How does a supernova affect humans?
A supernova depletes the ozone layer, exposing humans to radiation.
Just to be clear, in order for the supernova to deplete the ozone layer, the supernova would have to be 33 light years away or less from the Earth. These are called Near-Earth Supernovas. On average, a supernova like this occurs once every 240 million years.
The gamma rays caused by this supernova would reach the upper atmosphere. Here, a chemical reaction occurs where the molecular nitrogen turns into nitrogen oxides. This in turn depletes the ozone layer, exposing humans to solar and cosmic radiation.