"Old age" in of itself is not a direct cause of death; the direct cause can always in theory be attributed to a specific disease or failed organ system. However, over time, in most (but not all) organisms, many critical biological systems experience chemical or physical damage which eventually accumulate enough to cause failure. This is a significant problem in humans; it is estimated that of the roughly 150,000 people who die each day across the globe, about two thirds—100,000 per day—die of age-related causes.[2] In industrialized nations the proportion is much higher, reaching 90%.[2] Thus as an indirect cause, biological aging is by far the leading cause of death.