Problems in the disinfection of class 1 microbiology safety cabinets.

Abstract

It is accepted practice to disinfect microbiological safety cabinets with formaldehyde vapour particularly before such operations as filter changing. For some years at Shrewsbury the method described in PHLS Monograph No 61 in which 200 ml of an equal volume of commercial formalin and water were boiled away in a sealed cabinet and left undisturbed overnight has been used. The directions imply that the extra water is added to raise the humidity and so enhance the lethal effect of the formaldehyde. Unfortunately one of the effects of the extra water is to cause the inside of the cabinet to become wet from condensation. Because condensation on the load and in the chamber of low temperature steam/formaldehyde sterilisers is known to be a common cause of sterilisation failure,2 we tested our cabinet disinfection procedures.Among the many and varied recommendations for disinfecting safety cabinets (Table 1)' 3-6 only one quotes recent supportive experimental details.6 Because of the varied advice and the relative lack of published results we describe the tests we undertook and the results obtained.

A Comparative Study of Methods To Validate Formaldehyde Decontamination of Biological Safety Cabinets

“…The difficulties in obtaining effective decontamination by using formaldehyde gas demonstrated in this study are consistent with earlier reports (3,5). Bacterial spores inoculated onto stainless steel were the most resistant to decontamination.…”

“…In experiments undertaken with B. subtilis spores as an indicator, a successful decontamination, reflected by a total kill, was observed for seven pieces (of a total of 144 pieces). The decontamination conditions which were optimal for all of the biological challenge materials were 66% relative humidity, a minimum temperature of 28°C, and 10.5 g of paraformaldehyde/m 3 (Fig. 1).…”

B. Atrophaeus and G. StearothermophilusBiological Indicators for Chlorine Dioxide Gas Decontamination

“…Even without excess moisture, there is evidence of adsorption of CD into paper substrates that may subsequently reduce the number of viable spores during the incubation. Such an effect has been reported for CD (Jeng & Woodworth, 1990) and formaldehyde (Everall et al, 1982). For this reason, later validation studies for CD decontamination include a CD deactivation step consisting of the immersion of the BI into a sterile 1.0 wt% ST solution prior to its addition to growth media, as has been done by previous investigators (Han et al, 2003;Jeng & Woodworth, 1990).…”

“…Munro et al (1999) concluded that self-contained BIs with GS were a better indicator of formaldehyde decontamination failure than using BA on paper strips, while BA was more resistant on steel substrates at humidity less than 60% RH. Everall et al (1982) observed virtually no full deactivations by formaldehyde of BIs with 10 6 GS spores. Cheney and Collins (1995) have suggested that for formaldehyde decontaminations BA is a more sensitive indicator to show sufficient humidification, while GS is more sensitive to the amount of formaldehyde.…”

Use of Hydrogen Peroxide Vapor for Deactivation of Mycobacterium tuberculosis in a Biological Safety Cabinet and a Room

“…BSL 3 and containment level 3 laboratories must be sealable for vapor-phase decontamination. Formaldehyde fumigation has traditionally been used to decontaminate laboratories and biological safety cabinets (BSCs) used to handle M. tuberculosis (28), but there are significant safety and efficacy concerns associated with formaldehyde fumigation (6,7,11).…”