Canary in a Coal Mine

When we say something is a canary in a coal mine, it means it is an early warning of danger. Not surprisingly, the origin of the expression is literal – canaries were historically used to test for carbon monoxide and other toxic gasses in underground mines.

Air quality in underground mines is an important safety issue and modern mines use ventilation and a variety of sophisticated monitoring equipment to ensure air is healthy. As an industry, we believe the most important thing to come out of a mine is the miner, and our modern safety record reflects this. Injury rates in Nova Scotia’s mining and quarrying industry have been reduced 90% since the inquiry.

In the 1800s and early 1900s, Nova Scotian coal miners used flame - candles at first and safety lamps later - to test air quality in mines. Low oxygen levels would cause a flame to shrink or go out. If a flame grew, it indicated the presence of a gas such as methane, which is combustible, or particularly high oxygen levels. Flame also changed colour when exposed to certain gases. For example, methane, a common challenge in coal mines, burns with a blue flame. Using flame to test air quality was obviously a risky practice since it could trigger the fires and explosions testing tried to prevent.

The idea of using canaries instead of flame came from John Scott Haldane, a Scottish physiologist who invented oxygen therapy. He also invented an early version of gas masks in response to Germany weaponizing gas during World War I, and the system by which underwater divers decompress in stages as they return to surface.

Haldane investigated a number of mine disasters in the late 1890s. After examining the bodies of miners, he determined many had been killed by carbon monoxide (CO) caused by fires and explosions, not directly by the fires and explosions themselves. This discovery led to him designing respirators for mine rescue crews and suggesting the use of safety lamps that burned with a bright bluish tint when CO was present. However, even these lamps were not as sensitive to CO as canaries.

Canaries are good early detectors of carbon monoxide because they need immense quantities of oxygen to fly. Their anatomy allows them to get a dose of oxygen when they inhale and another when they exhale, by holding air in extra sacs. In other words, they get a double dose of air and any poisons the air might contain.

When affected by CO or other poisonous gasses, canaries would often stop singing or fall off their perches, often unconscious. This warned the miners to evacuate.

There are few references in historical records to canaries in Nova Scotian coal mines, but they were used here.

The Canadian Mining Journal’s January 1, 1911, edition discussed the Dominion Coal Company’s canaries: The “British Home Office recently passed an order directing that a sufficient number of canaries, linnets or other small birds, should be kept near the pit-top at every British colliery, in readiness for the use of rescue parties should the necessity for them arise. It has long been the custom or parties entering a mine after an explosion or fire to carry with them a small bird in a cage, and to judge by its behaviour whether or not the air was poisonous. The chief danger to such parties arises, of course, from carbon-monoxide, which cannot be detected by the ordinary evidence of the senses, as it is odourless and will support combustion. In the presence of carbon monoxide a canary or linnet will fall from its perch long before men are affected, and by so doing give the signal for retreat. The Dominion Coal Company has obtained a number of canaries that are to be kept at the Central Rescue Station at No. 2 Colliery.”

The 1912 annual report of the provincial Department of Mines said of the Dominion Coal Company, “At the No. 2 station, the Company have for some time kept canaries as suggested by the New Mines Act now in force in England.”

The 1912 annual report also says the Nova Scotia Steel and Coal Company kept “Canaries for gas testing purposes,” along with other safety and medical gear such as safety lamps and oxygen tanks, in its mobile rescue station, a refurbished Pullman railway car.

Despite the canaries’ dangerous role in coal mines, the birds were not just sacrificed for the sake of the miners’ safety. Haldane designed a special cage, pictured below, to protect the canaries. The circular door would be kept open and had a grill to prevent the canary escaping. Once the canary showed signs of carbon monoxide poisoning, the door would be closed and a valve opened, allowing oxygen from the tank on top to be released and revive the canary.

Taking care of the canaries was not only humane but was also done because miners grew attached to the birds and treated them as pets.

British mines stopped using canaries in 1987 when electronic gas detectors, which were more effective, were introduced. There were about 200 canaries in British coal mines at that time.