Containment Level 3 (CL3) facilities are vital to the safe operation of many organisations, such as laboratories and hospitals. They are designed and built to prevent the exposure of lab workers, the environment and other nearby people to a biological agent. All biological agents are categorised by the Advisory Committee on Dangerous Pathogens (ACDP) into four levels of Hazard Group; 1 being the lowest, and 4 being the highest risk. The level of containment required to work with these substances corresponds to their Hazard Group level – so CL3 is required for pathogens in Hazard Group 3.

The health risk attached to working with Hazard Group 3 substances is significant. As such, CL3 facilities have two physical layers to protect their users and the surrounding environment: for example a primary barrier – the safety equipment, such as a microbiological safety cabinet (MSC) – which contains the hazard at its source, and a secondary barrier – the room/laboratory itself – which is critical in protecting the lab worker and anyone in the nearby vicinity.

CL3 or CL4 Facility Uses and Specifications

CL3 or CL4 level laboratories must be used for work involving microbes which can cause serious and potentially lethal disease via inhalation, i.e. pathogenic biological agents. This type of work may occur in clinical, diagnostic, teaching, research, or production facilities.

CL3 facilities must:

In addition to the CL3 requirements, CL4 facilities must also:

Reasons for decontamination

Due to the nature of the substances worked with in these facilities, they require regular disinfection. Some circumstances after which disinfection may be required include:

Decontamination methods

The Control of Substances Hazardous to Health Regulations, 2002, recommend fumigation as the exclusive method of decontamination for CL3 facilities. Whilst any method of fumigation is inherently dangerous, it is only ever performed by trained personnel with specialist equipment, as is the only known method which achieves the required log 6 kill rate in all parts of a CL3 facility.

Formaldehyde fumigation is the longest-established method and is still widely used as for the decontamination of CL3 laboratories. The room must first be humidified to >70%, then gas is generated by vaporizing formalin solution. Depending on the size of the room, it may need to be exposed for up to 24 hours. Finally, the Formaldehyde is neutralised using ammonia before anyone can enter the laboratory and any residue is cleaned up.

Formaldehyde fumigation is much more effective than simply wiping down surfaces in order to decontaminate them, as the formalin/formaldehyde vapours will diffuse throughout the room and get to the most awkward to reach places of any CL3 facility, deactivating harmful microorganisms on contact. However, it’s still important to remove heavy soiling or detritus from surfaces prior to fumigation, as this process will not penetrate or remove thick layers of dirt or spillage. 

CL3 Validation

It is recommended that an annual validation is performed on a regularly basis e.g. during the annual or bi-annual maintenance visits, to ensure the facility is still performing as it was designed to and providing adequate protection for lab workers from dangerous pathogens.

This validation would include the key testing procedures performed during the commissioning of the CL3 laboratory:

A full validation should also be performed following major modification works to the CL3 laboratory.

Fumigation validation

Knowing the importance of having a sterile CL3 lab which protects its users and the surrounding environment from exposure to hazards, you might be surprised to learn that many operators do not regularly test the effectiveness of their fumigations.

It is important to prove a fumigation has been successful by validating the process using biological indicators.

A fumigation efficacy study uses biological indicators to verify the fumigation’s antimicrobial efficacy. Every CL3 lab should undertake a fumigation efficacy study as part of the initial commissioning or following any major modifications.  There are various types of biological indicator (BI) available and it’s important to use the correct type depending on the fumigant being used. For formaldehyde, we use the challenge microorganism Bacillus atrophaeus, which is the industry’s preferred biological indicator for this fumigation process. This challenge is applied to ‘spore strips’ which are placed around the room; if all bacteria on the strips are deactivated after the fumigation, then we know that the fumigation has been effective. Any failure in killing the Bis could be the result of environmental variations in the room and not the method itself.

It’s also crucial to test the sealability of your CL3 facility on an annual basis and prior to any fumigation. If there is a breach in the room’s seal, not only could harmful bacteria escape and potentially endanger others within the building or even the outside environment, but you would also not be able to safely and efficiently fumigate a room. This is why the Approved Code of Practice (ACoP) supporting the Control of Substances Hazardous to Health Regulations 2002 (COSHH) requires that, for containment level 3 facilities, the workplace must be sealable to permit decontamination.

There are three common methods for testing a lab’s sealability. The first is to use a small, smoke plume-generating device such as a smoke pencil, whereby leaks are detected by observing any deviation in the plume of smoke produced. The second is a whole-room smoke generating device, whereby any leakage to the areas surrounding the lab can be observed. The third, which is more applicable to the construction phase of a new build or refurbished CL3 facility, is to do a soap and bubble test by negatively pressurising the CL3 lab and applying a soapy solution to the walls, ceilings and floors. Any bubbles which appear when applying the soapy solution would indicate a leak, which would then be retested using the smoke pencil test to confirm the leak.

Don’t run the risk – validate your CL3 facility

You need to be sure that your CL3 lab can be effectively fumigated in the case of an emergency or spillage, in order to keep your staff and surrounding environment safe. That’s why we recommend you should always undertake a fumigation efficacy study, alongside any fumigation, to ensure its effectiveness in potential emergency circumstances.

As well as routine fumigation and efficacy studies, Crowthorne is able to offer an emergency fumigation scheme in the case of a spillage. Our trained engineers can attend your site within 24 hours to fumigate the potentially contaminated facility, thus reducing downtime.


If you wish to discuss regular inspections, verification and integrity testing of your CL3 laboratory, arrange a fumigation efficacy study or register for our emergency fumigation scheme, please get in touch