Background: The term "crystalline silica" refers to crystallized form of SiO2 and quartz, as the most abundant compound on earth crust, is capable of causing silicosis and lung cancer upon inhaling large doses in course of occupational exposure. Materials and Methods: In this study, airborne respirable dust samples were collected on mixed cellulose filters (25 mm diameter, 0.8 mm pore size), by using a cyclone separator at the flow rate of 2.2 l/min for a maximum volume of 800 liters. Infrared absorption spectrometry was used according to the "National Institute of Occupational Safety and Health" (NIOSH) method No. 7602 for analysis of samples. Risk assessment techniques predictive of silicosis and lung cancer were employed. Results: The geometric mean of workers' exposure to crystalline silica in ten industrial fields (stone milling and cutting, foundry work, glass manufacturing, asphalt, construction, sand and gravel mining, sand blast, ceramics, bricks and cement manufacturing) was in the range of 0.132 to 0.343 mg/m3 . Mortality rate of silicosis was predicted to be in range of 1 to 52 per one thousand exposed individuals. Risk of lung cancer mortality in exposed workers in the east zone of Tehran based on geometric mean exposure of industrial activity and 45 years of exposure was in range of 50 to 129 per one thousand. In terms of risk assessment of silicosis mortality, cumulative exposure of 21 percent of population complied with the notion of acceptable risk. In regard to lung cancer mortality, 100 percent of the population were expected to have an unacceptable risk after 45 years of active work experience. Conclusion: This study is the first of its kind in Iran demonstrating a profile of exposure in different groups of workers in the east zone of Tehran's greater city, covering 5.5 million populations. Considering the total population of one hundred thousand workers exposed to quartz in east zone of Tehran and aging of the current young workforce, numerous cases of silicosis and lung cancer are forecasted in near future. (Tanaffos 2009; 8(3): 43-50)