If reports that a combination of sarin and chlorine gases killed more than 40 people in Douma, Syria are true, many of the survivors should expect to recover physically. The chemical weapons themselves won’t linger in the town, but for some people, the fear and anxiety left behind by a chemical attack will.

There’s still a lot we don’t know for certain about the suspected chemical weapons attack said to have struck the town of Douma this weekend. Photos of limp children breathing through masks or covered in tubes have been pouring from the town, the victims of what might have been a combination of chlorine and sarin gases, Reuters reports based on photos, videos, and witness statements from the scene. The Syrian government has been accused of using both in the past, according to the Associated Press.

Syrian and Russian officials have denied that a chemical attack even took place — but government forces are keeping journalists and investigators from getting into Douma and checking for themselves, The New York Times reports. As a result, we don’t know precisely how many people have died, although reports tend to range from between 40 and 80. And at least 500 people who have survived the attack have sought medical care, The New York Times says.

We’ve seen chemicals leave behind toxic legacies before — like a compound in Agent Orange that has lingered in Vietnam for decades, harming the health of those exposed and of their kids. But unlike Agent Orange, neither chlorine nor sarin will contaminate the region for long. Chlorine gas blows away within minutes, says Cheryl Rofer, a former Los Alamos National Laboratory chemist who has now retired. Sarin sticks around a little longer than chlorine — because it’s not actually a gas, Rofer says. It’s a liquid that’s released in a spray of droplets, so it evaporates or breaks down within days, at the longest.

Both chlorine and sarin act quickly on the body, too; chlorine can burn the eyes, nose, and throat, and cause fluid to build up in the lungs — which can make people feel like they’re choking, according to the Centers for Disease Control and Prevention. Most of the people who die from chlorine gas exposure die of suffocation within a day, the CDC says. People who were healthy before inhaling chlorine gas and survive generally recover within two weeks — but sometimes, they can suffer from lingering trouble breathing. That’s relatively rare: Less than 5 percent of 700 British soldiers who were exposed to chlorine during World War I, for example, still had ongoing lung problems like bronchitis four years later.

Sarin, too, “presents an immediate but short-lived threat,” the CDC says. “Mildly exposed people usually recover completely. Severely exposed people are less likely to survive.” Much of what we know about sarin gas exposures comes from a long-term study of the victims of sarin attacks in Japan. The doomsday cult Aum Shinrikyo released sarin gas in Matsumoto, Japan in 1994, and in the Tokyo subway in 1995 — killing 21 people, and exposing more than 6,000 to the nerve agent. About 10 percent of the survivors experienced nerve damage and problems walking — but those symptoms cleared up by about three months after the exposure, according to the study published in the Journal of the Neurological Sciences in 2006. More than a third continued to report vision problems and eye pain for several years after the subway attack.

But it’s not just physical symptoms that linger. After all, chemical weapons are designed to inspire terror — and you don’t forget the times you couldn’t breathe. “The feeling of choking is an incredibly powerful one,” says Cindy Vestergaard, director of the Stimson Center’s nuclear safeguards program. “The long term psychological effects of being bombed and poisoned, those are things we can’t discount,” says medical toxicologist Peter Chai, a professor of Emergency Medicine at Brigham and Women’s Hospital.

About eight percent of the sarin attack victims from the 1990s were diagnosed with post traumatic stress disorder, the study says. The trauma kept people from going to back to life as usual: three years after the Tokyo attack, nearly 14 percent of victims surveyed told researchers  that they still couldn’t ride on the subway. That’s the point of chemical weapons: to inspire the kind of terror that keeps even the survivors from living their lives. In that way, chemical weapons have a much longer reach than the chemicals themselves.