The 007-meets-the-X-Files adventures in Cuba continue. Last week the US Department of State recalled non-emergency personnel and families home from the embassy in Havana, citing injuries and illness among 21 people—“hearing loss, dizziness, headache, fatigue, cognitive issues, and difficulty sleeping” according to a statement from secretary of state Rex Tillerson.

Those 21 people weren’t just cultural attachés. Some of the hardest-hit victims were US intelligence operatives, according to an AP story on Monday. Which is to say: Someone in Cuba has been remotely doing something mysterious to US spies’ ears and brains. Call it spook action, at a distance.

Most of the reporting on this story so far has talked about some kind of a “sonic weapon” or “sonic attack,” maybe a side effect of a surveillance technology. The problem is, physicists and acousticians don’t know how ultrasound (high frequency) or infrasound (low frequency) could do what the State Department says happened to its people. That leaves two possibilities: a new sci-fi sound gun or something else.

Here’s a hypothesis for the something else: poison.

Just to rewind a little bit: The reported injuries vary, from hearing loss (potentially permanent) and dizziness to confusion, headache, and even mild brain trauma. To audiologists and otolaryngologists, that suggests damage or injury in both the inner ear, which converts sound waves into neural impulses and regulates balance, and along neural pathways reaching into the brain. Some of the affected people reported hearing weird noises, sometimes only in specific parts of specific rooms—but others didn’t.

It may be that this was a test case, and they wanted to see how useful it was, whoever unleashed it on us. Edward Boyer, Harvard Medical School

Immediately that suggests some kind of focused acoustic attack. But nobody will admit to knowing about any technologies that can do all that. “Nothing about this story makes any sense to us,” says Robert Putnam, senior marketing director at LRAD, which makes the long-range acoustic device that a cruise ship deployed against pirates in 2005. But the LRAD uses audible—very, very audible—sound. Most of the Cuban attacks seem to have been inaudible. “If it’s infrasound, they’re not really hearing it, and you’d have to pump a tremendous amount of energy into the ground,” Putnam says. “If it’s ultrasound, it attenuates very quickly, and if you pumped a lot of energy into it, it’d heat the skin.”

And technologies that focus a beam of audible sound at a single spot don’t have the kind of range our nominal magic sci-fi sound gun would need—as of 2010 “you had to be within five or 10 feet of the emitter for it to have an effect,” Putnam says. Of course, maybe the magical sci-fi sound gun has made significant technological progress since then.

But sound isn’t the only thing that affects hearing and the brain. Chemicals do, too. It’s called ototoxicity—toxicity to the ear—and it’s a known side effect of, for example, some chemotherapeutics and antibiotics. The widely used cancer drug cisplatin can cause hearing loss, as can the category of antibiotics called aminoglycosides, which includes streptomycin and neomycin. Basically, those drugs get into the fluid-filled ductwork of the inner ear and damage the hair cells, which talk to the nerves that lead to the brain.

Chemotherapeutics and antibiotics need to be administered intravenously to have an effect, though, so they don’t fit the facts. Heavy metals like lead and mercury can be ototoxic, but they also stay in the blood for a long time; the State Department hasn’t released specific test results for its people—maybe it won’t—but it’s safe to assume they got blood tests.

Another class of ototoxins: solvents, like the cleaning product xylene or styrene, used to make glass-reinforced polyesters. Some linger in the blood, but others have a shorter half life. They off-gas from new carpet, paint, and furniture—it’s why some new homes aren’t immediately ready for occupation. “Some of these are going to be obvious. You’re going to smell it,” says Kathleen Campbell, an expert in ototoxicity at Southern Illinois University’s School of Medicine. “But when we study this, it’s usually through industrial exposure. We’re not looking at the possibility that it could be weaponized and have the aroma taken away.”