It is stated in many articles that
source 1, an article

I am making this post to prove this statement to be false, with a simple google search I have come to the conclusion that Cecil Kelley is actually the most radioactive man in history.

Everyone says Hisashi received the most lethal dose, yet he survived 83 days.
However Cecil Kelley was reported to have received a MUCH higher concentration of 36 Sieverts, over double Hisashi's dose. Cecil died in less than 48 hours.
A final measurement was taken from Cecil's blood after he passed, it measured 4,900 RAD which equals to 49 Sieverts. Article, Source,RAD to SV

Nobody talks about Cecil Kelley.

Now here is what I have figured out. There is an image of a man who appears to be missing all of his skin that went viral years ago. This image is not Hasashi Ouchi.
It has been thrown into articles about Hisashi to create buzz, and later believed by many to really be Hisashi. Hisashi's hospital stay of 83 days was well documented.
His well documented stay along with the viral image along with article titles stating "worlds most radioactive man!" Were combined to create buzz and generate views.
After a while people believed these articles as 100% truth even on wikipedia.

They are not.

Cecil Kelley is actually the most radioactive man in history.