Silicosis 101

Silicosis is a term describing pneumoconiosis (“dust disease of the lung”) caused by inhalation of dust containing respirable particles (< 10 µm in diameter) of crystalline silica.

Silica refers to silicon dioxide (SiO2), which occurs in crystalline or non-crystalline (amorphous) form. There are different forms of crystalline silica, including alpha quartz (“quartz”), which is the most common. Rock and soil commonly contain quartz. Other forms of crystalline silica, cristobalite and tridymite, are also found in soil and rock and can occur from heating alpha quartz or amorphous silica. Silica is the main part of sand. These three silica polymorphs are the most important in causing silicosis.

Silicosis has been recognized since antiquity. With the introduction of mechanical mining or other activities rupturing rock or sand into fine dust, the prevalence of silicosis increased. The risk of silicosis increases with dose (related to the intensity and duration of exposure) of respired crystalline silica. In the US, an estimated 1-2 million workers have had occupational exposure to crystalline silica dust. The precise number of those with silicosis, however, is not known. In recent decades in the US, the mortality rate from silicosis has been declining, suggesting recognition of hazards and more effective industrial hygiene measures.

People who work in jobs that expose them to silica dust are at increased risk for getting silicosis. These jobs include:

  1. mining, quarrying, tunneling and drilling
  2. abrasive blasting using silica-containing material (such as sandblasting)
  3. stone cutting and tombstone worker
  4. foundry worker
  5. refractory brick worker
  6. pottery worker

There are three basic types of silicosis:

  1. Chronic simple silicosis- usually resulting from long-term exposure (10-20 or more years) to lower concentrations of silica dust. This is the most common variety.
  2. Accelerated silicosis- which occurs 5-10 years after first exposure to larger amounts of silica dust.
  3. Acute silicosis- developing within a period of a few weeks to 5 years after exposure to high concentrations of respirable crystalline silica dust.

Silicosis can become “complicated” by the development of severe scarring (progressive massive fibrosis, or PMF) or by other disease, such as tuberculosis. Silicotics also have an increased risk for other Mycobacterial infections and fungal lung infections. Complicated silicosis is more common with accelerated silicosis than with the chronic variety. Silica is considered a human carcinogen, and persons with silicosis are at increased risk for lung cancer.

Symptoms of chronic silicosis include cough and shortness of breath (especially with exertion). Many in the early stages have no symptoms.  Some patients may experience weight loss, fatigue, or fever. Call your physician if you have been exposed to silica at work and you have symptoms of the disease.

The clinical diagnosis rests upon a thorough occupational and medical history, physical examination, x-ray and exclusion by your doctor of other potential diseases. The non-acute forms are marked by inflammation and scarring, in the form of small nodules, primarily in the upper lobes of the lungs. There may be enlargement of lymph nodes in the chest, sometimes with characteristic “egg-shell” pattern. Often your doctor will obtain pulmonary function tests (PFTs) to assess the impact of silicosis on lung functioning. Only rarely is lung biopsy required.

There is no known cure for silicosis. Acute silicosis may sometimes be treated with whole lung lavage, but this usually does not decrease the mortality of this severe variety of silicosis. After being diagnosed, you should have a baseline TB skin test (PPD) and if not positive (< 10mm induration), it should be repeated about every two years. For those with a positive PPD, one year of isoniazid prophylaxis is recommended. For those with active tuberculosis, the treatment is usually prolonged and may still lead to recurrent TB. Patients with silicosis who smoke should stop smoking. In the absence of effective treatment, primary prevention (control of exposure to respirable crystalline silica) is the only reasonable approach.

OSHA has recently adopted new guidelines for surveillance (screening) of workers who are exposed to silica regularly.

References:

  1. World Health Organization (WHO) – Silicosis fact sheet
  2. Occupational and Safety Health Administration (OSHA) – Safety and health topics, silica, crystalline
  3. American Thoracic Society (ATS) 1996 statement – Adverse effects of crystalline silica exposure
  4. Centers for Disease Control (CDC)/ National Institute for Occupational Safety and Health (NIOSH) 2002 review – Health effects of occupational exposure to respirable crystalline silica