An American in Afghanistan who claimed he was kidnapped by the Taliban may have actually voluntarily joined the terrorist group. He also may have offered them intelligence, most of it unremarkable—like the fact that U.S. soldiers’ torsos are protected by bulletproof vests. That got the Explainer thinking … Why don’t soldiers wear bulletproof face masks?

Because they’re hot, cumbersome, and ineffective against Taliban guns. There are lots of bulletproof masks on the market, and law enforcement agencies wear them in close-combat situations like hostage rescues, drug busts, and other raids. They’re especially useful against shotguns, which spray their pellets everywhere. But the masks don’t work well on patrol in Afghanistan. They’re uncomfortable, limit vision and mobility, and don’t breathe at all. While ballistic face masks protect against IED shrapnel, they can’t stop a bullet from an AK-47 Kalashnikov, the Taliban’s firearm of choice. Face shields would also make our soldiers look like Stormtroopers, which is not the image the Pentagon wants to present to the Afghan civilian population.

Because body armor can be so heavy, military commanders make mission-specific decisions about how much protection their charges need. The highest level of armor most soldiers wear in Afghanistan covers the entire thorax, parts of the neck, the biceps, shoulders, and the groin. There are special ceramic plates to protect the central organs from AK-47 fire—the rest of the system will stop only lesser projectiles. The full armor complement adds 33 pounds to a soldier’s weight. It’s not easy to lug that around Kabul at a 5,800-foot elevation, so most days the soldiers wear a lighter system that only covers the thorax.

If you wanted to, you could cover virtually your entire body in bulletproof armor. In addition to a face mask, you can buy a ballistic visor, forearm coverings, and leg protection that runs all the way down to the ankles. But, with the exception of eye protection, which many soldiers wear regularly, the military has tested all of these products and found that their burdens outweigh the benefits.

Stopping a bullet with a ballistic mask doesn’t mean you’ll walk away unscathed. The National Institute of Justice tests all bulletproof equipment for “back face trauma.” The researchers mold a piece of clay into the inside of the gear, then fire a shot at the front. If the bullet displaces the clay more than 44 millimeters, the NIJ fails the product. At that level of force and displacement, a shot to the face mask would really bruise you up and might even break your jaw. You’d definitely survive, though. Certified bulletproof gear is incredibly reliable. More than 3,000 police officers have been saved by bulletproof vests, while the FBI has confirmed one only vest failure, which seriously injured an officer. (Another police officer was killed by a shot to the torso while wearing a vest, but the circumstances of the failure are in dispute.) *

Facial armor has come and gone in military history. Most ancient Greek hoplites wore Corinthian-style helmets, which covered the entire head except the eyes. The Normans who overran England in 1066 wore helmets but fought with their faces exposed. Henry V’s knights cast themselves into the breach at Harfleur and Agincourt armored from head to toe. The development of armor-piercing bullets led to the downfall of the full suit. While Kevlar revived the popularity of body armor in the 1960s, the face shield mostly remains a thing of the past.

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Explainer thanks Lt. Col. Alayne Conway of the U.S. Army, Sheila Jerusalem of the National Institute of Justice, Shaun Paul of Bluedefense.com, and Pat Stallings of Point Blank Body Armor.Like Slate and the Explainer on Facebook. Follow us on Twitter.

Correction, Oct. 13, 2010: This article originally stated that the FBI has never recorded a vest being penetrated by a bullet it was designed to stop. In 2003, Pennsylvania police officer Edward Limbacher was wounded when a bullet penetrated his vest. In another 2003 incident, California officer Tony Zeppetella was killed by a shot to the torso while wearing a bulletproof vest. An investigation into the circumstances was inconclusive, though. The manufacturer claimed that bulletproof vests aren’t designed to stop bullets coming from certain angles, and the company reached a settlement with the deceased officer’s family. (Return to the corrected sentence.)