These satellites are made of metals such as aluminum and titanium and contain other potentially toxic substances. Scientists worry that as these satellites burn, they may release harmful chemicals that could damage the recovering ozone layer. 

"The disposal of satellites from these large low Earth orbit constellations into the atmosphere and their vaporization during reentry is a very significant potential impact both for the climate and the ozone layer," Martin Ross, a researcher at the Aerospace Corporation in California who studies effects of space systems on the stratosphere, told Space.com. "Right now, we know very little about what happens when these satellites vaporize and turn into particles and gases. We don't know how active these particles and gases are both chemically and radiatively. But we know that soon, there will be tons of this stuff coming down."