The Trump administration is considering a ‘heat ray’ weapon for the border—here’s what to know

The Pentagon has granted the Department of Homeland Security permission to test a controversial and little-used “heat ray” weapon at the U.S.-Mexico border, prompting questions over how safe it is and how it works.

The weapon is called the Active Denial System, and it is considered a nonlethal weapon: it causes pain but is not supposed to damage tissue. It works using millimeter waves, which—true to their name—have wavelengths between one and 10 millimeters. Millimeter waves are also used in full-body scanners, such as those in airports—but the military weapon puts out its 95-gigahertz waves at a higher intensity for just a few seconds at a time, says Kenneth Foster, a professor emeritus of bioengineering at the University of Pennsylvania.

This energy penetrates about 0.4 mm, the thickness of about four sheets of standard printer paper, of the skin, where it excites water molecules in cells and heats them up. The feeling is reportedly similar to standing in front of a hot oven. “It causes intense pain, but the exposure is not long enough to do damage,” Foster says.

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If the weapon is used at the border, it will mark one of the few uses of this technology ever since its development in the early 2000s, as CNN reported on Wednesday. According to the nonprofit Physicians for Human Rights (PHR), the system was used briefly in Afghanistan in 2010 but has not been employed in warfare since then. Prison officials in Los Angeles considered testing the device for riot control in 2010, yet pushback from civil rights groups and the Federal Communications Commission led to the program being scrapped.

“The weapon is politically toxic,” Foster says.

The system is bulky and temperamental; it doesn’t work well in rain, fog or mist because the moisture attenuates the heating effect on the targeted person. Foster suspects that the system’s cost, expense and bulk have a lot to do with why it hasn’t been used more; other nonlethal methods such as sound cannons are cheaper and more available, he says.

Persistent concerns about the safety of something that sounds suspiciously like a microwave oven are also a barrier. Microwaves, with their longer wavelengths, penetrate farther into skin and can cause more damage. Military testing suggests that exposure to millimeter-wave nonlethal weapons does not cause skin damage. Tests on volunteers in 2005 and 2006 did lead to a handful of people getting skin blisters and rashes, however. An incident in 2007 in which the weapon was set at the incorrect power and pulse duration led to one airman receiving second-degree blistering burns on 8 percent of his body. So it is possible that improper usage could cause injury.

“There is very, very little published on these weapons,” says Rohini Haar, an emergency physician at the University of California, Berkeley, School of Public Health and a medical adviser at PHR. Without published research, she says, there is little information on their safety in general or on potential differences in safety in real-world usage versus experimental or training exercises.

John Erath, Senior Policy Director for the Center for Arms Control and Non-Proliferation, a nonprofit, points out that testing such weapons in human trials for safety is “an ethical dead-end.”

PHR has also documented numerous cases of crowd-control weapons being used against people with no easy way to escape. Indiscriminate use of the system could cause injury not just from the heat itself but also from the reaction of those targeted, says Michael Crowley. He is a senior researcher at the U.K.-based nonprofit Omega Research Foundation, which tracks the use of military and police weapons and human rights abuses.

“Given its silent and invisible nature, the ‘pain ray’ makes avoidance of the weapon difficult, which could prevent people from dispersing safely, lead to panic-driven stampedes and consequent serious injuries,” Crowley says.

“Furthermore, those subsequently injured or crowd members who have existing mobility difficulties, including the elderly and very young, may not be able to leave the area and then be subject to repeated or prolonged exposure of this weapon, potentially leading to further intolerable pain and serious injuries,” he adds.