The  Environmental Working Group recently commissioned an independent laboratory test of 10 popular dog food brands, revealing that petfoods made with animal by-products contain higher levels of fluoride.

The test showed that eight of the brands contained fluoride amounts between 1.6 and 2.5 times higher than the US  Environmental Protection Agency's maximum legal dose in drinking water . All eight contained bone meal and animal by-products, presumably the source of fluoride.

The other two brands tested, one with vegetarian ingredients and from a small manufacturer, did not contain detectable levels of fluoride, as seen in the  study's chart .

Scientists have yet to determine a safe level of fluoride for pets.