How Geoengineering May Increase Passengers’ Exposure to Sulfuric Acid

Spraying solar-reflecting particles near the poles to combat global warming raises serious concerns about exposing passengers on commercial flights to hazardous levels of sulfuric acid.

As global temperatures could rise by up to 3.7°C by 2100, the concept of cooling the Earth by blocking sunlight is becoming increasingly relevant. The most discussed method involves injecting sulfur into the stratosphere to replicate the cooling effects of historic volcanic eruptions.

However, dispersing these particles into the upper atmosphere near the equator, where they are most effective, necessitates a new generation of aircraft capable of reaching altitudes of 20 kilometers—almost double the maximum altitude of current commercial airlines.

As a feasible alternative, researchers are investigating upgrading Boeing 777s and similar aircraft to release sunlight-reflecting compounds closer to the stratosphere, approximately 7 kilometers below. Yet, this practice could potentially introduce risks for flights operating between Asia and North America, which frequently traverse polar routes.

“If planes can reach those heights to spray sulfur, that’s also where passenger flights operate,” explains Alan Robock from Rutgers University. “I wonder about the implications of [sulfuric acid] exposure for both passengers and crew.”

Previous simulations have suggested injecting 6 million tons of sulfur dioxide into the stratosphere at altitudes of 13 to 15 kilometers near the North Pole from March to June and a similar injection near the South Pole from September to December.

Atmospheric models indicate that a total of 12 million tons of sulfur dioxide could sufficiently cool the Earth by approximately 0.6 to 1.0 °C. Robock and his team used sulfuric acid concentration results from these simulations to estimate how much of this noxious substance would infiltrate aircraft cabins.

Geoengineering flights disperse sulfur in narrow plumes that can take days to weeks to dissipate. In some cases, aircraft may pass through areas with sulfur concentrations as low as 7 micrograms per cubic meter—below levels seen in many urban environments like several major cities. However, there are also instances where sulfuric acid levels exceed 50 micrograms per cubic meter, considered hazardous by the European Union.

When inhaled, sulfur dioxide can irritate the throat and inflame the lungs. At elevated concentrations, it can lead to severe respiratory conditions and can narrow airways, significantly impacting breathing. Furthermore, exposure to high levels of sulfur dioxide carries an increased risk of stroke. Even lower concentrations can exacerbate asthma attacks. These health risks are especially pronounced for pilots and flight attendants who frequently fly these routes.

“If the developments outlined in this paper come to fruition, they are still decades away,” shares Wake Smith from Harvard University. He believes this timeline allows airlines and engineers the opportunity to enhance air filtration systems to better handle the anticipated sulfur dioxide and sulfuric acid concentrations, ensuring passenger safety.

“No prior research has focused on this aspect, making this study valuable,” notes Daniele Visioni from Cornell University. “While the preliminary findings are intriguing, they are not definitive game-changers. The risks likely exist elsewhere.”