Which primary contaminant is added to water supplies to prevent tooth decay?

The primary contaminant added to water supplies to prevent tooth decay is fluoride. Fluoride is a naturally occurring mineral that strengthens tooth enamel and helps prevent cavities. When it is added to drinking water, it enhances the remineralization process of teeth and helps to neutralize acids produced by bacteria in the mouth, which can lead to tooth decay.

Chlorine, while commonly used in water treatment for disinfection purposes to kill bacteria and pathogens, does not provide dental benefits. Nitrogen is not typically added to drinking water systems; it is more relevant in discussions about nutrient pollution and harmful algal blooms. Sodium, while present in some water supplies, is used in water softening processes but does not contribute to dental health. Hence, fluoride is uniquely effective and beneficial for dental health when added to water supplies.