The WHO draft guidance is a bit confusing. Section 5.5.1 states that HEPA are not required to obtain non-sterile operation. Then goes on to explain how to do HEPA testing throughout the rest of the document. Please bear in mind that WHO is actively searching for feedback and comments. In ISO 14644-2, recovery tests are listed as “optional” for all areas. I personally would apply a risked based approach to this. Do you have HEPA filters? Are there low air returns? What is the nature of operations (high particle generation?)
I’m a little surprised that WHO is asking for a recovery test for ISO 8 areas (especially when they state that HEPAs are not needed for ISO 8)
That being said. If a recovery test is requested. The ISO guideline is available for purchase, with the full equation. Unfortunately, the ISO is a copywritten document, and I cannot reproduce it (my apologies).
Steering you slightly here. Overall ISO (and WHO) first recommends doing the 100:1 test, where you start out with 100x the allowable concentration, Then turn the air handler on, and see how long it takes to get the air to within the allowable concentration. Using this method, there is no “correction equation” to correlate the “slope” method with the 100:1 method. The slope method is only used when the 100:1 method cannot be performed for some reason.
The 100x concentration can easily be obtained using an aerosol. In my experience a company called TSS was able to come and “fog” the room to obtain 100x the allowable concentration.
Overall, if you still need to do the recovery tests listed in ISO14644-3 B.12.3.2, It is listed very well, the calculations are not too difficult, and the testing method is straight forward.