Disinfection with gaseous formaldehyde. First Part: Bactericidal and sporicidal effectiveness of formaldehyde with and without formation of a condensing layer.

Suspensions of Staphylococcus aureus ATCC 6538, Streptococcus faecium ATCC 6057, and of spores of Bacillus subtilis var. niger DSM 675 dried on polished stainless steel carriers were exposed in a model chamber to 3.2 mg HCHO l-1 air at temperatures of 20, 25, 30, 35 and 40 degrees C and the kinetics of their inactivation were determined by successive colony counting during the exposures. The HCHO treatment was carried out once with the formation of a condensing layer on the carriers and once without recondensation, at a RH of about 90%. In both procedures the cells or spores were suspended and dried in saline peptone water and additionally, in the case of the exposure without condensate layer, suspended in peptone water only. For S. aureus and S. faecium, significant differences between the two processes were only observed at 20 degrees C, whereby S. aureus showed for example a D-value of 14.8 minutes and after an exposure with condensate a D-value of 28.1 min. However at higher temperatures the effectiveness of HCHO in gaseous or condensate form was rather similar. At 35 degrees C the D-values after exposure to HCHO in condensate and gaseous form was for S. aureus 4.1 min and 5.9 min respectively, whereas after treatment at 40 degrees C D-values of 3.2 min and 3.8 min respectively were determined. At 35 and 40 degrees C D-values for S. aureus suspended in peptone water and exposed to gaseous HCHO were 50% lower than when suspended in saline peptone water. At lower exposure temperatures large differences were not registered. For the B. subtilis spores exposure to formaldehyde without condensation showed D-values of 34.8 and 5.6 min at 20 and 40 degrees C respectively. These are 8- and 4-fold lower than those of a corresponding exposure with a condensing layer. No D-value differences were observed for spores suspended and dried in saline peptone water or in peptone water after exposures to gaseous formaldehyde.