Acid Mist FAQ
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Practical answers to the questions UK occupational hygiene, H&S and process teams most often ask about acid mist, corrosive vapour, alkaline aerosol and reactive chemical exposure under COSHH and HSE EH40. The answers below are general technical information and are not a substitute for medical diagnosis or legal advice on any specific site.
Acid mist fundamentals
Acid mist is a suspension of fine acid droplets in air, generated mainly by heated baths, electrolysis, gas-bubble bursting and aerosolisation during transfer or splashing. It is distinct from acid vapour (the gaseous phase of the same acid) and from alkaline aerosols (caustic droplets from alkaline cleaning, degreasing and CIP). On most plating, anodising, pickling and battery sites the breathing-zone hazard is a mixed mist/vapour cloud rather than a single phase.
Substance-specific questions
Sulphuric acid mist is the dominant exposure on heated sulphuric and chromic plating lines and in battery formation. It is sampled on an impregnated filter and analysed by ion chromatography for the sulphate ion, with the result compared against the EH40 limit for sulphuric acid mist.
Hydrochloric acid releases mainly as vapour from heated pickling and from acid spills, with secondary mist generation in agitated tanks. It is typically sampled with a treated filter or impinger and analysed by chloride IC against the EH40 limit.
Nitric acid releases as vapour, mist and — for fuming or oxidising conditions — as NO2. Sampling is paired: nitrate by IC for the acid, and a dedicated method (sorbent or direct-reading) for NO2 against its own short-term limit.
Alkaline aerosols from caustic cleaning, degreasing and CIP are sampled gravimetrically and/or by sodium hydroxide–specific methods. The hazard mechanism is irritation and burns rather than a classic acid profile, but the COSHH approach is the same.
Acid air sampling and monitoring
Personal pumped sampling at the operator's breathing zone, over the relevant averaging period, is the defensible default for COSHH evidence. Direct-reading instruments are valuable for diagnostics, leak surveys and short-term peak work, but they do not replace the laboratory-analysed personal sample for the EH40 comparison.
A typical acid air sampling programme defines the similar exposure groups, identifies the substances and limits, runs a full-shift personal sample on the worst-case operator and adds targeted short-term sampling for the tasks that drive the STEL. Results are reported against the substance-specific EH40 limits with a clear COSHH judgement.
Workplace corrosive exposure
Workplace corrosive exposure on UK sites is dominated by inhalation of acid mist, corrosive vapour and irritant gas, with secondary routes of skin and eye contact from splashes and surface contamination. Task-driven peaks during loading, dosing, rectifier shutdown, sampling and maintenance often matter more than the steady-state cloud, and short-term sampling is normally needed alongside the full-shift TWA.
Acid LEV testing
Acid LEV systems are subject to Thorough Examination at least every 14 months under COSHH Regulation 9. On corrosive duty the test must also reflect the chemistry — corrosion-resistant test equipment, scrubber check, drainage check and material-compatibility review. A passing Reg 9 report does not by itself prove adequate control; it must be paired with breathing-zone exposure data against the EH40 limit.
Chemical fume extraction
Chemical fume extraction is the engineering control that captures acid mist, corrosive vapour and irritant gas at source and routes it through corrosion-resistant ductwork to a scrubber and stack. Effective systems combine appropriate hood geometry, sufficient capture velocity for the task, balanced make-up air and a scrubber sized for the actual loading. Where breathing-zone results trend upward, the engineering case — not RPE alone — is normally where the response starts.
Acid COSHH assessment
An acid COSHH assessment identifies the substances, the tasks, the operators, the existing controls and the residual exposure, and records the judgement against the EH40 limits. For acid and corrosive work it should reference the substance-specific sampling method, the Reg 9 LEV evidence, the RPE programme and the planned re-assessment interval. It is reviewed on change of chemistry, process, throughput, controls or operator group.
Acid compliance and evidence
The COSHH compliance file for an acid or corrosive process typically includes the written assessment, the personal exposure monitoring reports, the LEV Regulation 9 reports, the RPE face-fit and maintenance records, the health surveillance records held by the OH provider, the training records and the action log linking findings to closure. The acidmist.co.uk acid compliance page describes how this evidence fits together.
Acid health risks (general context)
Acid mist, corrosive vapour and alkaline aerosol exposure is associated with respiratory irritation, eye and skin irritation and — at sufficient exposure to certain substances — sensitisation or more serious effects. This site provides occupational hygiene context for managing exposure under COSHH; it does not provide medical diagnosis. Operators with symptoms should be assessed through the site's occupational health route.
Geography and scope
acidmist.co.uk is a UK-focused resource on acid mist, corrosive vapour, alkaline aerosol, reactive chemical and irritant gas exposure in industrial workplaces. It works in the framework of the Control of Substances Hazardous to Health (COSHH) Regulations 2002 and the Workplace Exposure Limits in HSE EH40. It does not cover domestic air quality, environmental EPR permitting, or unrelated occupational hazards.
Frequently asked questions
What is acid mist?
Acid mist is a suspension of fine acid droplets in air, generated by heated acid baths, electrolysis, gas-bubble bursting and aerosolisation during transfer. It differs from acid vapour (gaseous phase) and from alkaline aerosols, and is assessed under COSHH against substance-specific EH40 limits.
How is sulphuric acid mist measured?
Sulphuric acid mist is typically sampled at the operator's breathing zone on an impregnated filter using a calibrated personal pump, then analysed by ion chromatography for sulphate. The full-shift TWA is compared with the EH40 limit for sulphuric acid mist.
How are hydrochloric acid fumes monitored?
HCl is sampled with a treated filter or impinger and analysed by chloride ion chromatography against the EH40 short-term limit. Direct-reading instruments support leak surveys and short-term peak work but do not replace the laboratory-analysed personal sample.
How are nitric acid fumes assessed?
Nitric work is assessed with paired sampling: nitrate-based IC for the acid mist/vapour, plus a substance-specific method for NO2 against its own EH40 short-term limit. Both results are read together because nitric chemistry can release reactive gas as well as acid.
What about alkaline aerosols?
Alkaline aerosols from caustic cleaning, degreasing and CIP are assessed under the same COSHH framework, typically with gravimetric and substance-specific sampling. The hazard is irritation and chemical burns rather than a classic acid profile.
How often does acid LEV need testing?
Acid LEV systems require a Thorough Examination at least every 14 months under COSHH Regulation 9. On corrosive duty the report must reflect the chemistry, scrubber performance and material compatibility, and it should be cross-checked against breathing-zone exposure data.
Is direct-reading monitoring enough for EH40 compliance?
Direct-reading instruments are valuable for diagnostics, leak surveys and short-term peaks, but the defensible COSHH evidence against EH40 limits is normally a personal pumped sample analysed by a UKAS-accredited laboratory for the substance in question.
When should acid exposure monitoring be repeated?
Re-monitor on change of chemistry, temperature, current density or throughput; on LEV or scrubber modification; on operator symptoms; following any spill or near-miss; and at a defined refresh interval — typically annually where exposure is borderline and every two years where it is well controlled.
Does acidmist.co.uk give medical advice?
No. acidmist.co.uk provides occupational hygiene information on managing acid, corrosive and reactive chemical exposure under COSHH and EH40. Medical assessment of operators is handled through site occupational health, not via this site.
How do I scope an acid mist programme for my site?
Start with the substances and tasks, define similar exposure groups, choose the substance-specific sampling method, run a full-shift personal sample plus targeted short-term work, and cross-check the result against the most recent LEV Regulation 9 report. The acidmist.co.uk contact route handles scoped enquiries.
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