Warning: Spoilers Ahead for Supergirl: Woman of Tomorrow #6
In a flashback, Supergirl reveals the twisted reason why Kryptonite is so deadly to Kryptonians. This revelation comes in issue #6 of Supergirl: Woman of Tomorrow, titled “Home, Family and Refuge.” The series is written by Tom King with art by Bilquis Evely and Mat Lopes.
In this 12-issue series, Supergirl travels the galaxy with a companion named Ruthye, whose father was murdered by a man named Krem. Ruthye seeks to hire Supergirl to take revenge against her father’s murderer. Supergirl initially refuses to get involved but changes her mind after Krem wounds Supergirl’s dog named Krypto with a poisonous arrow. Krem then escapes in Supergirl’s spaceship, and the pair are forced to track him across the galaxy.
Issue #6 is primarily devoted to a flashback as Ruthye recalls the origin story Supergirl told her. After Krypton’s destruction, Zor-El, Supergirl’s father, was able to build an atmospheric bubble over Argo City, which survived on a chunk of debris. This asteroid carrying the city floated through space aimlessly, absorbing ray’s of starlight from the yellow suns it passed along the way. This light changed the chemical composition of the land Argo City sat on, causing radiation to seep up from the ground. This radiation gave many of Argo City’s residents cancer and killed them, including Supergirl’s mother.
This leads to a startling revelation that shards of Kryptonite weaken Superman, Supergirl, and others by inflicting them with an extreme form of cancer-causing radiation poisoning. It’s also safe to assume that once the Kryptonite is a safe distance away, their super healing removes the cancer from their bodies. This situation is ironic, as the same yellow sun that gives them their powers is also what made the debris from their home planet so toxic for them.
There are many, minute details in this origin story that give small glimpses into the relationship between Kryptonians and their home planet. For instance, while the ground was able to absorb enough yellow starlight to become toxic, the residents of Argo City weren’t able to absorb enough to gain powers. This means that Kryptonite itself is far more sensitive to the light from yellow suns than Kryptonians. Another interesting tidbit is that Zor-El was able to hold back the radiation by crafting a makeshift shield of lead between themselves and the ground. Lead is also notoriously used by villains (and others) to shield themselves from Superman’s powers. So, Supergirl's father should be credited as the first to discover this Kryptonian weakness. Sorry, Lex Luthor.