Children up to age 8 can develop mottled enamel from taking tetracycline.

During this time he studied the mottled enamel situation which was affecting Native Americans.

Fluorosis leads to mottled enamel and occurs from overexposure to fluoride.

Fluorosis is a condition resulting from the overexposure to fluoride, especially between the ages of 6 months to 5 years, and appears as mottled enamel.

In the second half they speculated that fluoride would protect against tooth decay, proposed supplementing the diet with fluoride, and observed mottled enamel (now called severe dental fluorosis) without knowing the cause.

With the help of his fellow investigators and the cooperation of dentists and other health workers in the field, it was established that high amounts of fluorine in drinking water caused mottled enamel on the teeth, while at the same time precipitating lower rates of dental caries (cavities).