Reactive Chemicals
Acids & Corrosives · Acids & Corrosives overview
Reactive chemicals are substances that release energy and airborne products when they meet incompatible materials, heat, water or air. In UK industry they are encountered in acid–base neutralisation, oxidiser handling, metal-acid reactions, peroxide chemistry, polymerisation and specialty synthesis. Reactive chemical exposure is controlled under the Control of Substances Hazardous to Health (COSHH) Regulations 2002 and, where relevant, under the Dangerous Substances and Explosive Atmospheres Regulations 2002.
What makes a chemical reactive in workplace terms
From an occupational hygiene perspective, a reactive chemical is one whose principal hazard is not just toxicity but the airborne products of an intended or accidental reaction. Strong oxidisers (nitric acid, hypochlorite, peroxide), strong reducers (hydrazine, sulphide solutions), acid-metal couples that liberate hydrogen, peroxide-forming organics, and condensation chemistries that liberate HCl, HF or ammonia all sit in this category.
Reactive releases are often short, intense and operator-side — a tank top-up that meets a residue, a maintenance error that mixes incompatible drainings, an acid spill onto a metal floor. The short-term EH40 limits exist specifically to control this kind of peak.
Common reactive workplace processes
Reactive chemistry is present in many ordinary UK workplaces, not only in specialist synthesis houses.
- Acid–metal cleaning that liberates hydrogen and acid mist.
- Nitric passivation of stainless steel that liberates NO2.
- Hypochlorite and acid cleaning sequences that risk chlorine release.
- Mixed-acid pickling — nitric/HF and nitric/sulphuric.
- Peroxide-based brightening and disinfection.
- Neutralisation of acid or caustic spills.
- Polymerisation, condensation and reactive coating chemistry.
Why reactive chemical exposure matters
Reactive releases are short, intense and often unpredictable. The COSHH judgement therefore has to consider not only the steady-state airborne concentration but the foreseeable peaks generated by intended chemistry, maintenance and reasonably foreseeable error. Several reactive products — NO2, chlorine, HCl, HF, ammonia — have EH40 short-term limits set explicitly to control these peaks.
Reactive chemistry also drives some of the highest-consequence acute incidents in UK industry. Mixing hypochlorite cleaner with an acid descaler in a confined plant room, dosing acid into a tank still containing sulphide, or neutralising a caustic spill with an inappropriate acid can produce airborne concentrations many times the WEL within seconds.
Monitoring reactive chemical exposure
Personal sampling for reactive chemicals targets the relevant emission product — chlorine, HCl, NO2, HF, ammonia or peroxide — using a validated MDHS-equivalent method. The strategy is built around BS EN 689 but with explicit attention to short-term limits and to the tasks where reactive peaks are most likely. Direct-reading multi-gas instruments are valuable for monitoring known reactive zones — neutralisation pits, dosing skids, chlorine storage rooms — and for incident response.
Reactive risk assessment also needs a written incompatibility review of the actual chemicals on site, the order in which they are dosed, and the maintenance and cleaning procedures that bring them into contact.
Controlling reactive chemistry
The COSHH hierarchy applies, but for reactive substances the emphasis is on segregation and procedural control alongside engineering control. Substitute or eliminate incompatible pairs where possible; segregate storage and bunding; sequence dosing so reactive products are scrubbed before discharge; enclose and lid reactive plant; LEV with appropriately resistant materials and scrubbed discharge; and provide RPE rated for the worst credible release for tank cleaning, breakdown response and confined-space entry.
Permit-to-work and lock-out controls protect against the maintenance scenarios that drive most reactive incidents.
When to review reactive chemical risk
Review the assessment on any change of chemistry, dosing sequence or storage layout; after near-misses, spills or unintended reactions; after enforcement or insurer audit; on operator symptoms; and at a defined two- to three-year refresh. LEV serving reactive plant requires Thorough Examination and Test at least every 14 months under COSHH Reg 9.
Frequently asked questions
Is reactive chemistry only a chemical-industry issue?
No. Cleaning, water treatment, food, brewing, metal finishing and laboratory work all involve reactive chemistries — neutralisation, hypochlorite/acid sequences and peroxide dosing are typical examples.
How is a reactive peak measured?
By personal sampling against the relevant short-term EH40 limit, supported by direct-reading instruments during the activities where the peak is most likely (loading, dosing, cleaning, breakdown response).
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