Researchers said while the major source of fluoride is the presence of fluorine-bearing minerals in rocks and sediments, human activities such as use of fertiliser and the release of untreated industrial waste add to the level.
According to the study conducted by researchers from Bishop Heber College in Tiruchirappalli, heavy metals such as chromium, lead, and cadmium were found in the feathers of all house crows and house sparrows that were sampled.