Background: The purpose of the present study was to mechanically assess models of emergency tourniquet after 18 months of environmental exposure to weather to better understand risk of component damage. Materials and Methods: An experiment was designed to test tourniquet performance on a manikin thigh. Three tourniquet models were assessed: Special Operations Forces Tactical Tourniquet Wide, Ratcheting Medical Tourniquet, and Combat Application Tourniquet. Unexposed tourniquets formed a control group stored in a laboratory; exposed tourniquets were placed outdoors on a metal roof for 18 months in San Antonio, Texas. Two users, a military cadet and a scientist, made 300 assessments in total. Assessment included major damage (yes-no), effectiveness (hemorrhage control, yes-no), casualty survival (alive-dead), time to stop bleeding, pressure, and blood loss. Time, pressure, and blood loss were reported in tests with effectiveness. Results: Exposed devices had worse results than unexposed devices for major damage (3% [4/150] versus 0% [0/150]; ρ = .018), effectiveness (89% versus 99%; ρ = .002), and survival of casualties (89% versus 100%; ρ < .001). In tests for effectiveness, exposed devices had worse results than unexposed devices for time to stop bleeding (29 seconds versus 26 seconds; ρ = .01) and pressure (200mmHg versus 204mmHg; ρ = .03, respectively), but blood loss volume did not differ significantly. Conclusion: Compared with unexposed control devices, environmentally exposed tourniquets had worse results in tests of component damage, effectiveness, and casualty survival.