Problems in the disinfection of class 1 microbiology safety cabinets.

Microbiology safety cabinet disinfection procedures using formaldehyde have been tested. Tubercle bacilli were killed by concentrations of formaldehyde obtained by heating commercial formalin irrespective of whether the bacilli were in the cabinet free space or above the prefilters. However, Bacillus stearothermophilus spore papers for for the testing of low temperature steam/formaldehyde sterilisers were almost never sterilised and a strain of Staphylococcus epidermidis (NCTC 7944) showed a resistance intermediate between the B stearothermophilus spores and the tubercle bacilli. Tests using a vaccine strain of poliovirus type 3 indicated a considerable degree of resistance of the virus to the action of formaldehyde. No such resistance was demonstrated by vaccinia virus or echovirus 14. Chemical and biological evidence is presented which indicates that filter paper discs are an unsuitable carrier material for a challenge organism in testing the efficiency of any formaldehyde sterilising process. Recommendations are made towards developing a satisfactory test procedure.