in all probability, that bioload would somewhat be of a di erent microbiological character. We could also predict finding higher aero- bioloads in those areas of the room with the least air circulation, greatest occupancy, and highest level of activity. On the other hand, logic would tell us that walls, ceilings, light fixtures, and windowpanes would pose the lowest risk of disease transmission. All of this information would significantly a ect our approach to cleaning and disinfecting by establishing priorities as well as aiding in the selection of methods and materials needed to complete the task. This approach is quite unlike the overreaction we saw during the height of the COVID-19 pandemic. We can further refine our bioload estimates by adding time into the predictive model. We know that survivability and proliferation of organisms is determined by the six favorable conditions required for the growth of patho- gens: 1) food, 2) acidity, 3) time, 4) tempera- ture, 5) oxygen, and 6) moisture. By removing any single factor, survivability will decrease exponentially over time. By incorporating the concept of natural decay into our estimates, we can predict a more accurate risk of environmen- tal disease transmission. Using an aggregate of the bioload estimates, we can e ectively direct our suggestions for cleaning and disinfecting to those surfaces and environmental conditions that would have the greatest impact on the pub- lic health of people in these environments. Understand that bioload exercises are in stark contrast to that of laboratory analyses. It is not always possible to link laboratory results to the environment. We know this situation all
too well when we are confronted with looking for causality when it is not readily apparent. In clinical microbiology, the sample provides no information on environmental conditions or on the stochastic population dynamics pro- cesses. To fill in these gaps, we routinely use rapid screening field instruments and tech- niques such as adenosine triphosphate (ATP) monitors, UV lights, and other simple tools to provide us further insight into the ambient environmental conditions. We use these sur- rogates to estimate numbers, survivability, and movement of organisms of concern. Because bacterial growth and decay are universally presented as an exponential function, we report our bioload estimates in terms of their powers of 10—a logarithm (10 x )—without further refinement. For instance, in the school exercise, the bioload on the student desks would be presented as 10 2 to 10 3 , the common touch surfaces as 10 0 to 10 1 , and the walls and ceilings as somewhere between 10 -1 to 10 -4 . We also add as appropriate an area measurement desig- nation to the bioload numbers, such as per 1.0 ft 2 /ft 3 or 1.0 m 2 /m 3 . By way of further explanation, only di erences in log numbers are considered significant, rather than actual numbers that are generated by swabbing, contact plates, or ATP monitors. Because of the variability of the environment, there really is no di erence between the numbers 11 and 99 in the microbial world. There is, however, a significant di erence between log numbers such as 10 3 and 10 5 . In addition to our estimated numbers, we consider the four basic premises of
environmental microbiology in our final determinations. 1. Most microbes do not survive well outside of their natural environment or growth site. We try to characterize the microbes by their origin (human versus nonhuman) and their types by their ideal temperature for survival (mesophilic versus psychro- philic), as well as their significant meta- bolic characteristics (e.g., humidity, oxy- gen concentration, food substrate). 2. Microbes are found everywhere and in every environment, both natural and synthetic. We try where appropriate to provide esti- mates based on surface types. Examples include porous versus nonporous, rigid versus pliable, wet versus dry, and smooth (cleanable) versus rough (noncleanable). 3. There is no uniformity of distribution. Dif- ferent environments di er with the quality and quantity of microbes. A bit of detail related to the di erent and individual envi- ronments goes a long way. 4. Each environment can be considered a sep- arate biosphere, with a characteristic bio- load. We therefore try to report it as such by superimposing the desired outcome of prevention, contamination control, risk reduction, or simply the actions of clean- ing and disinfecting. Our next column will explain the practi- cal use of D-, Z- and F-values, followed by the principles of practical contamination control. In the meantime, please have fun with bioloads.
Contact: powitz@sanitarian.com
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July/August 2024 • Journal of Environmental Health
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