Previously, the chemical and mechanical deterioration of polyaramid and polybenzimidazolebased protective clothing fabrics that resulted from concurrent thermal, moisture, and simulated sunlight exposure were quantified. This manuscript discusses the individual impacts of ultraviolet (UV) light irradiation at ambient conditions and moisture and elevated temperature in the absence of irradiation on the performance properties of outer shell (OS) fabrics used in firefighters' turnout gear. The mechanical properties which most often determine the protective performance of firefighters' turnout gear, including unidirectional stretching, tear resistance, and elastic properties, were tested before and after an accelerated weathering process.