The gas weapon is one of the enduring horrors of World War One, a weapon of mass destruction. There were around 186,000 British casualties from it, the highest total among the Allies. The first mention of this kind of casualty in Shetland records is for a Tynesider of Shetland parents, Thomas Jamieson Ramsay. He served with the Northumberland Fusiliers and was gassed at Hill 60, probably in early May 1915. Hill 60 was part of the series of battles around Ypres. Thomas J. Ramsay recovered, to be killed in action on 29th January 1916.

The Shetland Roll of Honour lists three men who died of gas poisoning, or from a gas shell. The Roll of Service shows 27 men listed as having been gassed, including one sailor, George Wishart, who served on the monitor Sir John Moore off the Belgian coast. The Roll of Service probably underestimates gassing. At least one soldier, Walter Thomas Nicolson, of Nounsbrough, Aithsting, is listed in the Shetland News as dying as a result of gas poisoning in 1920. The lung scarring caused by gas enabled some lung diseases causing long term difficulties. The Roll of Service, however, doesn’t show W.T. Nicolson as being gassed.

Thomas Smith, originally of Reafirth, Mid Yell, served in the Cameron Highlanders. He wrote to his father, William Guthrie Smith, and the letter was published in the Shetland News, 13th January 1916. While it seems that he himself wasn’t actually gassed, he did experience an attack near Ypres.

It is hard lines to see your mates getting knocked out behind sandbags without having a chance. You would not mind being hit if you had them in the open.

His view was no doubt widely held, that gassing was an unfair method of warfare. Thomas Smith's sense of anger at unfair treatment comes through a little, but he felt the British would prevail in the end. 

They will take some shifting out of their trenches, but when we do get them on the trot, God help them.

Gas had practical difficulties and as counter-measures developed it did not prove to be a war winner. The use of gas (but not stockpiling) was banned in 1925. It remained a much feared method of mass destruction in the decades following the war, until even more thorough going means and practices developed.

More info: 

Shetlanders killed by gas, World War One
Shetland Roll of Honour, Soldiers Lost to Gas Attacks