I will now go into more detail on these issues below.
Concentration is measured in milligrams (mg) of fluoride per liter (1 mg/liter = 1 part per million or ppm). This can be controlled at the water works. Dose is measured in mg/day and this cannot be controlled as it depends on how much someone drinks – and some drink a lot – and how much fluoride they are getting from other sources. It is the total dose that has the potential to harm someone. The concentration (mg/liter) offers no guarantee of safety. It is actually worse than that, which brings us to part b) above.