Thermal lethality

The range of sterilization specifications calculable by these various approaches is summarized in Table 2. It is apparent that very brief sterilization specifications (on the order of 2-3 min holding time at 121°C) are obtainable when the microbiological contamination is completely characterized in terms of numbers and thermal resistances. In practice, such limits on hold times could be difficult to control precisely, are probably insignificant in terms of thermal lethality compared with heat-up and cool-down times, and could prove difficult to sell to regulators. Without complete thermal characterization of thermal resistances, specifications calculable by the bioburden approach are hardly significantly shorter than overkill specifications. Thus, it probably makes practical sense in most cases to choose only between overkill cycles for thermally resistant products and aseptic manufacture for heat-sensitive products. 

Often vast amoimts of thermal lethality calculated as Fo-values are delivered in these prehold stages of SIP. However, because these temperatures are being achieved in the presence of steam-air mixtures, it is not correct to assume that the biological lethality during the heat-up phase of SIP systems is equivalent to that achieved with pure steam. 

Liquid smoke, which contains approximately 10% acetic acid, has been used as an antimicrobial agent against the pathogens E. coli 0157 H7, Salmonella, Listeria, and Streptococcus. It has been confirmed that combined use of liquid smoke and steam had a synergistic thermal lethality effect on... 

Anderson, R.C. Alarie, Y. "Acute Lethal Effects of Polyvinyl-chloride Thermal Decomposition Products in Normal and Cannulated Mice, Abstracts... 

It was found that the amount of energy needed for the room fire to cause thermal decomposition of the PVC products in the plenum was larger than that needed to take the room to flashover. Furthermore, if the PVC products did eventually decompose or burn, somehow, they would cause a lethal smoke concentration only significantly later than a lethal (by toxicity) atmosphere had already been created by the fire itself. Thus, the PVC products did not add any significant fire hazard to that caused by the room fire. 

It is of interest to calculate, too the time required for both the fire itself and the thermal decomposition of the plenum PVC products to produce a lethal atmosphere. Table III presents such results for the fire, for heats of combustion of 20 kJ/g and 40 kJ/g, a range typical of most fires. In order to carry out this calculation it is assumed that the smoke is distributed instantaneously throughout the volume being considered, one or four room-plenums. The barriers represented by walls or... 

Solarization process increases soil temperatures up to levels lethal to many plant pathogens and pests and, therefore, direct thermal inactivation is the most important and normally expected mechanism. Some studies on the biochemical bases of sensitivity of organisms to high temperatures hypothesized that heat sensitivity is related to small differences in cell macromolecules, leading to a lethal increase of intra-molecular hydrogen, ionic, and disulfide bonds (Brock 1978). Sundarum (1986) suggested a reduced cell membrane function beyond an upper limit fluidity... 

The final column presents the radius of 50% mortality from fallout 1 hour after the explosion. Of all of the threats described, fallout is the hardest to predict because of the influence of local, regional, or even global weather patterns. The mushroom cloud can rise into the atmosphere as far as 80,000 feet, where wind and rain influence the time and location for fallout to occur.2 Individuals several miles from ground zero and well outside any radius presented in Table 5.1 can receive significant or even lethal radiation doses from fallout. However, while the air blast, thermal burns, and initial radiation are threats in all directions, fallout is a threat downwind from ground zero. Wind speed and direction vary at different altitudes, and it is safest to assume that fallout is a potential threat in all directions from ground zero. Individuals outside the blast zone generally will have several minutes to an hour or more to seek shelter before fallout arrives. 

Physical Pollutants Thermal Cooling water from power plants and industry Lethal to some sedentary species, displaces others. 

Figure 2 presents thermal resistance plot for a Z value of 10°C, the accepted standard for steam sterilization of B. stearothermophilus spores, and for a Z value of 20°C, the proposed standard [13] for dry-heat sterilization of B. subtilis spores. These plots are important because one can determine the I) value of the indicator micro-organism at any temperature of interest. In addition, the magnitude of the slope indicates the relative degree of lethality as temperature is increased or decreased. 

The product yields are particularly dependent upon the composition of the polymeric material, the temperature, and the ventilation conditions. Once the temperature of the surface is raised sufficiently (generally to around 300°C), then a process of thermal decomposition by oxidative pyrolysis begins. The products of nonflaming decomposition tend to be rich in partly decomposed organic molecules (many of which are irritants), carbon monoxide, and smoke particulates. This scenario presents a particular hazard to a sleeping subject in a small enclosure such as a closed bedroom, which can reach a lethal dose over a number of hours.6... 

Alarie, Y.C. and Anderson, R.C., Toxicologic and acute lethal hazard evaluation of thermal decomposition products of synthetic and natural polymers, Toxicol. Appl. Pharmacol., 51, 341 (1979). 

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