While the liquid oxygen used by the Falcon 9 does not harm the skies, the burning kerosene produces black soot that is immediately pumped into the stratosphere, a layer of air that is between 12 and 50 kilometers (7.5 and 31 miles) above the Earth. The soot stays there for up to five years, collecting heat, causing climate change, harming the ozone layer, and exposing the planet to harmful ultraviolet (UV) radiation. And SpaceX is not at all by itself.
A research by the National Oceanic and Atmospheric Administration (NOAA) claims that annual soot emissions from global rocket launches—of which there were 180 last year—amount to around 1,000 tons. According to NOAA, this situation will further worsen as the sector develops. The study's lead author, environmental scientist Christopher Maloney, said in a statement: "The bottom line is projected increases in rocket launches could expose people in the Northern Hemisphere [where most rocket launches take place] to increased harmful UV radiation."
Rocket launches alone only make a modest dent in the total amount of air pollution. Annual fuel consumption in the aviation sector is 100 times more than that of all rocket launches worldwide. However, there is a significant atmospheric difference: airplanes fly in the troposphere at an altitude of around 11 km (6.6 mi). In contrast to stratospheric soot, which lingers for a considerably longer time, soot from this region precipitates quickly. In fact, a single passenger on a rocket causes 100 times more climate-changing pollution than a passenger on an airplane, according to the NOAA analysis.