(1) Chronic Silicosis: Usually occurs after 10 or more years of exposure at relatively low concentrations. Swellings caused by the silica dust form in the lungs and lymph nodes of the chest. This disease may cause people to have trouble breathing and may be similar to chronic obstructive pulmonary disease. (2) Accelerated Silicosis: Develops 5 to 10 years after the first exposure. Swelling in the lungs and symptoms occur faster than in chronic silicosis. (3) Acute Silicosis: Develops after exposure to high concentrations of respirable crystalline silica and results in symptoms within a period of a few weeks to 5 years after the initial exposure [NIOSH 1986; Parker and Wagner 1998]. The lungs become very inflamed and can fill with fluid, causing severe shortness of breath and low blood oxygen levels. PMF can occur in either simple or accelerated silicosis but is more common in the accelerated form. Figure 1-1 shows a lung that has been damaged by silicosis.