We'll cover three World War I compounds, saving the latter-day nerve agents for a separate posting. 1915 was a terrible year, one among many, because it saw the advent of militarized chlorine, followed shortly by phosgene. Those two (though technically obsolete) are still in play, because their manufacture is so low-tech. Mustard gas (bis(chloroethyl)sulfide,) which is really a liquid for the most part, was also famously effective. I'm not going to talk much about the arsenical agents like Lewisite - they're nasty, but of less relevence, I believe, to modern warfare or terrorism.
Some of the other things from that era, ones that you'd think would be quite destructive, turned out to be useless. Gaseous hydrogen cyanide is a notable example, and illustrates some of the complications. For one thing, HCN is too light - the vapor disperses rapidly, instead of hugging the ground like phosgene. It's also a little-appreciated fact that HCN isn't really that toxic below a certain threshold. I've smelled the stuff myself in the lab - by accident, I should make clear. (Its aroma is distinctive, but not as bitter-almondy as advertised, at least to me.) People can survive just fine around low concentrations of cyanide gas, although I'm not endorsing it as a lifestyle choice. Achieving lethal concentrations of it in the field just isn't very practical. Not that it wasn't tried.
The three agents from the first paragraph, though, suffer from no such limitations. Chlorine and phosgene are heavier than air, and accumulate in low places (such as a World War I trench) if there isn't a strong wind. They're both reasonably persistent as well. Mustard gas is very persistent indeed, to the point that it couldn't be used against positions that attacking troops were going to occupy any time soon. All three have effectively no minimum concentration below which they stop doing some sort of harm. Very low exposure to any of them certainly isn't fatal, and you can even get away with no lasting damage, but it's still unwelcome.
While not the most effective chemical warfare agents (we'll get to those next week,) these things are all easily purchased or made in quantity. No doubt they have some appeal for terrorists, and as such they're worth looking at in a bit more detail.
Chlorine is a basic industrial chemical, prepared in immense quantities by electrolysis of brine. This is the chlor-alkali process, a classic of chemical engineering which has been refined but never superseded. This technique means that wherever they make chlorine, they make sodium hydroxide, too, and generally they go on to make sodium hypochlorite bleach for you, too.
There's been tremendous back-and-forth about chlorine use and production in Iraq, since it's also used to purify drinking water supplies. While Saddam Hussein certainly would have no qualms about using it, it's still not the most effective war gas by a long shot (we'll be getting to those in the next post.) I believe that the more likely use of chlorine would be as part of a low-tech terrorist operation involving its transport through a population center.
Its effects can be completely mitigated by reasonable protective gear, but that's just what ordinary people won't have. It has severe effects on the lungs, which is how it kills. One thing that keeps it from being more destructive is that it has a powerful smell, even at low concentrations. No one gets exposed to chlorine without realizing it and trying to get away.
Phosgene is worse. It also causes severe lung damage, but it's harder to detect. There's certainly a distinct phosgene odor (sort of an acrid rotting-vegetation smell, supposedly not so bad a low concentrations.) But that means the gas can be present at dangerous levels (especially over time) without being particularly noticeable or offensive. And if you don't know what the smell is (and what it means) you can be badly injured without realizing the danger. The majority of chemical fatalities in World War I came from phosgene.
On a personal note, perhaps the single longest day I've spent in my chemical career came some years ago, after I thought I might have been exposed to some phosgene one morning. I didn't really notice the distinct smell, but here was also plenty of HCl vapor present, and I worried that it had masked the phosgene. Surely, I thought, I'd notice some immediate effects if I'd been exposed. . .so I went and did some reading, and that's when I really started to get the cold sweats. The symptoms of even a fatal exposure to phosgene can be, initially. . .nothing. Only with time does it hydrolyze inside the epithelial cells of your lung tissue, causing increasing and irreversible damage. I spent a rather jumpy day, checking my breathing while trying to correct for the unusual general shortness of breath I was feeling.
Phosgene is also a large-scale article of commerce. It's synthesized from carbon monoxide and chlorine, using various catalysts. Not something that you would make in your basement, but if you're a country with a thermoplastics industry, you make phosgene or you deal with someone who does. As far as I know, it's not usually shipped around or stored in large quantities, which makes it less of a civilian terrorist threat. If you have an industrial phosgene synthesis, you generally make it on-site as needed.
Finally, we have mustard gas (something of a misnomer, as I've mentioned.) It didn't cause as many fatalities on the battlefield, but its insidious nature made it an effective weapon. It's not immediately irritating to the eyes or lungs, and it can be tremendously harmful at levels well below those that can be smelled. It's persistent, and penetrates ordinary clothing very well. A few hours after exposure is when the trouble begins. It attacks the lungs (although with a different sort of action than phosgene) and causes terrible burns to exposed tissue - all of this long after you have the chance to do much of anything about it. That's the main reason for its military effectiveness, since full body coverage is needed rather than just eye/lung protection. As a weapon of terror, this might be the worst of the three, if the people involved were to disperse it well enough.
Fortunately, it's also the hardest to get. There's no industrial application for the compound, and no legitimate reason to produce it in any quantity. However, its immediate precursor compound, thiodiglycol, is used industrially, although in quantities that don't come anywhere near chlorine or phosgene. It shows up as a component in inks and dyes, mostly, which means that an unsavory government could claim that a particular factory was making ball-point pen ink or the like. Synthesizing the mustard itself from the precursor wouldn't be much of a feat for any decently equipped lab. You might not get the cleanest product in the world, but it would be bad enough.
Trying it in a garage (or a cave!) though, would be another matter entirely. We've now crossed over from the list of "things you can buy" (or "things you can hijack a truckload of") into the territory of "things you have to make." And even a one-step synthesis like mustard gas, if it's to be done on any harmful scale, needs some equipment that you're not going to have in your kitchen - that is, if you're not going to gas yourself in the process. None of the problems is insurmountable, unfortunately, but they do raise the bar.
I mentioned the problem of dispersal. I don't want to gloss over it, but I don't want to give anyone a road map, either. The use of chemical agents in World War I, for example, evolved from opening cylinders and letting the wind blow the stuff across no-man's-land (in early 1915) to various types of sophisticated gas artillery shells. That was a direct result of experience - the wind could blow the gas all over the map, or right back at you, while shells could be targeted where you needed them.
As we'll see later in the discussion of nerve gas, the Aum cult in Japan found that even highly toxic compounds aren't as effective as they could be when poorly delivered. We can assume that Iraq has a real supply of chemical munitions (more on this later on, too,) while less infrastructure-rich terrorists probably don't (unless they buy them off of the Iraqis or any of the many other governments that are holding on to this stuff.) If they're going to use chemical agents on their own, though, they'll have to figure out a way to do so effectively. And that is, fortunately, not trivial. Beyond that I will speculate no more in public.