**6. Discussion**

Physical pathogen inactivation approaches have a number of advantages. First among these is the fact that these approaches display efficacy for a broad range of pathogen types, up to and including bacterial and fungal spores. In the hierarchy of pathogen susceptibility to microbicides (sometimes referred to as the Spaulding scale [82]), only infectious proteins (prions) may remain resistant to these physical approaches as normally applied [83, 84]. Per the established hierarchy with regard to viral inactivation [85–88], non-enveloped viruses display much greater susceptibility to microbicides, while enveloped viruses are considered to be among the most susceptible of all pathogens to microbicides. For physical inactivation approaches, this hierarchy may be somewhat different. As mentioned in the introduction to this chapter, the orthogonal physical approaches may display complementary efficacies for different virus families, and efficacy is not solely determined by envelope status or particle size.

Secondly, to a certain extent, the physical approaches require additions of photons to the inactivation matrix, not molecules—as in the case of chemical inactivation. This means that the physical approaches can be used without the necessity of removing the inactivating agent from the inactivation matrix. For example, gamma irradiation can be applied to finished product in sealed containers, ultraviolet irradiation can be applied through glass or plastic tubing, and heat can be applied to containers of liquids. Each of the methods can be applied to surfaces without the need to subsequently remove an inactivating agent.

The first-order behavior of physical inactivation approaches, discussed previously in this chapter, is also a useful attribute. For instance, gamma irradiation and UVC inactivation efficacies are typically first-order with respect to applied fluence. Efficacy of heat inactivation is typically first-order with respect to time at any given temperature. This means that once a *log10 inactivation per fluence* value is obtained, efficacy at a different fluence (gamma irradiation or UVC) can be estimated. Similarly, once a *D* value is obtained at a given temperature for heat inactivation, the efficacy for a different contact time can be estimated with some confidence.

In this chapter, we have attempted to convert, where possible, inactivation results from different reports into the *log10 inactivation per fluence* values and the *D* values discussed above, so that the readers can make informed estimates of inactivation efficacy for these approaches under non-evaluated conditions. These estimates are quite straightforward in the case of gamma and UVC irradiation. For example, if 2 log10 inactivation per kGy gamma irradiation or per mJ/cm<sup>2</sup> UVC is measured in a study, then 4 log10 inactivation should be expected at 2 kGy or at 2 mJ/cm<sup>2</sup> . For heat inactivation, if the *D* value at 65°C is 10 min, then 2 log10 inactivation should be expected after 20 min at the same temperature. The equations for the power function line fit of *D* vs. temperature plots [6] also allow one to estimate inactivation efficacy for non-measured temperatures. The plots shown in **Figures 1** and **2** can be thought of as depicting a 1 log10 inactivation *surface*. Any point on the line reflects the conditions necessary to achieve 1 log10 inactivation. Points to the right of this line will result in greater than 1 log10 inactivation, while points to the left of the line will result in less than 1 log10 inactivation.

As is apparent from this chapter, the three physical inactivation approaches discussed (gamma irradiation, UVC irradiation, and heat inactivation) each display *Physical Inactivation of SARS-CoV-2 and Other Coronaviruses: A Review DOI: http://dx.doi.org/10.5772/intechopen.103161*

efficacy for all members of the *Coronaviridae* family and for SARS-CoV-2 in particular. The different approaches may be useful, in particular, for different applications. For instance, in case of IPAC, of the three approaches, UVC is most useful for decontaminating indoor air. For such an application, indoor air to be recirculated is passed through a unit which exposes the air to an appropriate UVC fluence. This can be done while the indoor spaces are being occupied. For surface inactivation, each of the three approaches may be useful, depending upon the surface to be decontaminated. For decontamination of liquid matrices, again, each of the three approaches could be useful. The disadvantages of the three approaches are:


For each of these physical approaches, a balance must be achieved between the desired log10 reduction in infectious virus level and the need to retain the desired attributes of the material being decontaminated. This includes inanimate surfaces, such as plasticware in the case of gamma irradiation [11]. To put this in another way, users are not always free to use extremely high fluences of gamma or UVC radiation, or extremely high temperatures as a means of assuring decontamination. Each of these physical approaches are capable of causing unintended damage to biological solutions and material surfaces. Treatment of indoor spaces with UVC radiation must be conducted when those spaces are unoccupied by humans.
