Mercury and most of its compounds are extremely toxic (Hg is toxic in metal, vapor, salt, and organic compounds). Mercury is bioaccumulating heavy metal very harmful for people. Athmospheric mercury may be inhaled, but even if it falls into oceans and farmland it bioaccumulates and moves up in the food chain. Eventually into fish and people.
The collapse of the Roman Empire probably wasn’t caused by Mercury poisoning. But just being considered a legitimate contender for that role is impressively scary.
This also, of course, didn't cause the fall of the empire.
I believe it was also common to sweeten wine by leaving it in lead carafes.
It would be interesting if there was that kind of research but I think honestly with a noted rise in atmospheric mercury concentrations [1] we might err on the side of caution.
I'm half convinced we are going to mess space up in worse ways than mercury fuel but hey who listens to reason in these regards, they're blowing these things up just as a huge military dick swinging contest up there.
[1] https://www.dovepress.com/in-depth-review-of-atmospheric-mer...
The fish humans eat are often many levels of predation up the chain, so concentration of mercury in, say, tuna can be troubling. Even if you inject relatively small amounts of mercury in the ocean, much of this amount is not dispersed but returned in seafood.
The amount of mercury used by spacecraft engines is likely (not necessarily) too small to matter right now. With cheap reusable boosters to orbit, it may grow quite a bit soon enough to matter.
It seems to be specifically about emissions in low Earth orbit from maneuvering thrusters, such as in satellite constellations, because that will fall back into the upper atmosphere. Is there a climate change issue then, ozone layer, anything? No it just seems to be falling down that's the issue, this is mentioned further down:
> The environmental and human health implications of such releases have not been evaluated. Using an atmospheric chemical transport model, we simulate global deposition of mercury released from satellite propulsion systems. We estimate that 75% of the mercury falling back to Earth will be deposited in the world’s oceans, with potentially negative implications for commercial fish and other marine life.
So have or haven't environmental implications been evaluated? It seems to me this very paragraph does just that. And wasn't it not about propulsion in general but about OMS instead? Well, anyway, that's what mercury emissions are about.