Simple definitions of the terms Disinfection (sanitisation) and Sterilisation are: Disinfection; to clean in order to destroy or prevent the growth of disease-carrying micro-organisms. Sterilisation; the procedure of making some object free of live bacteria or other micro-organisms.
The cleaning process used by the decontamination plant removes and kills micro-organisms. The mechanical action of the washer extractor, together with the use of process chemicals, physically removes and suspends micro-organisms, many of which are damaged or killed by chemical action.
Part of the cleaning cycle is very likely to be at high temperature which will kill or damage most vegetative microbes.
The subsequent drying process further reduces the small number of surviving micro-organisms by desiccation and so the process greatly reduces the number and type of microbes (the bioburden) on the cleaned item.
All processing takes place in a controlled cleanroom environment and this ensures that the chance of re-contamination of the product by staff or from the environment is reduced to a minimum.
Items cleaned, dried and packed by a cleanroom decontamination plant will therefore be effectively disinfected and have a very low bioburden.
During the Victorian era heat was recognised as the main method for textile disinfection and ‘Steam Laundry’ employed boiling for all items.
Effective cleaning can be achieved at low temperature using modern wash chemicals and most domestic washing machines operate at 30ºC to 40ºC however this low temperature does not achieve disinfection and higher temperatures are required.
Guidance for the thermal disinfection of textiles has been provided in the UK by the Department of Health publication HSG (95)18 ref1: Hospital laundry arrangements for used and infected linen, which stipulates that consistent and acceptable disinfection can be achieved if the temperature of the wash load is held at 65ºC for 10 minutes or 71ºC for 3 minutes and this has become accepted practice for the disinfection of textiles.
Fungal and bacterial spores are not killed by these temperatures however.
Use of a high water-temperature as a disinfecting stage has several advantages.
Water can be easily heated by a variety of means which do not contaminate the water with particulate or chemicals.
Water temperature can be easily monitored using simple and easily validated temperature sensors and software.
The mechanical action of the washing machine gives rapid and consistent mixing of the contents. Any textile items submerged in the water are quickly raised to the water temperature evenly across all contact surfaces.
The high wash temperature increases soil removal and cleaning, reducing the requirement for chemicals.
To achieve drying the textile item is raised to a minimum of 100ºC and the majority of residual vegetative organisms, but not fungal or bacterial spores, will be killed by desiccation during this process.
Cleanroom handling and packing
Once items have passed through the disinfection stages all further processes are designed to minimise the possibility of recontamination of product by the staff, from the production environment or process equipment. All items are therefore handled, sorted and sealed in primary packaging within the main manufacturing cleanroom before being transferred to dispatch areas.
Disinfection process summary
Production of cleanroom garments and consumables with a consistent and very low bioburden therefore has several interlinked process stages;
- Mechanical removal or damage to micro-organisms by the wash process.
- Destruction or damage to micro-organisms by the process chemicals.
- Thermal disinfection from high temperature wash.
- Use of biologically controlled process water to prevent re-contamination.
- Desiccation of micro-organisms during drying.
- Cleanroom processing of disinfected items to prevent re-contamination.
- Cleanroom application of sealed protective packaging to prevent re-contamination.
Non-thermal disinfection methods
Chemical biocides may be added to the wash cycle. The correct dilution and distribution throughout the entire wash load together with an adequate contact time is necessary to achieve satisfactory disinfection of all items being treated. These parameters are more difficult to control and monitor within the washer extractor than temperature and it is therefore difficult to easily confirm effective disinfection of each item within each load.
Residual biocide may be retained on the fabric surface and give additional antimicrobial benefit during use. However residual chemical on the clothing may pose a contamination risk to wearers (especially on garments in contact with the skin), to the cleanroom environment and to the products being manufactured.
Chemical disinfection is usually only used therefore when materials will be damaged by thermal disinfection temperatures e.g. laminated fabrics.
Disinfection can be achieved at low temperature using ozone wash systems which saturate the process water with highly oxidizing ozone (O3) molecules. Small purpose-built washing machines are available which incorporate ozone injection systems with good reported results. In full size cleanroom washer extractors however producing and maintaining adequate levels of ozone throughout the wash load is very difficult to achieve and monitor. This technology is therefore currently not considered to be reliable for large scale
cleanroom decontamination plant use.
For some high grade bio-medical and pharmaceutical manufacturing procedures a further sterilisation process is required for garments or consumables following production and packing by the decontamination plant.
A sterile product is accurately defined as an item with a sterility assurance level (SAL) of 10-6. This means that there is a theoretical chance that if 1 million organisms were present on the product before the sterilisation process, 1 viable organism could remain after the process.
Cleanroom processed items presented for sterilisation by the decontamination plant will have only a very few organisms present which means that effectively no viable organisms (including fungal and bacterial spores) remain after sterilisation.
Sterility is usually achieved for textile items by gamma irradiation using a minimum irradiation dose of 25kGy.
The process is highly automated and controlled ref 2 with annual dose validations proving the effectiveness of the process for each product type. Additionally each sterilisation batch is monitored to measure the gamma dose throughout the product and a certificate of irradiation is issued to confirm that the minimum gamma dose has been received. Detex labels, which change colour from yellow to red on exposure to ionising radiation, are often applied to products and provide a quick visual confirmation of sterilisation.
Sterilisation is usually carried out by a specialist subcontractor employed by the decontamination plant. Gamma sterilisation is an effective method for producing sterile textile products for cleanroom use however the process physically damages fabrics over a prolonged period and will reduce the useful life of the sterilised products.
Autoclaving uses pressurised steam to sterilise items packed within a chamber, typically at a temperature of 121ºC. To be effective the steam must contact all surfaces to be sterilised and so steam permeable packaging is needed for any bagged items. The time required for sterilisation will vary with the product type and loading density and is usually validated using physical, chemical or biological sterility indicators placed within the load.
Ref 1 - HSG (95)18: Hospital laundry arrangements for used and infected linen.
Ref 2 - ISO 11137-2:2006 Sterilisation of healthcare products.