It is an atomic fireball. More specifically, it is the bright air shell that was first compressed into a growing fireball, about seven tenths of a millisecond after the explosion.
The air within this 450 foot shell is heated beyond the plasma incandescence of x-rays and / or gamma rays emitted by the explosion. If you were inside that fireball, you were dead - most of your body literally took off in a little time to think, "wha?"
So yes, people are clearly hurting gamma rays and x-rays. But on the other hand, traces of radioactive potassium inside your body produce several thousand gamma ray photons by the time you read it, and it will not hurt you. Gamma rays only communicate lightly with matter, and most of those coming from within you fly out of you without interacting with any of your atoms. To some extent, some can cause chemical changes in a single atom, and if that atom acts as part of a DNA molecule, it can lead to cancer.
But this is almost certainly not. We have evolved into a radioactive plan, and our cells have mechanisms to repair such damage. Those mechanisms are not perfect, but we have clear evidence that they depend on the task of handling such low levels of exposure like this.
Evidence:
Everywhere on Earth, soils and rocks emit radiation at low levels, but in some places, notably parts of Iran, India, and Brazil, the local environment emits more than ten times average background radiation. That radiation definitely injured the cells in people living in these areas — we know because we can see evidence of cellular repair mechanisms doing their thing. What we cannot determine is any measurable increase in cancer of other radiation-induced diseases.
So, we know for a fact that ten times that normal background radiation is essentially harmless. Radiation is not "magic death cooties." In fact, most likely in the absence of any radiation exposure, the mechanisms in our cells that are expected to continue to repair this damage may land and cause some type of disease, such as the case of autoimmune diseases that caused by a lack of normal parasites.
So, there is dangerous radiation, there is harmless radiation, and there is ‘suspicious radiation’ that we need to guard, but not too obsessed with.
Traces of uranium and thorium on granite countertops in my kitchen make it four times more radioactive than the rest of the house. I measured myself at a Gieger counter. But four times practical nothing is almost nothing. I, myself, am more radioactive than the rest of the house, so I don’t wear lead underwear while cooking breakfast.
On the other hand, the father-in-law puts his feet on a machine like this to see how his shoes fit:
These machines emit x-rays strong enough to pass a living foot and directly stimulate phosphors in a magnifying glass. That is clearly a dangerous and unacceptable exposure. Like gamma-ray photons, x-ray photons can tear atoms, break down chemical bonds, kill or injure cells, and damage DNA and cause cancer.
Tens of thousands of people used unregulated, high-power machines, and some almost certainly got cancer. But my father-in-law (God help us) is still with us. Meanwhile, digital x-ray equipment is now available that is very sensitive and very low power, the technicians in my dentist's office no longer come out of the room when they scan my teeth.
Gamma rays are the same and less dangerous than x-rays. It is less dangerous because it is more likely to interact with atoms. It is more dangerous because if they do, they can become more destructive of those atoms, reaching electrons beyond the outer shell of the electron.
But in the end, you have to weight risk against benefit.
Today’s dental x-rays carry almost no risk and a huge benefit. Those from my childhood carried a somewhat larger risk and the same huge benefit. The radiation from my kitchen countertop brings little real benefit, but also no real risk. Radiation used to treat cancers brings significant risk, but life-saving benefit. Radiation from the nuclear energy industry brings risks vastly smaller than most people imagine (the industry is actually safer than winds, solar, or hydro-power per kilowatt produced) but helped make possible the post-WWII era of prosperity, and can help stave off climate change.
Radiation is not something to be blindly feared. It’s fundamental to our world. It needs to be understood and managed.