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Beryllium: A toxic metal found in ores containing other elements that is used in making metal alloys for nuclear reactors and the aerospace industry. Acute exposure to beryllium fumes can cause a severe, sometimes fatal pneumonitis (inflammation of the lungs). Chronic overexposure to beryllium is more common and causes a diffuse inflammatory reaction in the lungs resulting in granulomas and fibrosis of the lungs and, in time, increasing shortness of breath. Beryllium poisoning is termed berylliosis.