The recent eruption of Hawaii’s Kilauea has generated apocalyptic scenes of bright red lava exploding hundreds of feet into the sky and burning buildings consumed by the molten rock. But there’s another danger, silent and often unseen, that has been with Hawaiian residents and visitors forever in varying degrees.

In Hawaii they call it “vog,” short for volcanic smog. It’s not a killer, in and of itself.

But it has made tens of thousands sick over the years, feeling as if they have pneumonia or a horrible headache or bronchitis. For those with asthma or other respiratory conditions, it’s worse.

In most of Hawaii, most of the time, there is no vog. People can breathe easy.

But if the winds are unfavorable, vog can spread far from the volcano on the Big Island to affect people as far away as Oahu, 200 miles to the northwest, as it did in 2008 and 2016.

The original source of vog is the sulfur dioxide now spewing from the fissures and vents near Kilauea, according to the Hawaii Interagency Vog Information Dashboard. When sulfur dioxide reacts in the atmosphere with sunlight, oxygen and other gases, the result is a form of air pollution not unlike that given off by sulfurous coal-burning power plants.

Where vog goes depends on the wind. When Hawaii’s famous tradewinds are active, it can be dispersed out to sea.

When tradewinds are light or disappear altogether, the sulfur dioxide “sort of pancakes out” from the fissure, Janet Babb, a geologist with the U.S. Geological Service, told The Washington Post.

Vog, which mainly consists of water vapor, carbon dioxide and sulfur dioxide, can appear as “hazy air pollution.” It can also contain several other compounds such as hydrogen sulfide, hydrogen fluoride and carbon monoxide, all of which are harmful to people, according to the Geological Service. However, of the three primary gases, sulfur dioxide, which has an acrid smell reminiscent of fireworks or a burning match, is the “chief gas hazard in Hawaii,” the service reported.

Vog is nothing new to people living on the Big Island or the surrounding islands. The summit of Kilauea has been emitting high levels of sulfur dioxide for the past 10 years, Babb said.

In past years when vog has plagued the islands, many reported suffering from debilitating symptoms.

To understand the health impacts of vog, try a Google search or a search of TripAdvisor.

Jennifer Griswold, an assistant professor at the University of Hawaii at Manoa, told KHON-TV in 2017 about her reaction to vog when she first moved to Hawaii.

“It felt like I had really severe tooth pain, or like I needed a root canal, or like someone was stabbing me in the face,” Griswold said. “I ended up going to a dentist who told me that my sinuses were so inflamed from the vog that they were essentially crushing the nerves of my teeth.”

On TripAdvisor, one visitor to Hawaii Island posted in 2016 asking for help after “suffering badly” from vog.

The user’s symptoms included a headache, a raw swollen sore throat and lethargy.

“We are planning on going to VNP Hawaii Volcanoes National Park today and if I had an oxygen tank I’d wear it!” the user wrote. “My question is will this get any better or should we just take our losses and leave?”

One day later, the same user provided a status update: “We are leaving today for Oahu. Hopefully I can recover enough to redeem the rest of our vacation. This has indeed been brutal!”

A Washington Post editor who frequently visits the Big Island coughed and wheezed for two weeks in 2016 before finally going for medical treatment. He was told he had all the symptoms of a bad case of pneumonia, thanks to vog.

According to the Hawaii Interagency Vog Information Dashboard, short-term symptoms could include eye, nose, throat and skin irritation; coughing and phlegm; chest tightness and shortness of breath; increased susceptibility to respiratory ailments; and in some cases, fatigue and dizziness.

Exposure is especially dangerous for people who have respiratory conditions such as asthma or emphysema, because they are more sensitive to the effects of vog, Jeffrey Kam, head of allergy and immunology at Straub Medical Center in Honolulu, told The Post. When vog occurs, Kam said he sees an influx of patients.

Studies have shown increased health risks stem from higher-than-usual amounts of sulfur dioxide emissions. According to an article published in 2010 in the Journal of Toxicology and Environmental Health, researchers found that a local clinic saw “three times as many headaches and twice as many severe sore throats” after Kilauea erupted in 2008. The researchers also reported a “six-fold increase in the odds of having acute airway problems,” which are more serious respiratory issues usually requiring immediate breathing treatments or transfer to the nearest hospital for emergency care, according to a news release.

Concentrations of sulfur dioxide, of course, are highest near fissures and immediately downwind, Babb said. In some areas, the acidic gas has exceeded 100 parts per million, she said.

“That’s in the dangerous zone,” she said. “That’s a very high concentration.”

Breathing the gas for even a short period of time can lead to long-term irritation and damage to a person’s nasal passages, throat, and even lungs and breathing tubes, Kam said.

Given these health effects, it would seem surprising anyone would choose to stay near sources of volcanic gases, but Hawaii County spokeswoman Janet Snyder told The Post there are at least a dozen people in the affected area who have yet to evacuate. She added that authorities are continuing to urge these people to leave.

However, leaving areas where there are high concentrations of volcanic gases can only do so much since the noxious fumes can be spread by wind, Kam said.

“These poor people are stuck down there,” he said. “You try to evacuate, but some of these evacuation centers are now getting inundated with the chemical smells and stuff and they have to relocate them.”

Other preventive measures, such as gas masks, also have limitations, Kam said.

To be safe in areas where there are toxic levels of gas, standard store-bought dust particle masks won’t cut it, Babb said.

Masks need to be properly fitted and equipped with the right cartridges to filter gases, she said. She added that even before purchasing a high-quality mask, people should still take a lung function test to ensure their lungs are “sufficiently robust and healthy.”

“People think oftentimes that they can put on those dust particle masks that you use when painting or sanding wood. That doesn’t work,” she said. “There’s more to it than just going to the local store and buying a gas mask.”

As scientists cannot predict when the eruptions will stop, Kam said, it’s important for people with respiratory illnesses to take proactive measures, such as stocking up on medication. Healthy people, he said, should simply avoid the pollutants as much as possible.

For people refusing to evacuate, Kam said the decision is “not wise.”

“They’re taking their life into their own hands,” he said.