Validation hierarchy and air change rates in differing facilities

To validate a facility, be it, cleanroom, dentist surgery, operating theatre or other, there needs to be a hierarchy of validation tests that are fundamental to the performance of area or facility. Of all of these, air change rates are crucial in providing accuracy for all the other performance test requirements. In a cleanroom environment, air change rate testing must be performed as the very first validation task.

If the air change rates are too low, having performed other validation tests, it will simply mean that all of the tests will have to repeated, assuming that the air change rate is corrected to the requirement.  We at TCA can advise you on the validation requirements of your facility. Please call us on 01737 924 700

What is the air change rate acceptance criteria?

Air change rate acceptance criteria can vary depending on the area classification or type of facility. Quite often it is decided by the end user or quality assurance personnel. When verification testing within an NHS department or other hospital environment, there are specific guidelines that many named areas have to achieve. These are listed on an Appendix table within the HTM 03-01 document. The theory behind having a set criteria for different areas, is the amount of dilution of clean air required to minimise pollution. After all, dilution is the solution to pollution. In areas such an anaesthetic room, an air change requirement is needed to control the level of medical gases that are in the atmosphere. You would not want your anaesthetist feeling drowsy after all!

If a cleanroom is under negative pressure, the air change rate is usually calculated using the extraction in the room. This can take the form of a regular extract vent or grille or calculated by a ducted air device. If the area under test is a dental surgery or similar, the air change rate requirement may be the only validation test applicable. Air change rates are calculated using the total supply, or extract, air volume in the room. The air volume is calculated by taking a balometer reading (if applicable) at all supply or extract grilles or by recording a velocity and multiplying that reading by the inlet or exhaust area.

The total air volume is calculated using all the supply or extract grilles and this figure is divided by the room or area volume (m³). A dental surgery may not have a specific acceptance criteria for example, but government guidelines may be introduced to help waiting times in between patients, in the wake of COVID 19 restrictions. Pharmaceutical, aseptic, or medical cleanroom areas are governed using EU GMP.

Although the acceptance criteria previously used to be specified as 20 air changes per hour, this figure was removed some time ago. Still now, many pharmaceutical cleanrooms still use 20 as a ‘rule of thumb’, and this is not a bad idea however, the guidelines do state that, if the particle count limits are sufficient within the cleanroom versus the number of technicians or personnel, then the criteria does not have to be specified.

However, unless you have a constant monitoring particle counter within the cleanroom area, you would not necessarily know. Keeping a limit on acceptance criteria might not be such a bad idea after all. According to HTM 03-01, one of the main one of the main current hospital standards, an aseptic pharmacy in a hospital environment, must achieve 20 air changes per hour. Other air change requirements in a hospital environment such as operating theatres or recovery rooms are subject to a rule whereby, they can achieve at least 75% of the design criteria. This allows for the air handling unit to decline over its lifespan. Naturally, an air handling unit that is running constantly, every minute of every day, is going to deteriorate over time. Be it, fans, fan belts (unless direct drive), pulleys, various stages of filtration and even duct work leaks. It is estimated that an air handling unit’s lifespan is approximately 20 years, depending on how well the AHU is looked after.

Maintenance is absolutely essential and can keep running costs down over time if they are looked after carefully. They are fundamental in providing the correct air supply to the source and thus delivering, hopefully, the correct air change rates. Another standard that is used within the NHS is HBN 04-01. This standard gives clarification on essential air change rate requirements for isolation rooms and utilities concerning infection control. The current global situation means that areas such as infection control are so very important and the acceptance criteria concerning air change rates are much higher than other areas of a hospital. They usually have certain levels of HEPA filtration, either directly in the room, or source, or as a dedicated safe change HEPA filter unit.

Total Clean Air - Testing & Validation Services

Contact us today and find out how we can best serve you on your next project.

How to perform Air Change rates

Measuring air change rates are usually performed using a variety of test instruments. These instruments should be calibrated annually and within the medical, pharmaceutical, and other industries that are governed with an internationally recognised standard, such as ISO or GMP, it is very important that the calibration dates are adhered to. Results can be deemed unusable unless the instrument measuring device is within the calibration dates. A suitable piece of test or validation instruments to measure airflow could be a rotating vane anemometer, a thermal anemometer (hotwire), a pitot tube or a balometer. Or it could be a combination of one or more of these, depending on the site, application, room, or access.

To accurately measure a supply volume from a terminal grille or grilles, it used to be thought that a correction factor should be measured. This is where a duct velocity is measured at a suitable test location in the duct, for one or more of the terminal supplies, using metres per second. The velocity is calculated in to either a cubic metres per second (m³/s), cubic metres per hour (m³/h) or litres per second (l/s) reading. The volume for the same terminal grille or grilles is measured at source, using a suitable balometer and measuring the reading using the same measurements. The difference between the two readings is calculated as a correction factor to account for the loss of air in the duct. Nowadays however, the best balometers come equipped with a device known as a back pressure compensator. The device is in the form a flap inside the base of the balometer.

When an initial reading is taken, the balometer will prompt the user to adjust the back pressure compensator and retake the volume reading in the exact same position. It sounds complicated but the backpressure device can be adjusted in the same swift movement. After taking several readings however, the arms may start to get a little tired, so it is always important to ensure that the balometer covers all the supply or extract grille. Of course, some supply or extract grilles will not need a back pressure compensation to be added to the final reading. Grilles that are directly fed, such as fan filter unit or a supply or extract that is not fitted with a diffuser, will not require this device.

Calculating Air Change Rates

Calculating air change rates utilises many pieces of equipment as sometimes the supply or extract may be in the form of a small duct or partially covered grille. This is where a rotating vane anemometer or hotwire can be really useful. These instruments tend to measure a velocity in metres per second (m/s) and this reading can be calculated by multiplying the velocity against the inlet or extract area. This will give a reading in cubic metres per second and then a further calculation can be performed, based on the room volume (m³). It is important to document the procedure and test equipment that is used to calculate an air change rate as instruments that measure in metres per second, tend to have a higher value.

Are Air Change Rates a critical Validation test?

As mentioned throughout this text, air change rates are highly critical in setting up a correct balance of an area or cleanroom suite. If a validation is performed in the incorrect order, such as HEPA filter integrity first, and the air change rates fall below the minimum requirement, the validation tests become useless as insufficient air flow through the filter would mean that the HEPA filters have not been integrity tested at their correct capacity rating and in a true, balanced condition. HEPA filter manufacturers are very strict with these requirements and will not warrant an integrity leak if the air volume is incorrect, should you be unlucky enough to discover one.

The various standards and regulations put upon the various industries can become a minefield of information when deciding on the validation requirements of your facility. To ease the burden on deciding on what you require please call us at Total Clean Air to discuss your facility. We can advise you and carry out your validation to you specified requirements. Please contact us on 01737 924 700.

Total Clean Air - Testing & Validation Services

Contact us today and find out how we can best serve you on your next project.

Phillip Godden

Phillip Godden is the Founder & Chief/Executive Officer at Total Clean Air.

Working Hours

  • Monday 08:00 - 19:30
  • Tuesday 08:00 - 19:30
  • Wednesday 08:00 - 19:30
  • Thursday 08:00 - 19:30
  • Friday 08:00 - 19:30