Fumigation

Overview

Introduction

Fumigation of microbiological safety cabinets and some animal and containment level 3 facilities is often carried out in the department. Fumigation with formaldehyde vapour is the recognised and most commonly used method for this type of fumigation procedures although an alternative system using vaporised hydrogen peroxide is available.

Formaldehyde vapour has irritant and toxic properties and carries a risk of respiratory and/or skin sensitisation reactions. At the higher concentrations used for room fumigations it is extremely hazardous and exposure to such levels, even for a short time, would likely result in very serious injury or death. Alternative systems using hydrogen peroxide are available and whilst the chemicals used are less hazardous than formaldehyde, there are still risks associated with the procedures and appropriate control measures are required in order to carry out the work safely.

Formalin is a commercially available 40% solution of formaldehyde vapour in water. When formalin is heated formaldehyde vapour is generated in quantity. Formaldehyde is a Schedule 1 chemical under the COSHH Regulations and has a Workplace Exposure Limit (WEL) of 2 ppm (or 2.5 mg.m^-3). Concentrations encountered during fumigation are many hundreds of times higher than this, so fumigation operations must be carried out only by trained personnel under strictly defined conditions. All workers using formaldehyde must be aware of safe handling procedures.

Under certain conditions formaldehyde can react with hydrochloric acid and chlorine-containing disinfectants such as hypochlorites to form bis (chlormethyl) ether, a potent lung carcinogen. Hydrochloric acid and chlorine-containing disinfectants must therefore be removed from rooms and microbiological safety cabinets before fumigation.

Formaldehyde vapour is an extremely effective biocidal agent. It acts as an alkylating agent, inactivating micro-organisms by reacting with carboxyl, amino, hydroxyl and sulphydral groups of proteins as well as amino groups of nucleic acid bases. A number of factors affect the efficiency of fumigation. For formaldehyde to act to maximum effect it must be able to penetrate (pre-cleaning is helpful if it can be done without jeopardising safety) and it must be able to dissolve at adequate concentrations in a film of moisture in the immediate vicinity of the organisms to be inactivated. Water vapour generated in the process of dispersing formaldehyde provides the essential optimum level of relative humidity and so it is important to ensure that water is added to the formalin prior to vaporisation. Too much formaldehyde results in the deposition of sticky deposits of paraformaldehyde and in cabinets may contribute to filter blockage. The amounts of formalin and water required for fumigation are given in the more detailed sections below. Fumigation is most effective above a temperature of 20°C and relative humidity of 65%. At temperatures below 18°C formaldehyde fumigation is less effective. Below 9°C, formaldehyde sublimes and is less easy to vaporise.

Agents causing the transmissible spongiform encephalopathy’s are resistant to inactivation by formalin and so its use is not appropriate in these cases. Workers handling these types of agents should ensure alternative decontamination procedures are put in place in advance of any work commencing. Further advice can be obtained from the Biological Safety Adviser.

Risk assessment

Risk assessment & procedure

A detailed risk assessment and procedure must be documented and approved by the Biological Safety Advisor in advance of all fumigation activities.

All laboratory groups are responsible for ensuring appropriate systems are in place to provide safe and suitable procedures for fumigation in biological laboratories and associated facilities. In conjunction they must ensure adequate and appropriate instruction, training and supervision is provided for workers undertaking fumigation procedures. They should also ensure that suitable arrangements are in place to monitor and review working practices.

Formaldehyde

Monitoring formaldehyde levels

If formaldehyde vapour is used as a fumigant, it is important that nobody is exposed to the 'workplace exposure limit' of 2 ppm. A formaldehyde meter is available from the Biological Safety Advisor to monitor levels during the fumigation procedure.

Safety cabinets

Fumigating microbial safety cabinets

Microbiological safety cabinets used to handle hazardous microorganisms (i.e. Hazard Group 2 organisms or above), must be fumigated in the following circumstances:

• after a major spillage or a spillage where inaccessible surfaces have been contaminated
• before any maintenance work on the cabinet where access to potentially contaminated parts is necessary (including filter and pre-filter changes)
• before carrying out filter penetration tests
• when there are any changes in the nature of the work that result in significantly different risks

Fumigation must be carried out only by a trained responsible person with adequate knowledge of the procedure and the precautions to be followed.

Where the cabinet has been used for hazardous micro-organisms, HEPA filters should be handled only with appropriate protective clothing (laboratory coat and heavy duty gloves) even after fumigation. Such filters must either be autoclaved prior to disposal in the general waste stream or be securely wrapped in yellow bags for disposal as clinical waste.

The cabinet must be sealed before fumigation to prevent leakage of formaldehyde into the laboratory. It should be checked to ensure that the closure panel/night door has been properly and securely located and a good seal has been achieved. Where necessary sealing tape should be used to ensure there is no leakage. With Class III or hybrid (Class I/Class III) cabinets a blanking plate should be fitted over the inlet filter.

If the cabinet is a recirculation type, there must be a safe means of venting the formaldehyde vapour to atmosphere (the outside) such as by the use of a fumigation adaptor kit (see below). This must be fitted to the cabinet prior to commencing fumigation.

The fumigation procedure should ensure inactivation of any micro-organisms that have penetrated the HEPA filter by adequate exposure of the downstream side of the HEPA filter and the ductwork to formaldehyde. In the absence of specific recommendations by the cabinet manufacturer the cabinet fans should be turned on for 10 - 15 seconds after about half the formalin has been evaporated and again after evaporation is complete. Passive migration of the fumigant through the filter can occur but this is not ideal. Some cabinets have automatic fumigation cycles programmed into the controls and in these instances the manufacturers' instructions should be closely followed.

The formaldehyde should be left to disperse within the cabinet for at least six hours (or preferably overnight) after which time the fumigant should be exhausted to atmosphere by switching on the fan and allowing air from the room to enter the cabinet. Before venting the formaldehyde in this way it is essential to ensure that no-one is in the vicinity of the exhaust outlet and that the exhaust air does not enter nearby windows or ventilation air intakes.

Following decontamination, the microbiological safety cabinet must be purged of all residual formaldehyde. With recirculation type cabinets a fumigation adaptor kit should be fixed over the cabinet exhaust to allow discharge of fumigant via the flexible trunking into either a ducted cabinet, fume cupboard, a fumigation port or out of a window. Care must be taken to ensure no formaldehyde is discharged back into the laboratory. Under no circumstances should recirculating type cabinets be fumigated unless there is a safe means for discharging the formaldehyde to the outside atmosphere.

Formaldehyde must not be recirculated into the laboratory; this would constitute an uncontrolled release of a hazardous chemical and be a reportable incident to the Health and Safety Executive.

The quantity of formalin required is approximately 2 ml per cubic foot (0.028 m³) internal air space. Typical quantities of formalin required for fumigation of the different types of cabinets are:

Whilst fumigation is in progress a large notice must be posted on the front of the cabinet to warn that the cabinet is being fumigated. An example of a suitable sign is provided below.

Manufacturers of microbiological safety cabinets should provide detailed instructions for fumigation of their particular cabinets and these should be followed. This is particularly important when the cabinet has an automatic fumigation cycle. An outline of the main principles of fumigation has been provided above and manufacturers' instructions should be consistent with these (if there are serious discrepancies please contact the Biological Safety Advisor for advice).

Groups must have a written procedure in place for cabinet fumigations. A summary of the main steps is provided below. This summary is a model only and should be modified as appropriate to tailor it to the particular cabinets in a laboratory. The written procedure must identify those individuals competent and authorised to carry out the fumigation process.

• Switch off the cabinet fans
• If the cabinet is a recirculation type, fit the fumigation adaptor kit to the discharge and position the other end to vent to atmosphere. Close the manual shut-off damper.
• Fill the vaporiser with the correct amount of formalin and screw on the aluminium cap - finger tight, having checked the gasket in the cap is undamaged. Place the vaporiser in the cabinet.
• Fit the closure panel/night door and fully seal the front screen and closure panel with sealing tape to ensure there are no leaks.
• Post a notice on the front of the cabinet indicating fumigation is in progress.
• Switch the vaporiser on.
• After approximately 10 minutes switch the cabinet fans on for 10 - 15 seconds.
• After a further 20 - 30 minutes switch the cabinet fans on again for 10 - 15 seconds.
• Leave the cabinet in this condition preferably overnight, but for a minimum of 6 hours.
• If the cabinet is a recirculation type check the exhaust of the fumigation adaptor kit is in a position to discharge safely and open the manual shut-off damper.
• Before venting the formaldehyde check no-one is in the vicinity of the exhaust outlet and that gas will not enter any open windows nearby.
• Exhaust the formaldehyde from the cabinet by switching on the fans and opening the closure panel/night door slightly (remove bung if fitted or crack open) until the majority of the formaldehyde has been exhausted. After about 10 minutes the night door may be removed completely.
• Any poly-formaldehyde residue in the vaporiser may be removed be heating with water containing a little mild detergent.
• Run the cabinet for at least a further 15-20 minutes to remove the last traces of formaldehyde.
• If the cabinet is a recirculation type the fumigation adaptor kit must be removed before the cabinet is used again.

Laboratories

Fumigation of laboratories

Formaldehyde fumigation of laboratories or other rooms must be carried out only by named trained personnel. Fumigation procedures for Containment Level 3 facilities must be validated to demonstrate decontamination efficacy. This involves undertaking a test fumigation with spore strips being placed at strategic locations around the laboratory.

Before commencing fumigation the laboratory/room must be completely sealed to prevent escape of formaldehyde vapour into other areas. Containment Level 3 laboratories are designed so that they are easily sealable for fumigation with minimal effort. In other laboratories/rooms that have not been designed to facilitate fumigation it will be necessary, before commencing fumigation, to seal service ducts and other holes through which fumigants may escape. If the laboratory has a false ceiling the void above must be checked to ensure that there is no interconnecting ducting to other areas and if there is these must be effectively sealed. There must also be an effective means of venting the formaldehyde after fumigation. Given the difficulties associated with fumigating such a room, any plan to undertake a fumigation must be carefully considered, fully justified, risk assessed and have detailed written procedures in place. Groups are advised to discuss any proposal to fumigate a room or area other than a Containment Level 3 laboratory with the Biological Safety Advisor.

Prior to undertaking fumigation with formaldehyde vapour, it must be thoroughly checked and confirmed that the room is properly sealed and there will be no release of gas to other areas. Any such escape of formaldehyde would constitute an uncontrolled release of a hazardous chemical and be a reportable incident to the Health and Safety Executive.

There must be a means of exhausting the formaldehyde vapour from the laboratory/room that avoids the need for any person to enter the area where the formaldehyde has been released. The extract must be a total loss system with no possibility of formaldehyde being ducted to other areas. This could be by the use of a ducted microbiological safety cabinet to exhaust the fumigant to atmosphere and via room extract ductwork. The recognised method is to have a switch external to the room to allow extraction of the formaldehyde without entering the room. The Biological Safety Advisor must be consulted before any other method is adopted. It should also be possible to operate, without entering the room, any dampers that may be installed if their opening is necessary to ventilate the room.

Once the formaldehyde has been generated the door must be locked to prevent anyone entering the room until the fumigant has been exhausted. Under no circumstances should anybody be allowed to enter a room whilst fumigation is in progress unless they are wearing full breathing apparatus that provides air from an independent source and they are fully trained in its use. Cartridge respirators are not appropriate for use in the concentrations of formaldehyde generated during fumigation.

Calculations should be undertaken to estimate how long it will take to purge all the formaldehyde from the room after fumigation is complete. This should be based on room volume and the rate of air extraction through the particular exhaust system that will be used. The calculation should also take account of the initial formaldehyde concentration in the room and the need to reduce it to well below 2 ppm. The following formula can be used:

time (mins) X 25 = room volume (m³) / extraction rate (m³.min^-1)

If a microbiological safety cabinet is being used to ventilate the room check the exhaust rate with the particular manufacturer. A Class II cabinet typically exhausts approximately 125 l.sec^-1 (to convert l.sec^-1 to m³.min^-1 multiply by 0.06).

After fumigation the level of formaldehyde remaining in the air must be below 2 ppm (2.5 mg.m^-3) before anybody is allowed to re-enter the room. The level of formaldehyde should be checked by, for example, sampling the air through a small port fitted in the door for this purpose. This can be done with either a hand-held direct reading instrument (such as a formaldameter) or appropriate air sampling tubes. If this sampling indicates levels are satisfactory to proceed (ideally less than 0.5 ppm) then further tests should be undertaken by a person entering the room to check the formaldehyde levels in all areas (often there are pockets of gas remaining following fumigation), this person must wear a laboratory coat, gloves and a full-face cartridge respirator (mask). Anyone wearing such a respirator should be aware of the level of protection afforded by it. As an example, a Class 1 full-face mask to EN136 (e.g. 3M 6000 series) with formaldehyde filters (e.g. 3M 6075 A1+formaldehyde) will protect up to 10ppm. The COSHH Regulations require that persons wearing full-face masks be subject to individual, and product specific, face-fit testing using a recognised quantitative test method. Further advice on this can be obtained from the Biological Safety Adviser.

An outline of the main principles of fumigation has been provided above. The following is a summary of the main steps but this should be tailored to the particular laboratory or area to be fumigated:

• Before commencing fumigation switch off all forced air ventilation systems, extract systems and any fume cupboards and microbiological safety cabinets in the room and deactivate fire alarm system smoke detectors. Seal up any external ventilation grilles as necessary.
• Check that the point from which the formaldehyde will be exhausted is free from obstruction and that the remote switch that will be used to exhaust the formaldehyde from the room is working (run the extract this way for at least 5 minutes then switch off).
• If undertaking a routine or test fumigation place spore strips at strategic locations around the room, following fumigation these should be collected and incubated. If there has been an accident any access in to the laboratory should be restricted to an absolute minimum and so this need not be done.
• Place the appropriate quantities of formalin and water mixture into the heater unit and when ready activate the heater and leave the room immediately. Use 100 ml formalin plus 900 ml water per 28.3 m³ (1000 ft³) of space.
• Lock the door and effectively seal around the edges with tape.
• Display a notice in large type on the door such as the following:  
  WARNING, DO NOT ENTER, FORMALDEHYDE GAS DECONTAMINATION TAKING PLACE
• Other warning signs may be required in the surrounding areas.
• After a period of not less than 12 hours (the procedure is best carried out overnight), the room must be well ventilated. Purge the space by using the remote switch to activate the extraction system (open dampers if necessary).
• Allow the room to purge for at least the time calculated as necessary to remove the formaldehyde.
• Check levels of residual formaldehyde in the room with suitable air monitoring equipment (formaldameter or air sampling tubes). Only enter if the level of formaldehyde is below 2ppm and wear a laboratory coat, gloves and a full-face cartridge respirator (with appropriate filter for formaldehyde gas).
• Check the room and all surfaces for formaldehyde residues and clean up as necessary.
• Remove all sealing materials.
• Only allow other staff to enter the room when formaldehyde levels are below 0.5 ppm. Levels should be as low as practicable before staff re-enter.

A record must be kept, for at least 5 years, of all fumigations of rooms/laboratories including date, personnel involved and the results of air monitoring on completion of the procedure.