3/4 March 2020
The question whether a disinfected surface has to be kept wet for the duration of the exposure time has reached us time and again. It seems like a statement keeps going around about this being a regulatory requirement.
However, this isn't described in any European regulations known to us. It is not correct that the EN method is described thusly, either. The EN generally doesn't intend for surfaces to be kept wet during the exposure time. In accordance with EN 13697, though, exposure times lie at 5 min for bacteria and 15 min for yeasts and molds. Since 100 µl of disinfectant are applied to the surfaces, they almost always stay wet during this short exposure time. The norms therefore do not specify the wetting time, but the amount of disinfectant to be applied in a test. After that, one lets the product take effect for the specified exposure time (no matter whether the surface dries during that time or stays wet). The same goes for longer exposure times as found in national listings like the disinfectant list of the VAH (Germany), the listing of AFNOR or the product specifications of many disinfectants.
Now, there may be concerns in practice that the application in rooms/cleanrooms with an elevated air exchange rate will result in significantly faster drying in practice than during test methods (or during validation on a laboratory scale) and that the efficacy of the disinfection procedure might actually be negatively affected. In that case the monitoring data of long-standing procedures should provide insight into whether there actually is a lack of efficiency. Is that the case - or when a new disinfection procedure is being implemented - the actual drying time on site could be determined and the reduction during that time assessed in the laboratory testing/validation. This would mean that during laboratory testing, the disinfectant would be deactivated after the determined drying time in practice and the efficacy tested. If this is case and the reduction actually isn't sufficient, an increased concentration could be a way to shorten the exposure time.
In general, however, it is valid that a disinfecting agent which has been applied to a microorganism cell maintains its effect on/in a cell even after the environment has dried.
It also needs to be considered that the microorganism concentration in a laboratory test is significantly higher than the germ count in an actual clean room (106 cells, depending in the method). For that reason, even with a lowered reduction rate on a laboratory scale, the limits normally set aren't exceeded in practice, and the monitoring values comply with the requirements.