The figures I've seen are much higher than that. By the end of the day, the lifeguard should have produced 20,000IU, but that is mostly because after the concen

The figures I've seen are much higher than that. By the end of the day, the lifeguard should have produced 20,000IU, but that is mostly because after the concentration reaches about 20,000, the sunlight begins to break down any excess, so it cannot climb higher than that. Interesting chemistry. So it begs the question - where did the "4000IU max intake" figure come from?