Horse, Dog and Soldiers With Gas Masks

Though many filtering masks existed for both mining operations and chemical exposure before the First World War, they were not widely known, and many of their designs could not have stood up to the chemical attacks that were perpetrated on a scale never before seen by humanity.

The first mass use of poison gas (chlorine, specifically) at the Second Battle of Ypres was a massive failure - though many Canadian soldiers were exposed, the prevailing winds shifted, and the German troops that deployed the gas were overcome. Still, this battle showed the Allies that the Germans were serious about using chemical agents, and both sides began to develop gas masks not only for themselves, but for their combat animals.

Dogs and horses both had specially-fitted gas masks, and while the canines learned quickly to work with theirs, horses had the significant problem of mistaking their breathing-boxes for feed bags, since the shape and feel was so similar. This was overcome by training and lengthening the gas masks, so that the filter boxes did not touch their lips.

National Museum of Health on Flickr. USA Army Signal Corps, ca. 1915-1918.

Classification of Chemical Agents

There are four primary classes of Lethal Agents that are produced as weapons. There are also two classes of Non-Lethal Harassing Agents.

Lethal Agents Lethal agents are classified by the affect that they have on the body, as all have the capacity to cause death.

Harassing Agents Harassing agents are less developed, as the line between incapacitation and lethality is often difficult to control with gaseous weapons, and incapacitation was seen as having little use in warfare for most of history.

The categories of chemicals  are also unrefined, and the difference is only how used they are - "Incapacitators" are currently only in very rare experimental situations, and have not had good effects or publicity in any of the known situations they were in.  "Riot Control" chemicals are considered non-lethal lachrymatory or vomit agents, and are intended for use against civilians or civilian groups seen as belligerent.

"As bad a mode of warfare as poisoning the wells of the enemy..."

In 1675, the first major agreement on chemical warfare was brokered between France and the Holy Roman Empire in Strasbourg. The treaty prohibited the use of poisoned bullets and missiles (such as arrows), and was, for the most part, actually fairly well-observed by all sides.

Chemical warfare, though still proposed from time-to-time by weapons engineers and strategists, was largely dismissed until near the end of the 19th century as “dirty” and “as bad a mode of warfare as poisoning the wells of the enemy”. Even the early snipers near the end of the US Civil War (though despised by many) were seen as more honorable and justifiable than chemical attacks. Various attempts at this kind of warfare were used in limited extent by small groups and nations not interested in such niceties, but there is little evidence that Western nations engaged in such tactics on large scale.

In 1899, the great powers of the world once again recognized the threat that emerging technologies and discoveries could pose to both peace and human (or at least civilian) rights in wartime. This resulted in the Hague Declaration of 1899, which prohibited the use of chemicals, toxins, or biological agents against others. However, the declaration was not binding to any nation, as it was not unanimously agreed upon. Still, a fragile agreement still stood among all, and chemical warfare was still off the battlefields for the time being.

That is, of course, until the Great War broke out, and one Fritz Haber would change the world forever…

More on the History of Chemical Warfare:

Center for Nonproliferation Studies: Chronology of Major Events in the History of Biological and Chemical Weapons

ScienceDaily: Siege Mine Smoke Attacks at Dura-Europos

Medical Management of Chemical Casualties Handbook: Introduction and History

Foreign Affairs [Canada]: History of Chemical Warfare

Chemical Stockpile Emergency Preparedness Program: A Brief History of Chemical Weapons

Avoiding Armageddon: Chemical Warfare

German medic in early-WWI gas mask

The first "masks" to protect against the deadly gasses such as chlorine (as they could actually result in fatalities, unlike the earliest gasses, which were only lachrymatory agents), were no more than damp cloths placed over the nose and mouth. It was believed that dampening the cloths with concentrated urine was more effective than simple water.

As the war went on, masks naturally got better, and easier to put on, so the effectiveness of gas weapons decreased significantly.

ETA: Forgot to mention what this guy was using. This is a gauze pad that was issued by the Germans shortly after they began using chlorine gas against the Entente power. It was filled with cotton waste, and was soaked in a bicarbonate solution. The tank and filter the medic carries are a basic oxygen tank and delivery system, but it couldn't be used at the same time as the filtering mask.