The risk of freediving blackout is not known as there are currently no rigorous data on freediving blackouts. However, the estimated, average, annual fatalities attributed to freediver blackout over a period of ten years in a population of approximately 135,000 divers in nine countries was 53 per year, or one in 2,547. The total number of fatalities appears to have remained unchanged in recent years but it is not possible to calculate the fatality rate because variables such as the number of dives or the diver population are not known.