Much of the concern related to ozone therapy revolves around the safety of blood ozonation. When inhaled by mammals in high levels, ozone reacts with compounds in tissues lining the lungs and triggers a cascade of pathological effects including pulmonary edema; however, ozone therapy does not usually involve inhalation of ozone gas. It has been argued that while peroxides (a product of ozone) are naturally generated inside phagocyte cells to kill bacteria, outside the cell they can damage tissue. Proponents suggest that its effects are tissue dependent, though the subject is still debated.