After a nuclear explosion, soot rises into the sky. It settles high above the clouds, in the stratosphere, and stays there for years. This soot acts like a thick curtain: it blocks some sunlight and alters the chemistry of the sky. As a result, ultraviolet radiation at Earth's surface changes — sometimes stronger, sometimes weaker, depending on the scale of the disaster.
Scientists ran two scenarios. In the first — a regional conflict between India and Pakistan—5 million tons of soot enter the sky. In the second, a war between the US and Russia, there is 30 times more soot. In the first case, the ozone layer, which protects us from UV, breaks down faster than soot can block the rays. UV at the surface increases, and safe sun exposure time drops sharply. This is especially dangerous for people with fair skin, who burn more quickly.
In the second scenario, there is so much soot that it blocks sunlight more than ozone is destroyed. UV at the surface decreases, and for a while, skin cancer risk even drops. But this is no salvation: soot settles, the ozone layer recovers slowly, and after years, UV rises again. Plus, lack of sunlight itself harms plants and agriculture.
If we take Earth's population in the year 2000 and apply these UV changes to it, we get these numbers. In a regional war, in the next 10–15 years, an additional 5 to 10 thousand people will die from skin cancer, and up to 75 thousand over a century. In a global war, in the first 15 years, there will be 8.5 thousand fewer cases, but then the curve climbs again. These deaths are not from blasts or famine, but from a sun that has changed.
UV doesn't just hit people. Plants that feed us and animals in fields and forests also get an extra dose. What happens to them hasn't been calculated yet, but one thing is clear: nuclear war doesn't end with explosions. It changes the very sky above us, and that sky remains different for the rest of several generations' lives.
The ozone layer, which protects us from UV, sits at an altitude of about 20–30 kilometers. If all the ozone were collected into a single layer at normal pressure, it would be only 3 millimeters thick, like a window pane.
In Nevil Shute's novel "On the Beach," people die from radiation after nuclear war, but only years later. Altered ultraviolet is just such a silent killer, acting not immediately.