A 60-year-old man has had worsening dyspnea and nonproductive cough over the past 2 years. On physical examination, his temperature is 37.4degC, pulse is 74/min, respirations are 20/min, and blood pressure is 110/70 mm Hg. A chest radiograph shows extensive interstitial lung disease and a prominent right-sided heart border. Spirometry reveals decreased FEV1 and FVC. His pulmonary disease is most likely caused by exposure to which of the following?