The chemicals that keep Legionella down can hurt the people dosing them. A biocide that kills bacteria in a cooling tower is, by design, a poison. The acid you use to strip scale off a calorifier will burn skin and eyes on contact. So the job is never simply “control the bacteria” — it is control the bacteria without turning the plant room into the next hazard.

That tension sits inside every water treatment programme, and it is why two separate legal duties land on the same task. Your Legionella risk assessment decides whether you treat the water and with what. The Control of Substances Hazardous to Health Regulations — COSHH — govern how that substance is stored, handled, and cleared up. Neither answers the other’s question.

Two duties stacked on one task

The Legionella side is familiar to most duty holders: a risk assessment, a written control scheme, a competent responsible person, and records that prove the scheme is actually being run [1]. Where that scheme calls for chemical treatment — continuous biocide dosing on a cooling circuit, a periodic chlorination, pH or inhibitor dosing — the technical detail of how you treat sits in HSG274 [2].

COSHH runs alongside all of that. The moment a hazardous water treatment chemical comes through the gate, you need a COSHH assessment for it: what the product is, who could be exposed, how, and what stops them being harmed. A clean Legionella risk assessment does not discharge that duty, and a tidy COSHH file does not prove your water is safe. You need both, and they have to agree with each other — there is no sense dosing a biocide your staff cannot store or handle safely. When you document the chemical controls, cross-reference them in the risk assessment report so the two records line up rather than contradict.

Where it bites in practice

Continuous dosing on a cooling system. Cooling towers are among the systems most likely to spread Legionella [3], and most are held in check by oxidising and non-oxidising biocides plus scale and corrosion inhibitors, dosed automatically. The COSHH exposure here is low but constant — changing drums, servicing dosing pumps, taking samples. Control it with engineering first: sealed dosing lines, bunded drums, automatic make-up, so a person rarely meets neat product at all. PPE is the backstop, not the plan.

One-off disinfection. After major works, a new installation, or a positive sample, the scheme may call for a high-strength chlorination using sodium hypochlorite. This tends to be the most dangerous chemical day on the site: concentrated product, hand-decanting, often in a cramped, poorly ventilated plant room. It is also where the classic mistake happens — bringing an acid descaler anywhere near the hypochlorite.

Daily handling by non-specialists. Spa pools fall under their own guidance, HSG282 [4], and are usually dosed by leisure or housekeeping staff rather than water treatment specialists. People whose main job is something else, handling disinfectant and pH correction every shift, are exactly who COSHH training is written for. The risk here is routine familiarity, not one dramatic event.

What good chemical cover looks like on site

If you audit one thing this quarter, audit the chemical store against a short, recordable list. Group it the way you would actually walk it:

Documents

  • Hold a current safety data sheet for every product on site, and confirm it matches the drum in front of you.
  • Keep a COSHH assessment for each substance that reflects how you use it here — not the supplier’s generic sheet filed unread.

Storage

  • Segregate incompatible chemicals: keep acid descalers away from hypochlorite and other oxidisers, because mixing them can release toxic chlorine gas.
  • Stand drums on secondary containment so a split container cannot reach a drain.
  • Keep the store ventilated, locked, and clear of water inlets and occupied spaces.

Handling

  • Specify the PPE on the assessment and check it is actually present — gloves, eye protection, and respiratory protection where the safety data sheet or task demands it.
  • Provide a spill kit sized for your largest container, with a written procedure people have read before they need it.

People and waste

  • Train everyone who handles or could be exposed to the chemicals, and record that they were trained.
  • Agree a disposal route for empty containers and spent solution that meets environmental rules — not the nearest gully.

The contractor question most sites get wrong

If a water treatment company does the dosing, plenty of duty holders assume the COSHH duty left with them. It did not. The contractor must control the risks their own work creates and bring their own assessments and trained people. But you still own the site, the storage, and everyone else who could be exposed — cleaners, contractors on other trades, occupants nearby. Your part is to be competent enough to ask for those assessments, read them against your building, and challenge the ones that are obviously a generic template with someone else’s address on top. Judging a provider properly is its own skill; the LCA Code of Conduct is a useful yardstick for what a credible one should already be doing.

Read this before you dose anything

This is general guidance, not a COSHH assessment and not a Legionella risk assessment. The hazards, exposure limits, PPE and storage rules change with the exact product and its concentration, so let the supplier’s safety data sheet and a competent assessor set the specifics for your site. Healthcare and other higher-risk premises carry extra requirements on top of the basics here. Where a chemical duty is unclear, check it against current HSE COSHH guidance rather than this page.

Start in the chemical store

Walk the store this week with a clipboard. List every product, pull each safety data sheet, and test three things against each container: is there a current COSHH assessment that matches how you use it, is it stored away from anything it should never touch, and does the PPE named on the assessment actually hang by the door? Anything that fails those three is your first fix. If the dosing is contracted out, ask the provider for their COSHH assessments and read them as if they were your own — because, wherever your people are exposed, they are.

FAQ

Does my Legionella risk assessment cover COSHH as well?

No. The risk assessment decides what treatment your water needs; COSHH governs how the resulting chemicals are stored, handled and disposed of. They are separate duties that have to be kept consistent, but one does not satisfy the other.

Our contractor does all the dosing — do we still need our own COSHH assessment?

You need to be satisfied that a suitable assessment exists and that exposure on your site is controlled, including for your own staff and other occupants. The contractor assesses their work; you remain responsible for the site and for checking their arrangements are real, not just filed.

Why can’t the descaler be stored next to the chlorine?

Acid descalers and chlorine-based products such as sodium hypochlorite are chemically incompatible. If they leak together or are mixed, the reaction can release toxic chlorine gas, so they are kept physically apart with separate containment.

Sources

[1] HSE, “Legionnaires’ disease. The control of legionella bacteria in water systems - Approved Code of Practice and guidance (L8)”. https://www.hse.gov.uk/pubns/books/l8.htm [2] HSE, “Legionnaires’ disease: Technical guidance (HSG274)”. https://www.hse.gov.uk/pubns/books/hsg274.htm [3] HSE, “Systems most likely to create legionella risk”. https://www.hse.gov.uk/legionnaires/risk-systems.htm [4] HSE, “Control of legionella and other infectious agents in spa-pool systems (HSG282)”. https://www.hse.gov.uk/pubns/books/hsg282.htm