Rhetoric around the use of such weapons has ramped up on both sides, culminating in President Joe Biden’s warning of nuclear “Armageddon” on October 6. There is no intelligence to suggest an attack is imminent—but what would happen to the world’s most-populated cities if a nuclear war began?

“It really depends on the size of the weapon,” Moritz Kütt, a research associate at the University of Hamburg’s Institute for Peace Research and Security Policy, told Newsweek.

“The size is usually measured in kilotons, which is 1,000 tons of explosive. The biggest conventional weapon we have is 10 tons…The Hiroshima weapon was 13,000.”

Today, weapons are even more powerful. “The arsenals include weapons that have hundreds of kilotons… even megatons,” Kütt said.

Their enormous size is not the only thing that separates these nuclear weapons from conventional explosives: they also send out radiation. “You would get up to third-degree burns from the light that is emitted from the weapon,” Kütt said. “If these burns are not treated, you will likely die because so much of your skin will be burned.

“Then, of course, there is the ionizing radiation… So you might not get killed by the explosion, but you might [instead] get killed by the invisible radiation that you cannot see.”

This considers only the immediate impacts of the detonation, and these will depend on how the bomb is detonated. “There are two ways to use a nuclear weapon,” Kütt said. “Air burst and ground burst. With an air burst, you can get the largest radius for this kind of weapon of destruction for cities and people.

“If you do a ground burst, the radius is smaller because the pressure waves hit buildings and don’t spread as far.”

However, ground bursts can have more long-term consequences on the areas surrounding the site of the explosion, in the form of nuclear fallout. When the weapon hits the ground, it takes dust and soot from the surface and catapults it up into the air, Kütt said.

It mixes with the radioactive material from the explosion to create radioactive dust particles that can be transported by the wind for tens or even hundreds of miles.

“It all depends on the wind direction and the wind speed, and also the amount of rain,” Kütt said. If the contaminated dust combines with rain clouds, it can be washed out of the sky as radioactive rain.

These effects will occur no matter where the bomb is dropped, but using one on a city would be particularly devastating.

Alex Wellerstein, a historian of nuclear weapons, created an interactive map, called NUKEMAP, in 2012 so users could visualize the impact of different kinds of nuclear weapons on a variety of geographical locations.

The global population is set to reach 8 billion people in November, and more than half that number—about 4 billion—live in cities. The United Nations estimates that 68 percent of the world’s population will live in urban areas within the next 30 years.

The top 10 most populated cities in the world are:

Tokyo, Japan – population of 37,435,000Delhi, India – population of 29,399,000Shanghai, China – population of 26,317,000São Paulo, Brazil – population of 21,846,000Mexico City, Mexico – population of 21,671,000Cairo, Egypt – population of 20,484,000Dhaka, Bangladesh – population of 20,284,000Mumbai, India –population of 20,185,000Beijing, China – population of 20,035,000Osaka, Japan – population of 19,223,000

Using Wellerstein’s NUKEMAP model, air-dropping a 350-kiloton W-78 bomb on Tokyo, the most-populated city in the world, would produce 760,490 fatalities and a further 2,805,170 injuries.

An air burst over Dhaka, Bangladesh, the most densely populated city in the world, would kill 1,834,390, while a surface burst would kill 1,061,850 and result in a fallout stretching over 7,000 square miles, into Myanmar. This model does not take into account the effects of the city’s infrastructure, or its people.

“In a sense, the human body is kind of a radiation shield, but only because it gets hurt by the radiation,” Kütt said. “You would have a more devastating impact in a city, even if it didn’t have quite as big a radius… The infrastructure would collapse, and there are more people who would be affected.”

Cities also tend to house the majority of a state’s healthcare facilities. “If cities are hit, hospitals are gone, and then there’s no medical treatment. All of these effects come from a single weapon,” Kütt said. “I think the biggest risk is escalation… Russia and the U.S. in particular are prepared to fire weapons at each other within minutes.”

This escalation would bring additional problems too: “Nuclear winter is an effect that comes from all the fires that will be produced by these nuclear weapons,” Kütt said. “[They] bring ash and soot all the way up into the high levels of the atmosphere. Then they stay there and they distribute globally.”

This would block out the sun and cause global food production to grind to a halt. “Most of us will starve.” Nuclear winter would be felt everywhere. “Even if the nuclear weapon doesn’t hit you, a couple of months later you will die anyway because you don’t have enough food.”

This will, of course, affect some sections of society more than others. “If you already have a stressful food supply, then any small change in this will be devastating,” Kütt said. “And the same is true for access to healthcare.”

Wealth disparities are often greater in cities than in rural areas, and so this inequality would be felt even more acutely in urban regions. However, when it comes to ground zero, it does not matter how wealthy you are.

“Nuclear war is very indiscriminate,” Kütt said. “There is some protection in place, but not any amount near to the level that will be needed. And it really depends on the country, and even the state. There’s no prevention for leaders to do this other than morals.

“The only real safeguard to prevent a nuclear explosion is not to have nuclear weapons. There is no small nuclear weapon.”