ASHRAE Journal - February 2014 - 19

TECHNICAL FEATURE
FEA

by Lidwell and his colleagues4,5 along with many other
studies6,7 have indicated a strong connection between
contamination in the air during surgeries and SSI rates.
Clinical trials carried out in Britain, Europe, and the
United States have confirmed that between 80% and
90% of bacterial contaminants found in the wound after
surgery come from colony forming units (cfu) present in
the air of the operating theatre.8 With respect to bacteria
transmitted to the surgical site through the air, squames
(or skin scales) are the primary source of transmission.9
Approximately 1.15 × 106 to 0.9 × 108 squames are generated in a typical two- to four-hour surgical procedure.6
Viral and fungal contamination also can be present in
these skin scales.
An interesting parallel can be drawn between the
control of these airborne contaminants in operating
rooms (ORs) and semiconductor manufacturing cleanroom environments. In each case, the design principle
of using unidirectional, controlled (often referred to
as laminar) airflow to contain and remove airborne
particles, particularly in the area targeted as the sterile
field, is generally accepted as the most effective method
of preventing airborne contaminants from landing in
that sterile field. However, cleanroom technology has
proven effective in virtually eliminating human particle
contamination, vastly improving product yields in the
semiconductor manufacturing industry.10
This article identifies the differences in air delivery
between semi-conductor manufacturing cleanroom and
operating room designs, and discusses how employing
the single-large diffuser (SLD) configuration used in
semi-conductor manufacturing cleanrooms can significantly reduce the incidence of airborne contaminants
reaching the patient in an operating room (OR).
There are three basic design concepts evaluated for
ceiling-mounted air delivery in a surgical suite: air curtain (AC), multi-diffuser array (MDA) and single-large
diffuser (SLD).
The AC design uses a laminar flow diffuser array over
the surgical table with a high velocity slot diffuser at the
perimeter of the sterile field. The concept is to create
a barrier of air from the slot diffusers, protecting the
sterile field from contaminants outside of that space.
The MDA uses multiple laminar diffuser panels that are
set into an array in the ceiling. According to ASHRAE
Standard 170-2013, Ventilation of Health Care Facilities,
minimum guidelines, the supply air array must extend a
minimum of 12 in. (305 mm) beyond the footprint of the

operating table, but up to 30% of the area encompassed
can be used for non-air delivery devices (booms, light
troffers, etc.). Light booms in the air field create blockages in the air, with turbulent zones beneath the lights.11
Similarly, gaps between diffusers allowed by Standard
170-2013 create turbulent zones beneath the areas where
airflow is not being delivered. The SLD design supplies
air from a single, large diffuser that concentrates the air
delivery in a controlled air field over the surgical table
and reduces these turbulent zones.

Semiconductor Manufacturing Cleanroom Design
Cleanroom and process equipment design has ultimately minimized the human impact through gowning,
careful control of filtered airflow directed to protect the
product from the human, and product isolation during manufacturing. The trend from 1985 to 2000 shows
that the cleanroom and cleanroom workers have been
virtually eliminated as sources of contamination, declining from 50% to < 5% of the wafer level contamination.
These results in product improvement are despite the
decreased circuit line widths by a factor of 85 and the
accompanying 17 fold allowable defect density reduction over the same 25-year period (from mid-1970s to
~ 2000.)10
One of the first large "state of the art" semiconductor
manufacturing cleanrooms was built in the late 1980s in
Santa Clara, Calif. It was a radical departure from any
cleanroom built previously in the United States. It was
an experiment - building a developmental fab incorporating all best known practices in order to evaluate
their impact on yield. The project philosophy could be
summarized as: "Hey, let's do this right from the bottom
up."12 Ultimately, it met yield expectation, achieving better than Class 1 (ISO M1.5) cleanliness by incorporating
more efficient HEPA filters (99.9995% efficient), filtering
outside air brought into the cleanroom with HEPA and
activated carbon filters, and using polytetrafluoroethylene cloth gowns that fully encapsulated the workers.
The gowns included helmets with battery-powered fans
to draw air in through the helmet face and discharge it
below the work surface after passing it through a small
HEPA filter. Other features were incorporated to reduce
particulate concentration including static dissipative
surfaces, perforated work surfaces, mouth rinsing, and
pre-gowning protocols to prevent gown and gowning
room contamination. There were many additional air
delivery process improvements as well that are now
FEBRUARY 2014

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ASHRAE Journal - February 2014

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