Improvement in space counts from increased outdoor air, use of HEPA filters or an UV-C disinfection system is subject to practical limitations. Increasing ventilation in terms of K values greater than 6 produce diminishing returns. As an example, adding more than 15 to 20 ACH to existing ventilation will only further improve space counts marginally (see graph below). The model equation for these outcome calculations is derived from work presented by Richard Riley and Edward Nardell in "Clearing the Air," American Review of Respiratory Diseases, 1989: 139: 1286-1294.

To determine K value, add the total ACH from the existing mechanical ventilation (A) to the added ACH from outdoor air, use of HEPA's or a UV-C disinfection system (equivalent or added) (B) divided by existing mechanical ventilation (A) or A + B / A. For example, adding 12 ACH with an UV-C disinfection system to an existing ventilation of 3 ACH (12 + 3 � 3) produces a K value of 5 or an outcome improvement of about 81%.

The degree to which space counts are improved is estimated, in theory, using the expression described above. It should be understood that this is an improvement over what currently exists. For instance, adding 15 ACH with UV-C disinfection system to existing ventilation of 4 ACH instead of 3 would improve the space counts by only 80% (15 + 4 � 4). For planning purposes any system's bacterial dilution, removal or inactivation along with its air moving capabilities must be quantifiable, and excellent air mixing must be achieved.

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