Nitric Acid Fumes
Acids & Corrosives · Acids & Corrosives overview
Nitric acid fumes are the mixed airborne release of nitric acid mist, nitrogen dioxide (NO2) and other nitrogen oxides generated whenever nitric acid attacks a metal surface or is heated, agitated or jetted. They are typical of stainless steel and aluminium passivation, mixed-acid pickling, etching, electropolishing, precious-metal refining and chemical reactor work. Both nitric acid and nitrogen dioxide carry UK Workplace Exposure Limits in HSE EH40 and are controlled under the COSHH Regulations 2002.
Where nitric acid fumes are generated
Nitric acid is a strong oxidising acid. When it reacts with metal — for example, stainless steel during passivation, copper during refining, or aluminium during chemical milling — the reaction liberates a brown-tinged plume of nitrogen dioxide together with a fine nitric acid mist. Heating, splashing or vigorous gassing greatly increases the release rate.
In UK industry the dominant sources are stainless steel and aluminium passivation lines, mixed-acid pickling (nitric/HF or nitric/sulphuric), nitric etching of electronic and aerospace components, precious-metal refining, and chemical processing involving nitration or nitric oxidation.
- Stainless steel passivation tanks and spray cabinets.
- Aluminium chemical milling and bright-dip lines.
- Mixed-acid pickling — nitric/HF for stainless, nitric/sulphuric for copper alloys.
- Precious-metal refining — gold, silver and PGM dissolution.
- Electropolishing of stainless steel medical, food and aerospace components.
- Chemical reactors involved in nitration and nitric oxidation chemistry.
- Laboratory acid digestion and nitric-based analytical chemistry.
Why nitric acid fume exposure matters
Nitric acid fumes contain two distinct hazards. The nitric acid mist itself is corrosive to skin, eyes and the upper respiratory tract. The nitrogen dioxide co-released during metal attack is far more insidious — NO2 is poorly water-soluble, penetrates deep into the lower lung, and can produce delayed pulmonary oedema several hours after a short, intense exposure. Both have EH40 Workplace Exposure Limits, including short-term limits set specifically to control acute peaks.
Because the brown plume from nitric reactions is often striking, operators may overestimate their ability to see and avoid the release. In practice the most damaging exposures often occur after the visible plume has dispersed but NO2 has built up in poorly ventilated tank-side or pit areas.
Monitoring nitric acid fume exposure
Nitric acid mist and NO2 are sampled separately and read against their own EH40 limits. Nitric acid mist is collected on treated filters or impingers with ion-chromatographic analysis. NO2 is captured on sorbent tubes or, increasingly, with diffusive samplers and read against both the 8-hour TWA and the 15-minute short-term limit. Personal samples on the most exposed operator give the COSHH judgement; static or direct-reading NO2 instruments support tank-side and pit-area diagnostics.
Sampling strategy follows BS EN 689, with attention to the short-term limit during loading, dragout, basket transfer and any maintenance that exposes a hot, reacting acid surface.
Controlling nitric acid fumes
Engineering control follows the COSHH hierarchy. Substitution to citric-acid passivation is well-established for many stainless components and eliminates both the nitric mist and the NO2 release. Where nitric remains essential, the controls are tank covers and minimised free surface, lip or push-pull LEV sized to the bath, alkaline-scrubbed discharge to remove NO2, and acid-resistant ducting and fans. Pit and tank-side ventilation must be designed for the dense NO2 plume.
Procedural controls — limit reaction temperature, control acid concentration, plan loading away from the operator, and use RPE with appropriate gas filters or supplied air for cleaning and maintenance — round out the control package.
When to review nitric acid fume exposure
Review the COSHH assessment on a change of acid strength, mix or temperature; a change to substrate or component geometry; modification of LEV; symptoms of cough, breathlessness or delayed shortness of breath; an enforcement or insurer visit; and at a defined two- to three-year refresh. LEV Thorough Examination and Test is required at least every 14 months under COSHH Reg 9.
Frequently asked questions
Why is the brown plume from nitric reactions a concern?
The brown colour is nitrogen dioxide. NO2 is poorly water-soluble, penetrates deep into the lung and can cause delayed pulmonary oedema hours after a short exposure. It is sampled and limited separately from nitric acid mist.
Can citric acid replace nitric passivation?
For many stainless steel components in food, medical and aerospace use, yes — citric passivation is recognised in ASTM A967 and eliminates the nitric and NO2 hazard. Suitability depends on alloy, finish and customer specification.
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