Particle counting in an operating theatre - what you need to know

Over my 20 year career of working for NATA accredited testing laboratories, I have commissioned thousands of HEPA filters in operating theatres throughout Victoria and learnt many valuable lessons along the way.

Performing test AS1807.6 (integrity testing of terminally mounted HEPA filters) is not enough to prove that an operating theatre will remain a clean space when “At Rest”. “At Rest” is a condition where the clean zone is complete with equipment installed and operating in a manner agreed upon, but with no personnel present.

Operating theatres are designed to be positive pressure to adjacent area’s, other than a sterile store or a setup room. Unfortunately, the ceiling space is not a controlled area and not monitored for any change in pressure. The ceiling space may become positive pressure to the operating theatre by something as simple as leaving a theatre door open.

A new operating theatre may pass test AS1807.6 at the commissioning stage, but any modifications to the air balance afterwards can completely change the performance of the filtration system. 

A change in air balance may cause the ceiling space to become slightly positive to the theatre and particles can then leak into the room through the HEPA filter housings, light pendants, access hatches and fire sprinkler fittings.

Microbes are transported around operating theatres attached to particles. Particles that are 0.5 microns can take up to 24 hours to fall 1 metre in still air. The void between an unsealed HEPA filter facia and the ceiling space often capture the HEPA filter test aerosol particles and linger for a long period of time. These lingering particles may cause Inexperienced HEPA filter technicians to fail a HEPA filter integrity test, not realising that it is actually induced leakage from an open manhole or a light fitting that is not sealed.

Whenever performing HEPA filter integrity tests, it is critical to:

  1. Seal the HEPA filter facia to the HEPA filter box before the HEPA filter is installed (not required for a 1 piece HEPA housing).
  2. Caulk around the HEPA filter facia to the ceiling 

These two steps will ensure the HEPA filter housing will not become an air leakage point after a change of air balance. Another major advantage is that you are removing a surface that particles can settle and that is critical to keeping a clean space.

Operating theatres are designed to have >20 air changes and this means that particles below 5.0 microns are easily entrained in air streams and removed from the operating rooms through low-level exhaust and return air.

ISO 7 - Maximum permitted number of particles/m3:

0.5 micron = 352,000

5.0 micron = 2,900

Performing Particle counting in an operating theatre to ISO 7 “At Rest” gives a good indication if there are any containment leakage issues. High counts in the 0.5-micron range is a good indicator that the operating theatre is not positive to the ceiling space and that fixtures in the room are not sealed. Other possibilities for high 0.5 micron counts is the theatre has low room air changes or poor supply air distribution.

High counts of 5 microns are often generated by HEPA filter technicians operating the particle counters with poor technique.

Opira uses Climet particle counters, the C-1050 particle counter samples 100litres of air per minute, meaning that an average size operating theatre takes less than 20 minutes to perform an ISO 7 particle count.  

Although particle counting is not a mandatory test, I recommend to all hospital facility managers that this test is performed. 

Contact Opira now to improve the performance of your operating theatres today. We have experts based in Brisbane, Perth and Melbourne. Check out our Collection of Particle Counters and Microbial Samplers.

- Written by Tim Mundy, June 2021.