Technical note · Healthcare mechanical services

Operating theatre ventilation design in South Africa

Outdoor air volume, pressure cascade and airflow pattern for a South
African operating theatre are set by two documents that a designer must read together: the
mandatory building regulation, and the gazetted health facility norms.

Two documents govern, and the mandatory one is stricter than most references

A theatre in a South African hospital sits under SANS 10400-O:2025, which is part
of the National Building Regulations and therefore mandatory, and under the IUSS Building
Engineering Services health facility guide, gazetted on 17 February 2016. They cover different
quantities and do not overlap much: SANS sets the outdoor air volume, IUSS sets the pressure
regime and the airflow pattern.

Where an international standard also covers a quantity, the more onerous requirement governs.
On outdoor air for a major theatre, the more onerous requirement is the South African one, by a
wide margin, and that is the single fact most often missed at concept stage.

Applicability as at August 2026. The highlighted row is the mandatory document.
This note does not cover filtration — see the scope note at the foot of the page.

SANS 10400-O:2025 Table 2 sets the outdoor air floor for a major theatre

SANS 10400-O:2025 Table 2, under Surgery and critical care, gives major operating
rooms in the ISO 5 to 7 range a minimum outdoor air rate of 10,00 l/s per person plus 35,00
l/s per m² of floor area. The rate is applied as
q_t = (n × q_p + A × q_b) × k_d,
with the diversity factor k_d at 1 for this room type.

The 35,00 l/s per m² floor-area rate is what makes the number large, and it applies to
the major category only. Every other theatre category in the same table carries 15,00 l/s per
m² — less than half. Categorising the room correctly therefore governs the answer more
than any other single decision on the sheet.

SANS basis: SANS 10400-O:2025 Table 2, row “Operating rooms (Major – ISO
5-7)”, Surgery and critical care. IUSS basis: IUSS Building Engineering Services, gazetted
17 February 2016, Operating theatre ventilation design, clause 1(g), p.27. ASHRAE basis: ASHRAE
170-2021 Table 7-1, “Operating room”, cited as two individual figures. Computed with LCE’s gated
design criteria engine. One geometry, as an illustration — change the area, the height or
the staff count and the answer moves.

The gap between 700 and 1 540 l/s is an air handling unit, a plant room and a riser. That is
why the SANS Table 2 check belongs at concept stage, while the shell can still absorb the plant:
once the building envelope is fixed, the space to correct it is no longer available at a sensible
cost.

SANS 10400-O:2025 Table 2, Surgery and critical care. The highlighted row is the
major theatre category and carries more than twice the floor-area rate of the other three. SANS
10400-O:2025 specifies no minimum exhaust rate for any of these rooms. Containment and sepsis
theatres are the exception on direction: they run negative, and the same table prohibits
recirculation in them.

The pressure cascade is 10 to 15 Pa, and the direction is a design decision

IUSS Building Engineering Services requires a pressure differential of 10 to
15 Pa between the theatre and adjacent rooms with all doors closed, and requires the theatre
to be positive relative to technical spaces at all times. Between those two clauses sits a
third: a theatre may be held positive or negative to the adjacent rooms depending on its
contamination-control requirement, so direction is decided per room, not assumed.

Because neither SANS 10400-O:2025 nor IUSS sets a minimum exhaust rate for a theatre, the
extract volume is a design output rather than a table lookup. It is derived from the supply
offset needed to hold the differential against the room’s actual leakage — door undercuts,
pass-through hatches, penetrations — which is why the same theatre in two different
buildings does not get the same extract fan.

IUSS also requires constant-volume systems for this reason: the differential has to hold, and
a variable-volume system that modulates supply without tracking extract will not hold it.

IUSS Building Engineering Services, gazetted 17 February 2016, Operating theatre
ventilation design, section 1 General requirements, p.27. Clause 1(a) also requires the
contamination-control concept to be developed to ISO 14644-4.

What makes a theatre unidirectional is a velocity, not an air change rate

A unidirectional airflow theatre — the ultraclean or laminar flow type
— is defined by the downward air stream over the surgical field, and IUSS specifies it as a
velocity, not a volume. The requirement is a mean air velocity of 0,35 to 0,45 m/s measured
below the UDAF screen at working height, with those mean velocities differing by no more
than ±10 % across the screen.

That is why a theatre cannot be judged unidirectional from its air change rate. Two rooms at
the same air changes per hour behave completely differently depending on how the supply is
arranged over the table, and the uniformity tolerance is what separates a working protected zone
from a screen that merely looks like one.

IUSS Building Engineering Services, gazetted 17 February 2016, Operating theatre
ventilation design, section 2 Unidirectional airflow or ultraclean theatres, p.28. The set-back
requirement exists to stop contaminants settling underneath the screen when the theatre is idle.
Clause 1(p), p.28, additionally prohibits internal ducting insulation.

Arithmetic on two published figures: the standard screen size from IUSS clause
2(j), p.28, and the velocity band from clause 2(g), p.28, against the SANS 10400-O:2025 Table 2
outdoor air rate for the worked 42 m², 7-person room. IUSS clause 2(j) notes the required
screen size may vary with the layout and function of the theatre, so treat 2 400 × 2 400 mm
as the starting point it is described as.

Temperature and humidity are gazetted as ranges, not setpoints

For unidirectional airflow theatres IUSS gives 18 °C to 24 °C and
45 % to 60 % relative humidity. For theatres generally it gives the same temperature range
with a minimum relative humidity of 45 %. Both are ranges. Neither is a setpoint, and neither
document specifies one.

The operating band inside that range is agreed with the clinical team for the procedures the
theatre is built for. Surgeons working under lights and gowns and a patient under anaesthetic
have opposite thermal requirements, and the resolution of that is a clinical decision the
engineer implements rather than an engineering decision the clinician accepts.

ASHRAE 170-2021 gives a wider band on both quantities than the gazetted South African norms
do. Where both apply, the narrower IUSS range is the one to design to, because a system that
holds 18 °C to 24 °C and 45 % to 60 % satisfies the wider band automatically.

IUSS Building Engineering Services, gazetted 17 February 2016, Operating theatre
ventilation design, clause 2(b), p.28 for unidirectional airflow theatres and clause 1(b), p.27
for theatres generally.

Common questions

What outdoor air rate does SANS 10400-O require for a major operating theatre?

SANS 10400-O:2025 Table 2 sets a minimum of 10,00 l/s per person plus 35,00 l/s per m² of
floor area for operating rooms in the major category, ISO 5 to 7. On a 42 m² theatre with
seven people that is 1 540 l/s. Minor theatres, containment and sepsis theatres, and the
ancillary areas of a theatre suite all carry 15,00 l/s per m² instead, on the same 10,00 l/s
per person, which on the same room gives 700 l/s.

How many air changes does an operating theatre need in South Africa?

Air changes are the wrong starting point in South Africa. IUSS gives 5 to 7 air changes per
hour of fresh air for occupancy, and ASHRAE 170-2021 gives 4 outdoor and 20 total for an
operating room. On a major theatre the SANS 10400-O:2025 Table 2 outdoor air rate is larger than
all of them: on a 42 m², 3,0 m high room it is 1 540 l/s against 245 l/s at 7 air changes
per hour. Size on the SANS rate first.

What pressure should an operating theatre run at?

IUSS Building Engineering Services requires 10 to 15 Pa between the theatre and adjacent rooms
with all doors closed, and requires the theatre to be positive relative to technical spaces at
all times. Direction relative to the adjacent rooms is a design decision: a theatre may be
positive or negative depending on its contamination-control requirement, and SANS 10400-O:2025
Table 2 specifically calls for containment and sepsis theatres to be negative.

How is the extract rate for an operating theatre calculated?

It is derived, not looked up. Neither SANS 10400-O:2025 nor IUSS specifies a minimum exhaust
rate for a theatre, so the extract volume is whatever leaves the supply volume enough offset to
hold the 10 to 15 Pa differential against the room’s actual leakage. That makes it a function of
the door undercuts, hatches and penetrations in the specific room, which is why two identical
theatres in different buildings do not get the same extract fan.

What air velocity does a laminar flow theatre need?

IUSS Building Engineering Services requires a mean air velocity of 0,35 to 0,45 m/s measured
below the unidirectional airflow screen at working height, with the mean velocities differing by
no more than ±10 %. Air is delivered downward from the ceiling to four low-level exhaust
outlets near the corners. A standard screen is 2 400 × 2 400 mm, which at that velocity band
moves 2 016 to 2 592 l/s.

What temperature and humidity should an operating theatre run at?

IUSS Building Engineering Services gives 18 °C to 24 °C and 45 % to 60 % relative
humidity for unidirectional airflow theatres, and the same temperature range with a minimum
relative humidity of 45 % for theatres generally. Both are ranges rather than setpoints. The
operating band inside the range is agreed with the clinical team for the procedures the theatre
is built for.

Which standard governs theatre ventilation in South Africa?

SANS 10400-O:2025 is the mandatory one, as part of the National Building Regulations, and it
sets the outdoor air floor. IUSS Building Engineering Services, gazetted on 17 February 2016,
sets the pressure differential, the airflow pattern, the ISO cleanliness class and the comfort
band. International standards such as ASHRAE 170-2021 are references rather than law here, and
where they cover the same quantity the more onerous requirement governs.

Who designs operating theatre ventilation?

The consulting engineer. The load calculation, the outdoor air and total supply volumes, the
pressure cascade, the plant selection and the specification are the engineer’s deliverable, and
they are complete before the tender goes out. Lombard Consulting Engineers designs the system and
writes the specification; a contractor prices and installs it against that specification.