The concentration of soluble Fluoride reduces rapidly through salivary dilution. Typically the concentration halves after every minute so that after around 90 minutes it is unlikely for any therapeutic level of Fluoride to be present.
In contrast, BioMin™ F is able to supply a low level of Fluoride for up to 12 hours after brushing. The slow and controlled supply of Fluoride is far more efficient in it's use of Fluoride. For this reason the level of Fluoride content in BioMin™ F does not need to exceed 600ppm.
The image below shows the soluble Fluoride decrease (light blue dots) over time. Compared to the Fluoride release of BioMin™ F toothpaste over the same time period (green line).