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67

You just need to have the appropriate chemireceptor which is triggered by CO molecules. Sensitivity is less of an issue: our nose can detect certain molecules at very low concentrations, like H2S, the molecule responsible for the smell of rotten eggs, which can be smelled at 0,0047 ppm. Since CO reacts better with hemoglobin than oxygen and CO2, a suitable ...


61

During WW1 chemical warfare was used to break the deadlock at the ocidental front, with cruel consequences but little to no true military value. The simplest gas used was Chlorine, that accumulated in pools and was quite deadly (and quite horrible as a weapon). I believe this one or any of the other gases used in WW1 might work as you want. Phosgene is a ...


57

It turns out it's not that hard to make hydrogen gas for an airship, and you can do it yourself. Take some dilute sulphuric acid. Put small pieces of metal in the acid - iron, zinc, and aluminum should all work. Capture the resulting hydrogen gas, and then put it in your airship. This method of hydrogen production was first specifically used for airships ...


28

Are you open to symbiosis? There are microorganisms that actively metabolise Carbon Monoxide. A creature that harbours colonies of such bacteria may not be able to directly smell atmospheric Carbon Monoxide but they could get feedback on it's local concentration based on the activity level of those colonies. This would be by way of sensing metabolic ...


27

Ok, I misunderstood the question, you want nasty, seeable stuff. The natural part is a bit problematic. Here we go: Nitrogen dioxide Brownish stuff which is caused by burning which is a major air pollutant and has this sharp, biting odor. Poisonous, of course . To be exact, the atmospheric nitrogen is bonding with oxgen, first causing nitrogen monoxide ...


27

You're looking for some sort of (micro-)parasite. Parasites are nature's masters of reprogramming other animals to behave in odd ways. Expecially toxoplasma gondii is known to infect humans and it's possible that it affects us similarly to mice, which get attracted to feline urine, so that the cats can eat the mice and thereby get infected with the parasite....


26

A cubic meter of air weighs about 1.2kg. A cubic meter of 15% O2 and 85% He would weigh about 20% of that or about .25 kg. so a cubic meter of the O2/He mixture would lift about 1kg. Ignoring the weight of the bubble itself, a 70kg person would need a 70 m3 bubble, which would be about 5 meters in diameter. Since the bubble material would have to be pretty ...


25

Los Angeles? Salt Lake City? Or maybe even someone local by 1931. The first neon sign in Las Vegas went up in 1928. http://captainhistory.com/wordpress1/2018/02/25/the-first-neon-signs-in-las-vegas-nevada/ If you want there to be some adventure and finagling involved your characters could go on a road trip to Los Angeles, which would be a good contrast ...


24

If you happen to be on a Space Station, you could simply vent the atmosphere. They may have some internal injuries, but they should revive when you pump the air back into the room. The important thing to consider is that any "knockout" gas is a potential killer in a large enough dosage, and merely pumping it into the room is a very imprecise way of ...


23

Another possibility is Bromine. It's denser than air, so it will pool nicely, and has the literary advantage that it's red, with all the symbolism that entails. It's toxic at high concentration, but it also stinks, so it's hard to ignore and hard to imagine anyone getting killed inadvertently, as opposed to silent killers like CO2.


23

They can just smell it like anything else. "Odorless" just means "human noses didn't happen to evolve the ability to smell this particular chemical." If humans or other animals evolved in an environment where being about to smell carbon monoxide was beneficial, they'd just be able to smell it. People are talking about particular mechanisms, which is fine ...


20

The caves flourish with sulfur oxydizing bacteria: Microbial oxidation of sulfur is the oxidation of sulfur by microorganisms to produce energy. The oxidation of inorganic compounds is the strategy primarily used by chemolithotrophic microorganisms to obtain energy in order to build their structural components, survive, grow and reproduce. Some inorganic ...


19

I read How to Design a Plague that Causes Insanity? and Would a prion be the most likely cause of zombies?, yet I think the question is different here, and very interesting. It's reality-check, so I went to the wikipedia. 1. Not capable of having verbose interaction with not-transformed humansThis seems to be quite straightforward: Communication Disorder. My ...


17

Hydrogen is the lightest gas there is. In its common diatomic form (H2), it consists of two protons and two electron, making it extremely light, even compared to the next lightest gas, helium (although the difference in lift between them is minimal). There's nowhere better you can go, unless you choose to simply use Hydrogen-1. Plus, if you heat it up a bit, ...


17

A common tactic in old mines was to set up multiple pairs of shafts. One shaft would have a fire lit at the bottom of it. The hot air in that column would rise, forcing cold air to flow down the paired cold shaft and setting up a flow of air through the mine. How does this help you? Well, any way to set up a temperature difference can create this kind of ...


16

Microorganism-powered aerial flight is plausible, but with caveats. As you point out in your question, hydrogen is the best lifting gas we have. So this becomes a question of "What's the most efficient biological process that produces H$_2$?" The answer, of course, is algae. Normally, algae get their energy from photosynthesis- taking in sunlight, water, ...


14

You actually have quite a few choices. Given your parameters, Argon (Ar): It’s a naturally occurring gas on Earth (it comes from the radioactive decay of a few other elements that naturally occur) and our senses can’t detect it (it’s tasteless, odorless, and invisible). It’s not really a poison in and of itself, but it replaces oxygen in low-lying areas and ...


14

Fog is just a suspension of minute droplets of water/liquid in the atmosphere. If the atmosphere was breathable before the fog, it stays breathable also with the fog. There are cities in our world where fog is notoriously a major aspect of their life, still they flourish (think London or Frisco). Water based fog can be highly beneficial for life: in the ...


14

Hot air. Cheap, readily available, nonflammable. The hotter, the liftier. You can replenish your supply anywhere you can breathe. Gaseous water. Cheap, readily available, nonflammable. The hotter, the liftier. You can replenish your supply a lot of places. You will need a container that can withstand the steam. Anhydrous ammonia. Dark horse ...


12

Not without any additional means. In the chemistry question that you mentioned the selected answer discussed entropy as one of the reasons gases won't separate in the atmosphere. This means that even in the absence of turbulence gases in the atmosphere would remain mixed. However, you can use non-chemical means to separate gases: Gravity. Different gases ...


11

Not really. The lift is not actually provided by the lifting gas, it is provided by the air it displaces, so lift can't exceed air density. The lifting gas simply provides the pressure to sustain an air displacing structure with large volume with minimal mass. The differences in lift between lifting gasses are actually simply the differences in the weight ...


11

Bath Salts is a broad classification for a type of drug that has been linked to zombie like behavior in humans. It is a psychoactive drug that often triggers extremely violent and anti-social behavior. There was a famous zombie like attack that took place in Florida a few years ago. Face-Eating Cannibal Attack Bath salts are an alternative drug for Heroin ...


10

A physics note before I start: Every atmosphere is escaping, awalys. The catch is just that some bodies have escape rates so low, that we don't notice on the timescale of the age of the universe. That being said, what you're searching for here sounds like a cold trap. You need a layer of a chemical species S at some height $H_0$ that forms at $H_0$ from ...


10

How about Titan? It has 14% the gravity of Earth, but 1.45 times the atmospheric surface pressure. Titan can achieve this because the atmosphere it does have is much heavier than Earth's atmosphere. Although Titan does not have its own magnetic field to protect its atmosphere from solar wind, it's protected by Saturn's relatively large magnetic field. If ...


10

It's not the same as "smelling" it, and as far as I can find, there are no studies testing this ability, but there are countless stories about cats (and sometimes dogs) who saved their human families (and dogs) from carbon monoxide poisoning. In these cases, the CO levels were high enough to cause severe symptoms in humans, but they were asleep and didn't ...


10

About 15 seconds and they'll hear wind. I tried calculating this exactly and ended up with a lot of "depends". Human bodies swell pretty quickly when exposed to vacuum and this could partially seal the neck, slowing the wind to a long hiss. Theres a reserve oxygen supply in the backpack. They mostly filter co2 for the 8 hours of use but they can ...


10

A very likely risk is that the heavy gases would stratify at the lowest heights, displacing lighter gases and making it an anoxic environment. A situation where normal atmospheric mixing is stopped is called inversion Given the right conditions, the normal vertical temperature gradient is inverted such that the air is colder near the surface of the Earth. ...


9

Yes But, there are many things working against this forming - especially for a terrestrial world. Here are the issues that spring to mind: Chemistry You can subdivide chemicals using many different strategies. I'm going to subdivide them by calling them: reducers (fuels) oxidizers inert If you have an atmosphere with a high quantity of oxidizers (...


9

NO2 or nitrogen dioxide is toxic, heavier than air and visible (opaque) in high concentrations as your question asked. It is reactive with organic materials (life) as it is an excellent oxidizing agent. Also forms a strong acid on reaction with water (nitric acid), and ozone on reaction with volatile organics in the presence of heat and light. Wikipedia on ...


9

I have been pondering this one and I think I have a candidate. Fog. • /Denser than air/ – water as a gas is not denser than air, but can exist at earth temperatures in equilibrium with its liquid phase. The presence of microscopic droplets of liquid condensing from and evaporating back into the gas makes the fog more dense than air. Additionally (as ...


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