What happens to bullets fired directly upwards?

Thanks to Yousef Hyder and Luis Rafael Moscote Salazar for the answer!

James -  Celebratory gunfire is fairly widespread as you suggest, with examples across the world from the Americas to the Middle East.

Bullets leaving a gun barrel experience explosive acceleration. Depending on the firearm, a bullet fired straight upwards can rise several kilometres before gravity halts its climb and pulls it back down. Because bullets are often made of lead, which is very dense, falling bullets encounter relatively less air resistance and reach high terminal velocities, up to 180 metres per second.

At these speeds, they can penetrate the human skull. Yousef Hyder is a neurologist at the University Hospitals in Birmingham. I asked him what the effect of such an impact could be.

Yousef - The bullet will damage the skull and impact on the brain itself, but then following on from that you'll get secondary effects. So they'll be bleeding inside of the skull, which we call intracranial hemorrhage, and then the actual brain itself will start to swell up. When that happens, the pressure inside of the skull will start to rise. So because the skull is basically a rigid box, it can only handle so much pressure and eventually the body is not able to compensate for that and the pressure will start to spike up exponentially. What can happen is herniation, which basically means that the brain is being pushed out of its normal position and it can even move through openings in the skull. Basically, parts of the brain which are very critical for health, like the areas that control breathing and your heartbeat, those will get squashed and that's when it becomes life-threatening.

James - A penetrating injury is thus very serious. But aside from these most extreme cases, less severe damage could also lead to lifelong consequences for a patient.

Yousef - So if the bullet's falling in such a way that it's tumbling and it's not at such a high speed, then it can impact the head without causing a penetrating injury but still definitely can cause severe trauma. It can knock a person unconscious and it can cause what we call concussion as well. They are prone to having very long-term effects, months and even a year later, long-term headaches, problems with balance, sleep, thinking, even emotional problems, all because of this injury.

James - So how common are these injuries? Well, my thanks here goes to Luis Rafael Moscote Salazar, director of the Colombian Clinical Research Group in Neurocritical Care. For his consultation when composing this piece, he conducted a literature review on this very question. He found that if you're unlucky enough to be struck, the mortality rate from a falling bullet is estimated at around 32%. One reason for this is that victims of falling bullets rarely have protective equipment such as helmets. After the Iraqi football team won the Asian Cup in 2007, celebratory gunfire killed three people. In Puerto Rico, New Year's Eve gunfire in 2004 caused 19 injuries and one death. So, Dr Moscot Salazar's conclusion?

Luis - My advice would be try finding a different way to celebrate.