There is mounting evidence that some casualties arriving at clinics and hospitals in Syria have been exposed to the nerve agent, Sarin. However, there's been no mention of the large numbers of fatalities in a fairly small radius that would might expect from an attack on an urban area with Sarin-filled military munitions. This has caused David Cameron and other Western leaders to display their technical ignorance by talking about "small scale" use of nerve agents such as Sarin. What matters with chemical weapons is not scale, so much as concentration. For some reason, the concentration of Sarin gas in the affected area was so low that many (most?) of the casualties were still alive when they reached hospital. This is also what happened when an extremist cult released quantities of homemade Sarin onto the Tokyo underground rail system.
In Tokyo, cult members hurled the contents of sarin-filled jamjars and glass bottles around, in very crowded underground tunnels. The consequences ought, on the face of it, to have included hundreds if not thousands of passengers dying within a few minutes. This did not happen. The probable explanation was that by the time the attack was mounted, the jars of "Sarin" contained relatively little actual Sarin. Sarin isn't hard to make, but it is hard to make Sarin that's particularly pure, and it apparently took months for the cult to make and distribute (to followers) all of the Sarin that was used in the coordinated mass attack. Sarin is not very stable, and unless made from exceptionally pure ingredients, it can degrade (that is, a substantial proportion decomposes into simpler chemical breakdown products) within months, or even weeks. In the days when Iraq actually had a chemical warfare programme, the Sarin munitions that Iraq was making had a shelf life of weeks. So, assuming that the cult's chemists couldn't do better than an Iraqi government factory, it's probable that most of the Sarin which the Tokyo cult made (over a nearly two year period, according to some sources) had degraded by the time it was used.
It's important to note that the breakdown products of Sarin are themselves quite toxic, but not in the same immediate way. They tend to result in chronic health problems and cancers, rather than prompt shutdown of the body's vital functions. The lethal dose would be many times higher, too.
The United States, having made a huge stockpile of Sarin and Sarin-filled munitions in the fifties and early sixties, stopped making new Sarin in the late sixties. Instead, the US Army redistilled degraded Sarin from existing weapons and storage containers, yielding a smaller amount of pure Sarin each time, along with a lot of breakdown products which were disposed off, not always very sensibly, around Denver, Colorado. By nineteen-eighty, there simply wasn't enough actual Sarin left for this to be worth doing anymore, and binary weapons were introduced as a replacement. (The binary weapons often involved a more modern nerve agent, VX, too.)
Syria is known to have received Soviet Chemical Weapons, and as importantly, gas masks, protective clothing, warning equipment and antidote kits, during the "Yom Yippur" war against Israel in 1973. Indeed, the automatic antidote injectors which NATO subsequently developed were inspired by surprisingly advanced devices issued to Syrian tank crews and captured by the Israelis when the tide of war turned in their favour.
Most of the actual weapons seem to have taken the form of chemical munitions for rocket launchers and heavy mortars which the Syrians already had, as well as aerial bombs. In all cases, there's a lot of commonality between Soviet chemical munitions and other weapons; for example: a Sarin or Mustard gas mortar bomb will be identical to a phosphorous incendiary bomb, except that instead of plastic capsules of white phosphorous being packed in the casing around a small bursting charge, there will be triangular capsules of a poison, usually Sarin or Mustard gas. Aerial bombs for poison gas have a lot of parts in common with explosive cluster bomb dispensers, and so on. This reduces training and industrial costs, and allows one type of launcher to deliver chemical, incendiary and explosive attacks. It also makes accidental or unauthorised release of chemical weapons a more than theoretical possibility.
Mustard gas munitions from 1973 might very well prove lethally effective, assuming that the propellants and bursting charges were in good condition, but Sarin munitions of that vintage would almost certainly contain mostly breakdown products diluting a quite small amount of actual Sarin. This would still cause casualties, including some fatalities, and quite a lot of long-term health problems amongst those living in the affected area afterwards. But a decisive military effect would be most unlikely.
Recent Israeli intelligence/propaganda has emphasised, not the Syrian legacy stockpile of Soviet-era chemical munitions, but a more recent alleged programme to build a laboratory capable of making bulk nerve agents in a short time, ready for prompt use on the battlefield. (They also heavily suggest attempts to make much more modern and sophisticated agents than Sarin, too.)
If true, and if the laboratory is still able to function after two years of civil war, this would represent a genuine capability to use nerve agents on the battlefield to genuine military effect. It is this which would represent the sort of threat to the wider region, and world, which might justify international military intervention. And it is this which remains unproven. Indeed, the nature of the incidents which do appear to have occurred, rather suggests that freshly-made, pure and viable Sarin is not actually available to the Assad regime.
The Soviet legacy chemical weapons stockpile is not harmless, however. Degraded Sarin will not drop thousands of people dead in their tracks, but it will make hundreds or thousands chronically ill and seriously shorten their lives in the long run, as well as causing economic impoverishment. And experiences on the Great war battlefields of France and Belgium inform us that forty, or even sixty years after manufacture, mustard gas munitions are still horribly dangerous.
These dangers might well justify the provision of gas masks and protective clothing, and even more importantly, first aid and medical guidance, perhaps to rebel groups, certainly to any third party such as the UN or Red Crescent, able to operate in contested areas.
Footage shown on the BBC shows supposed Sarin victims, being given oxygen in hospital, but still wearing their street clothes, presumably as they were when the gas hit them. This is a very basic and avoidable mistake, which can't have helped any of those casualties recover.
Sarin gets into clothing, and into hair. It is absorbed through the skin, and is inhaled. Clothing contaminated with Sarin can continue to exude Sarin into the air and into the skin, for half an hour or more after the casualty is moved into clean air. Cloth contaminated with mustard gas is slightly less dangerous, but stays dangerous for weeks.
The first thing that has to happen to a chemical weapon casualty, is that their clothes come off and are left outside the hospital. Oxygen may be given as they are stripped and washed, but if they are not stripped and washed, the oxygen is only going to buy them a few minutes extra. In a Muslim country, there's a reluctance to strip casualties in a public place where others may see. But where the clothes may harbour Sarin, or Mustard gas, the longer the patient wears those clothes, the more harm will be done. Not just to the casualty, but to others, including those trying to help them.
Ideally, there needs to be a compartmentalised process, where clothes come off outside, there's a tent in which casualties are washed, and only then do they enter the hospital. Oxygen, and if it is available, an antidote might be given outside, but every item used on a casualty in one stage, has to stay in that stage and not follow the casualty into the next stage. The only thing passing down the line needs to be the patient.