I saw a headline saying that donating blood lowers the level of forever chemicals in your body. This post will give a back-of-the-envelope calculation to show that this idea is plausible.
Suppose there are chemicals in your bloodstream that do not break down and that your body will not filter out. Suppose you have about 5 liters of blood and you donate 500ml of blood at a time, 10% of your total blood volume.
Presumably the blood you donate has the same proportion of forever chemicals as the blood you keep, so you lose 10% of your forever chemicals in a blood donation.
Assume you don’t absorb more forever chemicals after you start donating blood, and your body replaces the donated blood with new blood free of forever chemicals.
The quantity of forever chemicals in your blood after n donations is 0.9n times the original amount. How many donations would it take to reduce your level of forever chemicals by half?
We need to solve
0.9n = 0.5.
Taking logs we find
n = log(0.5)/log(0.9) = 6.58.
So after 7 donations, you should have reduced the level of forever chemicals in your blood by about a half. Assuming you donate every 8 weeks, this would take a little over a year.
This is just a simplistic calculation. The result could be inaccurate or even entirely incorrect for any number of reasons. But it does show that the idea that blood donation lowers forever chemical levels is plausible.