Healthcare Room Pressure: Operating Theatres at +25 Pa, Isolation at −5 Pa (NHS HTM 03-01, 2021)

+25 Pa. That is the nominal pressure NHS HTM 03-01 (2021) gives an operating theatre in Appendix 3, against 0 Pa for the clean corridor. Get the direction wrong and every door opening draws corridor air across the sterile field. This guide covers room-by-room pressure targets, the operating suite cascade, PPVL isolation rooms, and design errors that show up in submittals.

Which edition? UK values are from NHS HTM 03-01 Part A (2021). US values are from ANSI/ASHRAE/ASHE Standard 170-2008 as printed in the FGI 2010 Guidelines (Part 6, Table 7-1) and, for anterooms, FGI 2010 Part 2. Later editions of Standard 170 and the released 2026 FGI Guidelines may differ. See the section on which edition applies.

Key pressure numbers at a glance

HTM 03-01 Appendix 3 note (a) says nominal room pressures are given to set up pressure-relief dampers, calculations and transfer device sizing; the resulting pressures are not immutable provided the desired airflow rates and directions are achieved.

What does healthcare room pressure control?

Room pressure is the differential, in pascals (Pa), between a room and adjacent spaces. Positive pressure rooms push air out and protect patients from air entering from outside. Negative pressure rooms draw air in and keep infectious aerosols from leaving. Neutral rooms run balanced supply and extract.

HTM 03-01 §8.18 describes two methods of door protection: closed door protection, where a pressure differential across a closed door makes any leakage flow from the clean to the less clean side, and open door protection, where the differential drops on opening and is replaced by an airflow through the doorway from clean to less clean. Closed-door leakage rates are in Appendix 4, and Appendix 4’s designers’ notes assume door gaps of 4 mm along the bottom, 3 mm at the top and sides, and 2 mm between double leaves. These are design assumptions used to size transfer devices, not a door specification.

How does the operating suite pressure cascade work?

Air cascades from cleanest to least clean. When the preparation room is used as an instrument lay-up room, HTM 03-01 §8.35 regards it as cleaner than the operating theatre and puts it at 10 Pa above the theatre, with a design air supply that results in not less than 22 ac/h, discharged into the theatre through a pressure stabiliser.

Appendix 3 sets the nominal pressures: operating theatre 25 Pa, scrub bay 25 Pa, sterile pack store 25 Pa, lay-up 35 Pa, anaesthetic room 15 Pa (design, with 10 Pa or more suitable when commissioning, per note c), recovery room 0 Pa, clean and general access corridors 0 Pa, and utility room -5 or 0 Pa.

Pressure stabilisers set the room differential when the doors are shut (closed-door protection). When a door is opened the stabilisers close and force the air through the doorway (open-door protection), per §8.19. Open-door airflow rates are in Appendix 5.

What pressure levels apply to isolation and critical care rooms?

Bone marrow transplant, haematology/oncology and organ and tissue transplant rooms are supply only, with at least 10 ac/h, +15 Pa to the corridor, and an EPA12 final filter (Table 3). Level 2 and 3 critical care individual rooms have at least 10 ac/h, +5 Pa to the general area in Table 3, and an EPA10 final filter. Appendix 2 lists critical care at +10 Pa; use the table that governs the room.

Category 2 isolation rooms (infectious patients) are extract only in the patient room and en-suite, at least 10 ac/h, -5 Pa to the general area (Table 4). The neonatal intensive care unit is 10 ac/h, neutral to the corridor (Table 5). Endoscopic procedure rooms are -5 Pa (Appendix 2), and the bronchoscopy and endoscopy rooms are ventilated to control staff exposure to airborne pathogens and waste anaesthetic agents (Table 2).

What is a PPVL room and how does dual-mode isolation work?

A PPVL (positive pressure ventilated lobby) room is the isolation room type described in HTM 03-01 Table 4, with details in Health Building Note 04-01 Supplement 1. It serves patients who may be infected, immunocompromised or both.

Table 4 gives the design parameters: supply in the lobby, flowing through a pressure stabiliser into the patient’s room and on, through a door undercut or transfer grille, to an extract in the en-suite. Bedroom air change at least 10 ac/h; lobby pressure +10 Pa to the corridor; bedroom pressure neutral; en-suite pressure negative. The table adds that the facility to fit EPA12 (BS EN 1822) filtration should be provided; the PPVL entry is Air quality SUP2 with the facility to fit EPA12.

How do pressure stabilisers and transfer grilles differ?

HTM 03-01 §8.19 describes pressure stabilisers as devices that set the room differential pressure when doors are shut and, on door opening, close to force air through the doorway. A transfer grille lets air pass between rooms through a fixed opening; where the pressure difference must be held, the transfer device is sized for the design leakage, including door gaps (Appendix 4 designers’ notes). If doors are fitted with cold smoke seals, the notes recommend factoring the design leakage into the size of the transfer grille or pressure stabiliser, otherwise air-gap whistles and doors held partially open by air pressure can result.

Why do these specific pascal values exist?

HTM 03-01 Appendix 3 sets a hierarchy of cleanliness (sterile, clean, transitional, dirty), with air flowing from the cleaner to the less clean class and the dilution criteria met. The pascal values are nominal: note (a) says the resulting pressures are not immutable provided the desired airflow rates and movement directions are achieved. The direction test is the compliance check, not a single pascal reading.

US vs UK: how do the two pressure systems compare?

ANSI/ASHRAE/ASHE Standard 170 defines protective environment (PE) rooms as positive and airborne infection isolation (AII) rooms as negative. In Table 7-1 of the 2008 edition (FGI 2010) a PE room is positive with 2 outdoor and 12 total ac/h, and an AII room is negative with 2 outdoor and 12 total ac/h. Rooms with reversible airflow for switching between PE and AII functions are not permitted (Table 7-1 note t). The UK PPVL room achieves the dual purpose with a positively pressurised lobby instead of a reversible room. The anteroom rules and the combination AII/PE room are in FGI airborne infection isolation room requirements.

What does operating theatre monitoring require?

HTM 03-01 §8.67 says all operating theatres and rooms where surgical interventions are carried out should have a control panel mounted on a wall with its screen centre at 1.5 m high, in the direct line of sight of staff at the normal operating position. Per §7.24, where room differential pressure gauges are required they are mounted directly adjacent to the entry door at 1.5 m above floor level, in the eyeline of staff entering the room.

Under §8.68 the panel includes readouts of 25 mm for air temperature and relative humidity, a red indicator light when the supply AHU fails, is switched off or is in set-back (legend: “Theatre not to be used in this condition”), and a green light when the supply AHU is operating at full speed (legend: “Conventional theatre mode”). Per §8.73, if the supply fails while the theatre is in use the extract should shut down to prevent reverse airflows, and an alarm should sound and show on the theatre control panel.

Which edition applies to your project?

In the UK, design to the current HTM 03-01 and the project’s NHS technical requirements. In the US, which edition applies depends on the state’s adoption: states adopt different editions of the FGI Guidelines (2014, 2018, 2022 and others) and through them different editions of Standard 170. The 2026 FGI Guidelines are the current release. The 2008 table values here are shown because that edition is printed in the FGI 2010 text. Confirm the adopted edition and state amendments with the authority having jurisdiction.

The part everyone gets almost right

1. Door seals too tight. HTM 03-01 sizes airflow with assumed gaps of 4 mm at the bottom and 3 mm at the top and sides. Cold smoke seals reduce door leakage when new and undamaged, so the design leakage must still be factored into the transfer grille or stabiliser size (Appendix 4 designers’ notes).

2. Shared AHU between operating suites. HTM 03-01 §8.71 says each conventional operating theatre suite should have its own dedicated AHU. §8.72 allows, in retrofit, with approval, sharing between not more than two operating suites, each with its own temperature control, but not between a conventional and a UCV suite.

3. Misreading the lay-up pressure. The lay-up room is 10 Pa above the theatre (§8.35); the Appendix 3 value of 35 Pa is a nominal figure against the theatre’s 25.

4. PPVL rooms without provision for EPA12 filtration. Table 4 calls for the facility to fit EPA12 filtration; reserve the space even if SUP2 filters are installed first.