A gentle breeze sends waves lapping onto the sand as a smattering of sunbathers spreads lazily out on beach towels; a pair of children splash in the water.
A typical tourist scene in the Canary Islands. But it is possible only because of an unprecedented network of monitors, testing the air for poisonous levels of carbon dioxide.
For two years, the black sand beach here was deserted, its ice cream stands and souvenir shops shuttered with signs reading “Peligro!” (“Danger!”). Only now are tourists starting to return to Puerto Naos, this slice of paradise in La Palma.
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Lava wasn’t the only hazard from the 2021 Cumbre Vieja eruption in La Palma, Spain. Concentrations of carbon dioxide spiked as well, threatening locals’ health. But an innovative network of CO2 detectors is helping keep them safe today.
In September 2021, the Cumbre Vieja volcano erupted a few miles away, spewing lava down the hillside toward Puerto Naos and the nearby fishing village of La Bombilla and forcing the evacuation of over 1,000 people.
Months later, scientists discovered something far more insidious: The volcano had caused an unexpected seismic reaction that released hazardous levels of CO2 – once trapped in the ground – into the air. During the evacuation period, some areas in town had reached air concentrations of CO2 as high as 480,000 parts per million – over 400 times the acceptable limit.
“There were dead birds and cats on the ground close to garages and basements,” says Nemesio Peréz, scientific coordinator at Involcan, a geological institute in Tenerife. “That’s when we realized we had a problem.”
Before long, Mr. Peréz joined a working group of Spanish authorities and other geological experts. In 2022, they launched Alerta CO2, a €4 million ($4.5 million) project funded by Spain’s national government, and the first of its kind globally to monitor gas levels on a large scale.
Now, there are more than 1,300 CO2 detectors in Puerto Naos and La Bombilla, making the villages the most monitored in the world for the gas.
The project has not only allowed around 80% of homeowners to return, but could also provide lessons for other communities facing environmental disaster going forward.
“We’re constantly monitoring the air quality here,” says Javier Llamas, the mayor of Los Llanos de Aridane, the town most affected by the volcanic eruption. “People shouldn’t have to worry about the basic necessity of breathing clean air.”
On watch for CO2
Jutting out from a rocky cliff is the majestic Meliá La Palma, Puerto Naos’ only hotel and the area’s biggest draw. After the volcano erupted and the town was evacuated, there were questions about whether the hotel would get back on its feet. Three years later, it is at full capacity.
“Many guests write us to ask if there is still gas in the area, if businesses are open, and if it’s safe,” says Irving Ribot, manager of Meliá La Palma. “We just try to reassure them that when they’re in the hotel, they’re monitored all the time.”
There are 55 CO2 detectors spread throughout the hotel, discreetly placed in hallways, inside the restaurants, and in the lounge area. Most guests will never even notice the grapefruit-sized detectors. But if concentration levels get too high, an alert goes off in the offices of Alerta CO2 just up the road.
There, Mr. Peréz and his team have set up a makeshift office in an elementary school left empty after the COVID-19 pandemic.
“We’re here 24 hours a day, seven days a week,” says Marisol Gonzalez, project coordinator of Alerta CO2, as she pulls up a spreadsheet worthy of a sci-fi film. On her computer screen is data from each of the homes in Puerto Naos and La Bombilla using their detectors.
Normal levels should be below 770 ppm. If the CO2 concentrations go above 1,000 ppm continuously for 30 minutes, firefighters in the office next door make a call to the home or business. Those who need help get a personal visit; otherwise, they’re encouraged to go outside and air out the space.
That can mean simply opening the windows or, in some buildings, using the air purifiers that the Alerta CO2 team devised – imposing machines that introduce clean air in and flush bad air out. Some residents have complained that the machines are ugly or noisy. But, asks Mr. Peréz, “What do you want, gas or noise?”
Homecoming for Puerto Naos locals
There’s no way of knowing how long it will take for the ground to stop releasing CO2 because, says Mr. Peréz, there are no scientific tools to forecast the end of the problem. But levels consistently went down in the last half of 2024, which has allowed people to return to their homes and businesses.
The first 139 people returned in December 2023, and returnees have progressively increased since then. Over 900 homes and businesses in Puerto Naos and La Bombilla have now been authorized to reopen.
That’s reassuring for Margaret Peréz, who spent her entire adult life in Puerto Naos and raised her children here. When the town was evacuated, she went to live with family in nearby Los Llanos. But she always had her eye on returning.
“Some people are scared of coming back, or they ended up in a bigger town and decided they liked it better there,” says Ms. Peréz, who works at the only grocery store in the village. “Not me. I love it here.”
There are still zones closed off around Puerto Naos and La Bombilla, and several underground parking lots and basements remain sectioned off with sticker tape and “Danger!” signs. Along the boardwalk, a rhythmic, green light flashes on the outdoor CO2 detectors – a constant reminder that the area is not out of the woods yet.
The La Palma regional government has since created the Smart Island app, where people can see outdoor CO2 levels in real time. Local business owners are doing their best to reassure clients.
“One girl came in and saw the CO2 detector and said, ‘Whoa, it’s at 800 ppm in here!’ She was nervous,” says Maria Malik, a local hairdresser. “But in reality, if you have any enclosed room full of people, your CO2 levels are going to go up from simply breathing.”
