Which Gases are the Most Toxic?

I have had carbon monoxide poisoning before. Do NOT recommend! Get the damn detector. While visiting my aunt, her furnace had apparently malfunctioned in the middle of the night. I woke up one morning to find that I had a splitting headache. I have migraines, so this wasn't uncommon. I noticed that the pets had all been sick overnight too, but I was late and panicking about the exam I had first thing in the morning, so I dragged myself to school. Halfway through the first period, I got called to the office. Turns out my aunt and cousins had woken up and realized that something was very wrong, and were diagnosed with CO poisoning. They had me go immediately to the hospital for treatment. Again, DO NOT fool around with CO poisoning!

When you mention methyl isocyanate, you really need to bring up Union Carbide and the slaughter it still hasn't been held responsible for in Bhopal.

You need to do a full “would you rather breath in”-tournament instead of a simple tier list. There are too many factors to consider in tier lists. But having to choose between plausible life threatening accidents should be easy enough.

Phosgene survivor here: it has a wet/grassy smell, doesn't smell 'bad' in any overt way. I don't recall if my eyes were irritated, but I do remember that I didn't feel sick right away. Source: Thermal decomposition of chlorinated paint-remover on a stove-top. I had a light exposure, the only lasting damage was to my sense of smell.

H2S needs more respect, it's really quite terrifying. You can smell it at low concentrations, but it quickly kills your sense of smell at higher concentrations. I work with it a lot in petrochem, and if we get so much as a whiff of H2S we shut everything down and evacuate the building. That stuff is no joke. If you're in an area you'd expect low levels of H2S, being able to smell it is sometimes more reassuring than not being able to smell it, as it's possible that there's too much in the air to smell. Either way, I won't go near the stuff without a fitted full face respirator.

I have breathed hydrogen selenide. It was the worst experience in my life. It feels like you lose feelings and functionality progressively from you throat down your airways, millimeter by millimeter. You just stand there and think "It will reach my lungs, and I am going to die". And I don't think there were more than 2 ppm in the air at the worst moment. I read it smells horrible but I honestly couldn't smell it at all, or was too scared to notice.

One day in a high school chem class we were studying your typical reactions of metals with acids. In the fume hood was a bottle of some hypophosphorous acid which my dumb 18 year old brain thought this was just some spicy phosphoric acid since I’d never come across such a reagent at the time. Me and my lab partner decided it would be a good idea to add a bunch of it to a couple of grams of zinc powder we had to do the experiments with and we thought the reaction was giving off hydrogen gas because our syllabus strictly said metal + acid = salt + hydrogen… a few minutes go by of this mixture bubbling away until our lab supervisor sees the bottle of the hypophosphorous acid and the violently frothing mixture and turns purple before putting the test tube holder at the very back of the fume hood and slamming it in shut. The fume hood remained untouched for the best part of a month with the mixture still there and after a bit of Googling what had happened I quickly discovered that the reaction didn’t produce hydrogen but instead was essentially a phosphine gas generator which scared the living sh*t out of myself and my lab partner. The morale of the story is never work with ‘spicy’ reagents until you have carefully thought out the products of what such a reaction could be instead of being a flammable gas it was horrifically toxic abomination which with our high school chem knowledge we were completely ill equipped to deal with. Please respect the reagents you work with and carefully assess the risks before mixing anything since things could have gone very t*ts up very quick that day. Myself and the 10 others in that lab that day are lucky to be alive. Chemistry is no joke and I have learned to give chemicals the proper respect they deserve. Edit: our PPE was a hoodie, some nitrile gloves and a pair of lab goggles old enough for me to call grandpa

Phosgene story: While working as an HVAC tech a lifetime ago, I was tasked with repairing a pinhole in a residential AC unit line set. I used a recovery machine to evacuate the system (so I thought) and then started repairing the hole with an oxy-acetylene torch. I immediately knew something was wrong when the flame started gently blowing away from the hole and in my direction and the flame turned bright green. I moved my torch and went to move away but it was too late. I caught a bit of it while breathing and thought I was going to die. My eyes, nose, throat, and lungs felt like they were on fire and I couldn't stop coughing long enough to catch my breath. EMS arrived and gave me oxygen and after the coughing stopped they told me to follow up with my Dr. My sinus cavities, throat, and lungs were burning for a week and I coughed up sooo much phlegm. Worst experience ever.

Don't forget the fact that once Radon finishes decaying it becomes Lead 206, so not only do you breathe radioactive gas but down the road all that gas becomes lead, fun stuff

You should do a tierlist of all the 4-4-4 compounds. Back in my undergrad days, we used to call those compounds "12s" and made a game out of hunting wikipedia for em.

I use to work next to the local hazmat people and ended up just gabbing with them, there was a railroad next to us and HF tank cars from Vulcan chemical came through there. They told me if one of those ever crashed we would know, because all the hazmat people screaming and running the other way.

Vinyl chloride is really interesting : it causes very exotic cancers (liver angiosarcoma) and it's one of the very few chemicals experimentaly proved to trigger sclerodermia-like lesions.

Controversial opinion: Ozone smells amazingly good. Good to the point that I have probably inhaled more of it over the years than is really ideal. Those "Ionic Breeze" air purifier things were great ozone generators for getting rid of annoying odors in rooms. Also have unfortunately smelled hydrogen selenide gas that I accidentally produced when disposing of some cold bluing solution. Not the worst smell ever but the lingering I pact was pretty strange. Everything smelled/tasted slightly of garlic for at least a week afterwards (my breath didn't smell garlicky to my girlfriend though thankfully).

"If you smell it and you can escape with your life, it can't go into S-tier" LOL

Methyl Isocyanate might not be a gas at your lab's RT but it can be a gas at the outdoor ambient conditions of, say, India. Seriously, how the substance that caused the most lethal industrial disaster in history (Bhopal) didn't make S tier is beyond me.

The Bhopal disaster was a methyl isocyanate gas leak from a pesticide plant in the middle of a state capital in India. It injured over 500,000 (!!!!) people. Offical death-toll is 3787, but more realistic estimates are around 16000, which includes the longer-term deaths from MIC-related diseases.

I'm only about halfway through this, but I propose some changes. 1. Ethylene oxide. SUPER useful compound. Not only do they sterilize medical equipment with it, it's an ingredient in a lot of useful chemicals. Glycols, glycol ethers, ethanolamines, ethoxylates and acrylonitrile all contain ethylene oxide. The problem is that it's an explosion hazard and extremely flammable. NFPA rates it 3 for health, but it's pretty much at the top of 4 for flammability. Somewhere between A and S would be appropriate for ethylene oxide. 2. Mustard gas. This stuff needs to be in C tier, and here's why: During World War II the Italians had a big stockpile of nitrogen mustard at Bari, which they were holding as a chemical warfare agent. Naturally, the Allies bombed it and the populace was exposed to it. After the war the Allies went in and discovered the population had a LOT less cancer than the rest of the Italian citizenry. This, and some research into using it for cancer treatment at Yale during the war, led to nitrogen mustard becoming the first chemotherapy agent. Research since then has focused on making nitrogen mustards that are as toxic to the cancer while being less toxic to the patient and caregivers than the real agent is. 3. Silane. Oh my. S tier. Most definitely. It's pyrophoric. They use it to make silicone compounds, but a huge use of it is making solar cells - you spray silane on a substrate in a clean room with a high-oxygen atmosphere, it catches fire, and when the fire goes out you have a nice film of very pure elemental silicon. Very slick. They make this in Moses Lake, WA, at the REC Silicon factory and transport it in tube trailers clearly marked SILANE. When I see one on the road I give it an extremely wide berth. Also...sulfuryl fluoride...that's not Klapötke's lab, is it?

I feel like part of the consideration should also be likelihood of exposure. CO isn't extremely dangerous in and of itself, as it takes a good few hours to kill you and it's fairly easy to treat people who've been exposed; but because it's an extremely common byproduct of inefficient oxidisation aka any and all processes that involve fire, rust or decomposition outside of a controlled, well-ventilated environment, risk of exposure is super duper high and as a result CO poisoning kills numerous* people every year. H2S is similarly terrifying, and while from a mere toxicological standpoint it's 'only' an A, the fact that it's so hard to detect by the body (yeah you can 'smell it and live' but that window is extremely small as once you've stopped smelling it you're more than likely boned), and the fact that untrained people can and regularly do come into contact with it should catapult it to (H2)S-tier without a second thought. The sheer number of stories of "someone fell over, someone else ran over to help them, now they're both dead" as a direct result of exposure to H2S should be enough. *I tried googling statistics for how many people die of CO poisoning every year globally, but while apparently articles, foundation webpages and even a paper I found agree it's 'one of the most common causes of death as a result of exposure to a poisonous gas', none of them actually gave any numbers. So, even if I had been ready to give in to confirmation bias, I couldn't because apparently statistics aren't important or something.

I think OsO4 should be way higher in the list due to its enormous toxicity. A chemist who discovered it was hospitalised for two weeks with heavy poisoning after just a small exposure. Really terrifying thing, I’d put it together with arsin

1:55 "F tier will still eff you up" That got me laughing. Here's something with some of these toxic chemicals you might find amusing that I've read about. During the early days of rocketry, someone had the bright idea of trying liquid ozone as an oxidiser. Has a higher density than liquid oxygen, and it has a higher boiling point. Unsurprisingly, the researchers had severe problems with keeping such large amounts of liquid ozone stable and gave up eventually. Another bright idea for a rocket fuel oxidiser was chlorine trifluoride. Yes, that ClF3. I mean, it's a very powerful oxidiser and a very energetic oxidiser, it would've made liquid ozone look tame. Then someone realised that the exhaust would be hot HF and HCl. (I read about these in a book called Ignition, an informal history of liquid rocket propellants by John D Clark, definitely worth a read)