Canaries
Limnic eruptions are another example of how dangerous large concentrations of CO2 can be.
Wandering Wikipedia, I see that there’s actually a geologic term for pockets of dangerously oxygen poor air: mazuku. “Evil wind” in Swahili.
We kill enormous numbers of animals daily using CO2.
I’ve talked to people doing this. Universally they believe that it’s painless. The boss said it’s painless. The literature says it’s painless. I tell them to try it briefly themselves.
They simply refuse to believe that so many people are killing animals painfully, and that nobody knows about this.
In addition to the pain, the article I linked to above says that high CO2 levels cause uncontrollable lung spasms. Your body will start coughing wildly in an attempt to drive the CO2 out of your lungs. The researchers had to use special mixtures of gas to eliminate this spasm reflex during their tests.
But not without prompt symptoms.
At a party as I kid I was trying to speak with much higher pitch in “helium speech.” So I emptied my lungs as much as I possibly could. Then I inhaled from a He balloon while trying not to suck in any outside air at all.
Within about five seconds I heard roaring “jet planes” approaching from behind me.
“Black Sparklies” started encroaching on my peripheral vision, leaving a central patch of vision which became more and more narrow.
I had to grab the table to keep from falling over.
By panicked violent breathing, I made the “tunnel vision” open out again, and the roaring-squeal tinnitus seemed to move off into the distance behind me.
I expect that “Helium Blackout” is probably identical to “N2 blackout.”
Also, I once fainted during a long blood draw. Symptoms were identical to the above, but also I tried to say “you better take it out” as I was falling over, but it came out “Yubbbtoodduuu.” So slurred speech yes. Nightmare of syringe needle broken off inside artery.
Oh, also it’s interesting to wake up after fainting. It feels exactly like waking up after many hours of sleep. You know it must be morning. But there’s a giant plane of linoleum pressed against your face. With white med workers’ shoes glued to it. With legs sticking sideways out of the shoes. I guess the classic “where am I” question instead is really this one: “It’s obviously morning and I’m just waking up after a full night’s sleep, so how did all you people get into my bedroom. Wait a minute, this isn’t my bedroom. It looks like a doctor’s office. I went to the doctor, was it days ago? Yesterday? Hey, I don’t remember coming home! What day is it anyway? Hey, WHERE AM I?”
so do you think they should they switch to killing animals using nitrogen instead? Or is nitrogen significantly more expensive than CO2?
if a person dies from exposure to nitrogen atmosphere, will examination of the corpse readily determine the cause of death? Or is the pattern of damage from this similar to some other causes of death, so that there would be no certainty as to the true reason?
Someone you’d want to get rid of? 
It’ll be suffocation, but without the signs of violence that might accompany strangulation, smothering, etc.
According to this MSDS sheet, CO2 would be lethal at far lower concentrations:
As wbeaty describes, inhaling CO2 or nitrous oxide first causes the ears to wring awfully loud, like there are high-pitched planes buzzing around your head. Then the dizzyness and unconsciousness comes. You’ll hit the floor but you won’t feel it until later. You’ll feel dissociated, like you’re floating out of your body or like your limbs aren’t attached properly. You’ll seem to simultaneously float through the air and sink like rock. Should the cat happen to begin pawing at you interestedly, she’ll feel like she’s 10 cats with a million paws each, poking you all over. Oh, and it’ll seem hilarious at the time, but will seem incredibly stupid and dangerous 10 years later.
Not that I’d know what a whippet was like, or anything… ::shifty eyes::
As for a stopped heart, anyone that’s ever had a proper blood choke done on them can attest that it’s certainly lights-out in seconds…no more than 3, I’d say.
when people die because their heart stopped for no clear reason, does that also look like “suffocation but without the signs of violence”? Or are there distinct features in “death from heart failure” to separate it from “death from no oxygen in the air”?