What is the optimal fluoride concentration in drinking water set in relation to?

The optimal fluoride concentration in drinking water is set with consideration to the annual average maximum daily air temperature. This relationship is based on the understanding that temperature affects water consumption patterns and, consequently, the exposure of individuals to fluoride. In warmer climates, people tend to drink more water, which increases their fluoride intake. This is why regulatory agencies take temperature into account when determining the optimal fluoride levels, as it helps to balance the benefits of fluoride in preventing dental cavities with the need to avoid excessive exposure.

The other options do not directly influence the optimal fluoride concentration in the same manner, as they relate to different environmental or demographic factors that do not have the same relevance for establishing safe levels of fluoride in drinking water.