HSG274 Part 1 Update: What It Means for Your Legionella Risk Control

In March 2024, the Health and Safety Executive published the first significant revision to HSG274 Part 1 since the guidance was originally issued in 2014. If you operate or have responsibility for an evaporative cooling system, whether that's a cooling tower, an evaporative condenser or a hybrid dry/wet system, this update affects you, regardless of whether you're running a full-scale legionella management programme or simply keeping on top of a single unit.

It's worth saying at the outset that this isn't a dramatic overhaul, and the law itself hasn't changed. Most of what's different is about how you test, inspect and protect people, rather than the underlying legal duties. Think of it as a compliance checkpoint: an opportunity to confirm your current practices still hold up against what the HSE now expects to see.

It's also worth knowing that the HSE's own bulletin announcing the update is selective. It flags two points, DPD No 1 testing and the effect of pH on biocide efficacy, and leaves out the rest, including the inspection frequency and RPE changes below. If you've only seen the bulletin, you've seen a small part of what actually changed.

What Actually Changed in the Revision

The headline point is that only Part 1, covering evaporative cooling systems, was formally reissued. Parts 2 (hot and cold water systems) and 3 (other risk systems) remain as previously published as documents. That said, some of what's changed reflects testing and biocide chemistry that isn't unique to cooling towers, so if you also manage Part 2 or 3 systems using the same halogen-based dosing, the DPD No 1 point below is worth reading regardless of which document technically governs your system.

Within Part 1, three changes carry the most day-to-day weight for duty holders and their water treatment providers:

• A pH-adjustment requirement for DPD No 1 testing. The guidance now makes explicit that DPD No 1 results, used to measure the residual level of chlorine and bromine-based biocides, must be pH-adjusted. This is because the active, biocidal forms of these halogens become progressively less effective as system pH rises.

• A more prescriptive inspection schedule, including daily checks. A new table sets out specific inspection tasks against specific frequencies, including two tasks that are now explicitly daily.

• A new requirement for appropriate respiratory protective equipment (RPE). Anyone carrying out inspection, cleaning, disinfection, sampling or monitoring where aerosol or mist could be generated now has a clear RPE expectation, with direct reference to HSE guidance HSG53.
  

Alongside these, the guidance also adds a new expectation around photographic records to support cleaning decisions, clarifies record-keeping wording for smaller organisations, adds detail on wet/dry hybrid systems, and introduces new material on rapid testing methods such as qPCR. But the three points above are the ones most likely to require an actual change to what happens on site.

DPD No 1 Testing: A Change Many Water Treatment Providers Still Miss

This is arguably the most consequential change in the update, and one that's easy to overlook precisely because it sits inside a technical info box rather than a headline clause.

Chlorine and bromine-based biocides work through their active acid forms, hypochlorous and hypobromous acid. As cooling water pH rises, these active acids increasingly convert into their far less biocidal ionic forms, hypochlorite and hypobromite. A DPD No 1 test measures total residual halogen, but it doesn't distinguish between the active and inactive forms. The result: a system can show a biocide reading that looks perfectly healthy on paper while the water is, in practice, under-protected because pH has quietly eroded the biocide's real effectiveness.

The 2024 update makes clear that DPD No 1 results should be pH-adjusted to account for this. In practice, that means:

• Confirming with your water treatment provider that pH-adjustment is actually built into how DPD No 1 readings are interpreted, not just recorded.

• Checking system pH is being measured alongside biocide residual, not treated as a separate, lower-priority parameter.

• Reviewing recent test history for any pattern of rising pH, which may mean a biocide reserve that has looked stable is in fact becoming less effective.
  

If your current water treatment reporting doesn't reference pH-adjustment when presenting DPD No 1 results, that's worth raising directly, since it's one of the areas where general practice hasn't yet caught up with the updated guidance.

More Prescriptive Inspections, Including Daily Checks

The new inspection table sets out a clearer frequency structure than the 2014 edition offered:

• Daily: confirming the system is operating as described in the operation and maintenance manual, and a visual check of the cleanliness of the water in the system.

• Weekly: assessing the physical condition of the system, and dip slide analysis or equivalent testing.

• Every one to three months, based on risk: central control function and conductivity sensor calibration, and, where accessible, checking uniformity of water distribution, spray condition, drift eliminators, pack and base tank.

• As determined by risk assessment: cleaning and disinfection.
  

The daily checks are the detail most likely to catch sites out. Where a system was previously checked on a less structured basis, a genuinely daily visual check of water cleanliness and system operation is a real, measurable change to what a responsible person or their contractor needs to be doing, not simply a documentation update.

Alongside this, the guidance adds a new expectation around photographic records: photographs taken before and after cleaning, and during pre-clean inspections, are now recommended to support decisions on cleaning frequency and to evidence that cleaning has actually been effective. This sits alongside the inspection table rather than replacing it, so photographic evidence should complement, not substitute for, the written inspection log.

New RPE Requirements for Inspection, Cleaning and Sampling

The update also strengthens the guidance around protecting the people carrying out this work. Appropriate respiratory protective equipment is now explicitly required wherever inspection, cleaning or disinfection generates aerosol or mist, such as flushing with high volumes of water or disinfectant, with direct reference to HSG53 for selecting the right RPE.

This extends to sampling and monitoring too. New sections set out that a risk assessment should be carried out before any sampling activity, that staff prone to increased risk shouldn't be involved in sampling or outbreak investigations, and that RPE should specifically be worn when sampling as part of an outbreak investigation, where a system has previously yielded high legionella results, or where a sample point is used infrequently and considered higher risk.

For duty holders, the practical question is whether your contractor's method statements and your own in-house procedures already reflect this, or whether RPE has, until now, been treated as good practice rather than an explicit expectation tied to specific circumstances.

What This Means for Your Legionella Risk Control Programme

None of this changes the underlying duty. A documented, defensible control programme has always been what HSE inspectors, and in the event of an incident, courts measure duty holders against. What this update does is tighten the evidential and technical bar across testing, inspection and protective measures, all at once.

That's precisely where an independent review earns its keep. A fresh set of eyes on your current controls, checked directly against the 2024 wording rather than assumptions carried over from 2014, is the most reliable way to confirm your existing programme still meets the standard, before an inspection or an incident tests it for you.

If it's been a while since your legionella control management approach was independently reviewed, this update is a natural prompt to do so.

Reviewing Your Current Water Safety Plan Against the Update

A practical starting point is to work through your existing documentation and provider reporting against the revised guidance:

• Confirm your water treatment provider's DPD No 1 reporting explicitly accounts for pH-adjustment, not just raw biocide readings.

• Cross-check your current inspection schedule against the new daily, weekly and one-to-three-monthly frequencies, rather than the schedule you may have inherited from an older risk assessment.

• Review method statements and in-house procedures to confirm RPE requirements for inspection, cleaning, disinfection and sampling are explicit rather than assumed.

• Confirm photographic evidence is being captured at the right points, before and after cleaning, and stored somewhere it can be matched to the relevant log entry.
  

More broadly, this is a good moment to review four documents specifically: your legionella risk assessment, your written control scheme, your safe systems of work, and any vendor or contractor documentation, to confirm each one reflects the 2024 wording rather than the 2014 original.

This is also a sensible moment to revisit your wider water management documentation, since these expectations rarely sit in isolation from the rest of your water safety plan.

Common Gaps ISHEM Sees in Cooling System Documentation

Working across a range of sites, a handful of gaps come up repeatedly when we review existing cooling system documentation:

• DPD No 1 results recorded without any reference to pH, leaving no way to confirm the biocide reserve is actually effective rather than just present.

• Inspection records that exist but aren't dated or attributed to a named individual, which weakens their value as evidence if they're ever scrutinised.

• RPE requirements referenced generically in a risk assessment but not tied to the specific inspection, cleaning or sampling tasks where they now apply.

• Missing before-and-after photographic evidence for remedial cleaning, even where the cleaning itself was carried out competently.
  

Staying Ahead of the Next HSG274 Review

Updates like this one tend to formalise practices that well-run sites, and well-run water treatment providers, were already moving toward. The real exposure sits with organisations relying on a control programme, and a water treatment provider, that predates 2024 without having checked either against the current wording, particularly around how biocide test results are actually interpreted.

If you'd like an independent review of your cooling system controls against the updated HSG274 Part 1, including how your DPD No 1 testing is being reported, or want to discuss training for your Responsible Persons, get in touch with ISHEM.