Symbolism, Imagery, Allegory
In the book, there are lots of references to keloid scarring, which a lot of Hiroshima residents experienced as a result of the bomb/their burns. As Hersey notes: "Most burns healed with deep layers of pink, rubbery scar tissue, known as keloid tumors" (4.17).
Because schoolgirls had been released from school to work on the construction of fire lanes on the day of the atomic bomb explosion, there were a large number of girls who got severe, disfiguringly burned as a result. Girls in this position were known as "Keloid girls" or the "Hiroshima Maidens."
It doesn't take Sherlock Holmes to figure out the symbolism here. In emphasizing the physical scars that the inhabitants of Hiroshima suffered, Hersey draws attention to the more metaphorical scars that the city and its residents endured—and were still reeling from when Hersey encountered them.
If you want to experience a really gut-wrenching, visceral reaction to the bombing of Hiroshima, check out images of "Hiroshima Maidens." For an (very minor) example of this horror, check out this.