Microbiological contamination of air in the operating theatre is generally considered to be the most common risk factor for surgical site infections in clean surgery. The link between postoperative infection and operating theatre air quality has been well established.

Evaluation of the quality of air in operating theatres should be performed routinely by microbiological sampling and particle counting.

This is the reason for the Dept. of Health’s directive in their National Core Standards that state/district and provincial hospitals conduct particle counts in their operating theatres twice annually (every 6 months) and the results are kept for auditing purposes.

What is particle counting and how does it reduce the risk of microbiological infection?

Particle counting is the method of sampling the airborne particle concentrations in a controlled environment (operating theatre) by means of a laser light scattering instrument and comparing the results to an international standards system.

This enables a classification of the environment (theatre) to an international standard.

In South African hospitals the standard most commonly used is the International Standards Organisation or more commonly known as ISO 14644-1

Theatres generally fall primarily into two categories, which are:  Minor theatres –ISO class 7 and  Major theatres –ISO class 5.

Why do we sample the air?

Micro- organisms are transferred by particles in the air. Simply put, if we reduce the number of particles in the theatre then we reduce the risk of infection at the surgical site.

The way we reduce the number of airborne particles is by:

• Filtering the air that is supplied to the theatre or controlled environment.
• Dilution of the contaminated air in the theatre or controlled environment.
• Preventing entry of contaminated air from outside the theatre or controlled environment.

Filtration of the air is done by the use of primary, secondary and tertiary filters which are housed in the Handling Unit or in the case of Major theatres, the tertiary filters are located above the laminar flow unit in the theatre.

Should the filters not be maintained regularly, then poorly filtered or unfiltered contaminated air will be supplied to the theatre, placing the patient at risk of wound site infection.The filters also dilute the contaminated air that is carried into the theatre by theatre staff.

Preventing the entry of contaminated air from outside the theatre is done by ensuring that the theatre is pressurised to a higher degree that that of the adjacent areas. Air will then move out of the theatre when the theatre doors are opened. It is, therefore, important to keep all the theatre access doors closed.

Particle count and air velocity test being conducted in a Major Laminar flow Theatre.

In essence, we keep the theatre environment sterile by controlling access, wearing the correct attire, sterilising instruments then we put the patient at risk by not monitoring the air supply to these environments.

Particle counting when conducted by an experienced person, will determine the state of the filtration, the theatre pressure and whether the theatre complies with the certification awarded to the theatre.

This is done by following a strict format as laid down by the ISO body. (International Standards Organisation)

I sincerely hope that this summary sheds light on the importance of particle counting and how the process reduces the risk of infection in theatres.

Eric Lundberg

ParticleScience – CEO