The formaldehyde/low temperature steam sterilizing procedure

Abstract

The sterilization of heat-labile equipment still remains a problem in the health services. Low temperature sub-atmospheric steam and formaldehyde has many advantages over ethylene oxide (EO), but the machines and cycles available have been rather uncertain in their reliability as sterilizers (Cripps, Deverill & Ayliffe, 1976). There have been differences of opinion on the relative roles of steam and formaldehyde in the sterilization process and also on how these chemicals should be applied. Nordgren (1939) studied the sporicidal activity of gaseous formaldehyde and found that the procedure was more effective in an atmosphere of low humidity. Alder, Brown & Gillespie (1966) described a sterilization system whereby a dry evacuation of air from an autoclave chamber was followed by an injection of heated formalin vapour and then by sub-atmospheric steam [low temperature steam (LTS)]. Alder (1968) and Mitchell & Alder (1970) also used a dry system for the evacuation of air, but modified the method of Alder et al. (1966) by injecting and evacuating small amounts of formalin vapour followed by LTS in a super-heated environment (unsaturated steam). The jacket surrounding the autoclave chamber was used to radiate the extra heat required. Weymes (1982) assumed that the latent heat of LTS and the formaldehyde monomer had a synergistic and sporicidal effect, that the steam was in its optimum state when it was dry and saturated, the latitude on either side of this phase boundary was unknown. Hurrell (1980) thought that the principal requirement for sterilization was a homogenous mixture of monomeric formaldehyde gas and saturated steam. Gibson (1977) and Marcos & Wiseman (1979) recognized the importance of a low humidity in their studies of this sterilizing process. Line & Cutts (1983) described a process in which a controlled injection of steam, sufficient to raise the pressure in the chamber to 80-100 mbar, and introduced at an early stage before the formaldehyde, was effective.