HTM 03-01 Operating Theatre Ventilation Outside the NHS

HTM 03-01, Specialised ventilation for healthcare premises, was republished in 2021 and supersedes the 2007 edition.^[1] Part A covers design and validation and applies to new installations and major refurbishments, to be considered as the standard to be achieved. Part B covers operational management and performance verification and applies to every system, whatever its age. For healthcare facilities outside a hospital, such as clinics, the guidance asks for a risk assessment of the treatment delivered, the condition of the patients and the intensity of use, to decide how far it applies.

The law that sits underneath it is shorter. Every enclosed workplace must be ventilated by a sufficient quantity of fresh or purified air, with a warning of plant failure where health or safety needs one.^[2] Exposure to waste anaesthetic gases must be prevented or adequately controlled, with engineering controls ahead of protective equipment.^[3] A provider registered with the Care Quality Commission must keep premises suitable for the purpose they are used for.^[4]

None of those three says how many air changes a theatre needs. HTM 03-01 does, and that is why it is the document a private hospital, a day surgery unit or a veterinary referral centre is built to.

Paragraph 8.36 of HTM 03-01 gives the operating theatre's supply four jobs: dilute airborne microbial contamination, remove and dilute fumes and waste anaesthetic agents, stop air from less clean areas getting in, and control temperature and, where needed, humidity.^[1] The air-change rate covers the first two, the pressure differential and door protection the third, and the heat and moisture calculation the fourth. The design supply volume is whichever is greater.

The pressures are Appendix 3's nominal values. HTM 03-01 gives them to size the pressure relief dampers and transfer devices, and says the resulting pressures are not fixed provided the intended airflow rates and directions are achieved.

The figures are the design basis for a conventional theatre. HTM 03-01 also covers ultra-clean ventilation, which adds a canopy over the operating table and its own validation tests.

Appendix 3 of HTM 03-01 sets out the hierarchy of cleanliness for an operating suite: the lay-up preparation room and the theatre at the top, then the anaesthetic room and scrub, then recovery and the corridors, and the service corridor and utility room at the bottom.^[1] Each class sits at a lower nominal pressure than the one above it, so air leaving the theatre always moves towards somewhere less clean, never the other way.

Door protection is the part most often under-designed. HTM 03-01 calculates it on the basis that during use only one door, or one leaf of a double door, is open at a time, and that the worst case is usually the door between the theatre and the utility. The fewer doors lead off a theatre, the less traffic, and the simpler the air movement scheme it needs.

That is a planning decision as much as an engineering one. A suite drawn with five doors into the theatre needs more air, larger plant and a more complicated balance than one drawn with three, and the difference is set on the first layout, not in the plant room.

Paragraph 8.62 asks for a temperature adjustable within 18 to 25 degrees by the staff, and a system that can hold 20 degrees at summer design conditions and 22 degrees at winter design conditions.^[1] Where those are not appropriate, for example for children or patients with low body mass, the guidance asks for the design temperatures to be agreed in writing with the Ventilation Safety Group. Paragraph 8.69 sets the humidity range when the department is in use, and allows it to fall lower out of hours but not to rise above 70 per cent.

The control panel is specified in detail in paragraphs 8.67 and 8.68. It is wall mounted in the line of sight of the operating position, shows the temperature and humidity large enough to read from the table, has a means of adjusting the temperature set point, and carries a red light when the supply plant has failed or is set back, labelled so that nobody operates in that condition, and a green one when it is running at full speed.

Paragraphs 8.71 and 8.72 set that out, and the shared-unit exception comes with conditions: no more than two suites, each with its own temperature control, and an airflow measurement point in the branch to each so the air delivered to each suite can be checked.^[1] The guidance also asks for controls that are simple and robust, and that let staff keep the ventilation running until a procedure is finished, rather than complex automatic pressure control that only an off-site specialist can interpret.

For an independent hospital or a veterinary referral centre adding a second theatre, this is often the decision that sets the programme. A dedicated unit needs plant space, a riser and a route for supply and extract to a theatre that may be in the middle of the building, and finding all three is easier on a survey than after the layout is fixed.

Chapter 12 of HTM 03-01 separates validation from commissioning.^[1] Commissioning sets the system to work. Validation, in paragraphs 12.1 to 12.4, examines the complete installation and the fabric of the rooms it serves, measures the ventilation performance actually achieved, and confirms the system is fit for purpose and will need only routine maintenance to stay so. The validator would normally be the client's Authorising Engineer for ventilation, or someone of similar standing.

For the contractor that means building for the test from the first day: airtight rooms with sealed penetrations, accessible dampers and test points, ductwork and plant installed so the validator can reach and measure them, and a commissioning record the validator can rely on. A theatre that fails validation is rarely failed by its plant. It is failed by a leaking ceiling, a door that does not close or a damper nobody can reach.