The terrorist bombings in Brussels on Tuesday that killed more than 30 people were a reminder of more than just the vulnerability of urban infrastructure to dedicated individuals with explosives. They also pointed out that different kinds of infrastructure come with different kinds of dangers.

Case in point: an enclosed space like a subway station can magnify the effects of an explosive. Blast injuries like the ones people suffered in Brussels’ Maelbeek subway station are much worse than those incurred in open air. It’s a matter of physics and physiology.

After two decades of attacks in subways in London, Paris, Madrid, and Israel, the somewhat gruesome field of inquiry into blast injuries in enclosed spaces has yielded some changes. Station platforms have fewer trash cans and more open space now, and less glass at ground level. That means less stuff to turn into projectiles. But underground rail stations are still one of the most difficult types of spaces to protect and make safer.

That’s because of how explosions work. “The initial kinetic energy generated by the blast is a gigantic pressure wave, like a sonic boom,” says David Lemonick, a retired Pittsburgh emergency room physician who wrote a blast injuries primer for ER docs. “There’s an initial overpressurization that blows out your eardrums and your lungs, knocks you down, sends you flying, and then picks up other stuff in its wake and shoots it all out in different directions.”

The gas-filled spaces of the body–lungs, sinuses, the inner ear—are particularly vulnerable to the overpressure. And traumatic damage to them isn’t always visible or immediately apparent to first responders as they try to triage the injured.

Walls and hard surfaces reflect the blast wave, magnifying the effect.

That initial high-energy pressure wave only lasts about 10 milliseconds. After the initial compression of the air, the blast wave passes, and the air is under-pressurized. If that happens in an enclosed space, walls and other hard surfaces reflect the blast wave back, creating secondary pulses. Windows break outward, but floors reflect and magnify the effect.

After these fast-moving waves subside, then come flying projectiles, pieces of the bomb (apparently the Brussels bombs were packed with nails), parts of whatever exploded, broken glass, and other objects. Getting hit with any of that causes what physicians call secondary injuries; tertiary injuries—usually head injuries and amputations—occur when a person gets thrown through the air by blast winds and hits the ground or another object. The final, fourth, level of trauma comes from burns.

“The same blast in the outdoor world would have an effect,” Lemonick says. “But the overpressurization in a subway is particularly devastating.”

How much worse are enclosed spaces? Researchers in Israel compared bombings on Jerusalem buses, and found that in two cases—during a chilly winter when the buses had their windows shut—passengers had a significantly higher risk of death and serious injury than people waiting for the bus at a stop.

While the United States has been mostly free from subway terrorist attacks, they have been part of urban life in parts of Europe, the Middle East and Asia for decades. “The challenge in protecting subways is that they are open targets,” says Christopher Cherry, an engineer at the University of Tennessee. “They are crowded and accessible. To design security into subways is difficult in the conventional sense of active security, like bag checks and metal detectors.”

Still, Cherry and colleagues did come up with a few ideas for making subway stations safer. In an 2008 article in the Journal of Architectural and Planning Research, they proposed making interior spaces more open and cleared of obstructions. That lets passengers see what’s going on around them, and it lets a blast wave disperse, reducing secondary injuries. Less glass means less broken glass. They also found that train cars and seats with materials that don’t burn very well, such as in a 1996 Paris Metro bombing, saved lives.

Not all ideas for making subways safer are necessarily good ones. Cherry often visits China where he studies rail transit systems that have become more like our airports. Passengers face pre-platform screening and metal detectors, which might lower the risk of attacks on the platform but also creates yet another chokepoint for potential terrorists.

Belgian officials say the subway bomb went off inside a subway railcar in the middle of a three-car train that was leaving the Maelbeek station. The conductor managed to stop the train and evacuate passengers from the two outer cars, but 20 people died. After the London Underground bombings on 7/7 in 2005, designers proposed flimsier subway cars that would (in theory) fall apart and disperse a blast on board rather than contain it. “You have to weigh that against the utility of a subway car in the first place,” says Eddie Chaloner, a London vascular surgeon and former military doctor who wrote about blast injuries after 7/7. “It would be hazardous in everyday use.”

No “see something, say something” awareness campaign can change that. “Whatever system you devise to prevent this or mitigate the damage, if the opposition is smart they will always come up with way to go around it,” Chaloner says. “You can’t protect everyone completely.”