Complex sentence: In the United States, an estimated 6 to 8 million animals are brought to shelters each year, of which an estimated 3 to 4 million are subsequently euthanized, including 2.7 million considered healthy and adoptable.
Split into simpler sentences: In the United States, an estimated 6 to 8 million animals are brought to shelters each year. Of these, an estimated 3 to 4 million are subsequently euthanized, including 2.7 million considered healthy and adoptable.