blue canary
n.— «Altschul said that the first responders to any terrorist attack would likely be police or firefighters. Like the canaries once used by miners to detect poisonous methane gas, the “blue canaries,” as police and firefighters are sometimes called, would probably suffer heavy casualties until the agent is identified.» —“Army giving cities fighting chance against chemical weapons attack” by Ron Martz Atlanta Journal-Constitution (Georgia) June 29, 1997. (source: Double-Tongued Dictionary)
Further reading
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