report does more than print concentration numbers. It links each result to the engineering context that produced it, identifies the specific failure where a result is high, and proposes a proportionate next step in the COSHH control hierarchy — usually a tank cover, LEV upgrade, scrubber service, fume suppressant change, or selected and face-fit tested RPE.
Frequently asked questions
Does acid mist monitoring cover the gas as well as the mist?
For acids that exist as both gas and aerosol — notably HCl and HNO3 — the sampling train is selected to capture both phases (for example a denuder or combined filter/sorbent). The laboratory report distinguishes the contributions where the method allows.
How often should acid mist monitoring be repeated?
There is no single statutory frequency. Many UK plating and chemical sites repeat acid mist monitoring annually as part of their COSHH review cycle, with additional visits after process or LEV changes. The interval should be set in the COSHH assessment, not assumed.
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