Silicosis is a serious disease caused by inhalation of crystalline silica particles. The most common industrial exposures to crystalline silica occur in mining, rock drilling, stonework and quarrying, foundry work, sandblasting, glass blowing, and pottery making. This article will focus on the requirements for medical surveillance of silica workers and the need for education of workers and their employers to reduce the incidence of this preventable disease.

Silica
 Oxygen is the most abundant element in the earth’s crust. Silicon is the second most abundant element. With so much of each of these elements around, you just know they are going to interact. Silica is made up of one atom of silicon and two atoms of oxygen. Silica can be free, unbound to other minerals, such as quartz, chert, flint, opal and diatomite. If the silica is combined with other minerals, the combined forms are called silicates, which include substances such as asbestos and talc. Amorphous silica is non-crystalline and does not cause silicosis. If the silica molecules are lined up in an orderly, repeated pattern, the silica is crystalline in form. There are several types of crystalline silica depending on the type of repeated pattern in the molecular chains. Quartz is the most common crystalline silica in the environment, occurring as sand and as a component of many rocks. The crystalline forms of silica are formed from heating quartz or amorphous silica. The heat con-version of amorphous silica to crystalline silica occurs frequently in industries such as foundries. Cristobalite and tridymite can occur from this conversion, and are much more fibrogenic than quartz.

Silicosis
Silicosis is the disease that may be caused by inhalation of crystalline silica particles of respirable size. The interaction of crystalline silica and alveolar macrophages leads to the fibrosis that is characteristic of silicosis. The development of fibrosis takes time. For most exposures the average time for development of fibrosis is about 20 years. Acute silicosis from massive short-term exposure is the exception. The most common symptoms of silico-sis are: dyspnea with exercise, fatigue, fever, weight loss, loss of appetite, mild cyanosis of the lips and ear lobes. The three types of silicosis — chronic, accelerated and acute — are determined by the intensity of the exposure to crystalline silica. Silicosis is characterized by the chest radiographic findings. Chronic silicosis may be simple chronic silicosis or conglomerate silicosis, also known as Progressive Mas-sive Fibrosis (PMF). In simple silicosis the chest x-ray shows small, discrete radio opacities that are less than 10 mm in diameter and predominantly occur in the upper lung zones. Typically, lung function is normal in simple silicosis. There may be mild abnormalities in forced ex-piratory flow. However, there is rarely any disability. In PMF, also known as conglomerate silicosis, the small opacities coalesce into large opacities greater than 10 mm in diameter. Those opacities are most predomi-nant in the upper lung fields. Advanced conglomerate silicosis produces a characteristic butterfly-shaped opac-ity in the upper lung areas. Chronic silicosis occurs after 10 or more years of mild exposure and the radiographic changes may not be apparent until after 15 years of exposure to silica. Hilar adenopathy is common. The hilar nodes may be outlined with a thin shell of cal-cium. This eggshell calcification is highly suggestive of silicosis. Once conglomerate lesions are present, lung function becomes impaired and further deterioration may be rapid. Accelerated silicosis develops within 10 years of ini-tial exposure to more intense levels of silica. Symptoms may be absent or be mild, presenting with chronic cough and dyspnea, but may become severe with the de­velopment of PMF. Radiologic findings appear between 5 and 15 years of exposure showing the same pattern as chronic silicosis.

Acute silicosis occurs within weeks to a few years after massive exposure to crystalline silica. Acute silico­sis is generally fatal. In acute silicosis, the radiographic appearance is of basilar alveolar proteinosis and occurs from short-term overwhelming exposure to free crystal­line silica.

Associated Conditions
The increased risk of tuberculosis in individuals with silicosis has been recognized since the 16th century. This increased risk of infection also applies to other mycobacterial infections. The increased prevalence of tuberculosis in populations of individuals with silicosis has been reported to range from 5% to 75%. Since there is increased incidence and prevalence of tuberculosis in the presence of silicosis, surveillance for tuberculosis is a necessary part of the medical surveillance of silica-exposed workers.

The association of silicosis with pulmonary carci­noma remains controversial. There is an association between silicosis and autoimmune disease reflected by an increased incidence of connective tissue disease such as scleroderma.

Smoking markedly increases the pulmonary injury of silicosis.

Exposure Limits
The current OSHA permissible exposure limit (PEL) for general industry, 10 mg/m3 divided by (% silica quartz + 2) as respirable dust, is based on a formula recom­mended by the American Conference of Governmental Industrial Hygienists (ACGIH) in 1971. The PEL for construction and maritime was based on particle count­ing technology (from the ACGIH’s 1962 Threshold Limit Value), which is now considered obsolete.

Granite workers in environments that were carefully monitored and found to be consistently below the ex­posure limit of 0.1 mg/m3, have, over time, developed radiographic lesions consistent with silicosis.1 Together with the results of similar studies,2,3 these led NIOSH to establish a lower recommended exposure limit (REL) of 0.05 mg/m3.

Currently, the American Conference of Governmen­tal Industrial Hygienists also recommends a threshold limit value (TLV) of 0.05 mg/m3 for respirable crystal­line silica.

Medical Surveillance
Prevention of silicosis is the paramount goal for silica-exposed workers, because silicosis is 100 percent pre­ventable. While primary prevention through exposure control to silica is critical, medical surveillance of exposed workers plays an equally important role in prevention and early detection. It is vital to insist on the use of respiratory protection, even when respirable crystalline silica levels in the air are low. Engineering controls and improved industrial hygiene have lowered the number of affected people; however, there is still a significant incidence of silicosis. Individuals with potential exposure to silica need to be provided with appropriate respiratory protection and the require­ments of the OSHA Respiratory Protection Standard must be met. The mandatory OSHA Respirator Medical Evaluation Questionnaire must be administered prior to the use of the respirator and fit testing is required. Re-evaluation and repeated fit testing can be ordered by the physician if deemed necessary.

The surveillance recommendations I provide in this article are similar to OSHA’s Special Emphasis Program on Silicosis but are more conservative and cautious. Medical surveillance for silica exposure includes:

1. Respiratory protection review of the mandatory OSHA Respirator Questionnaire:4 The OSHA respirator questionnaire is an excellent tool for monitoring the crystalline silica-exposed workforce and covers the majority of the questions that would be pertinent to silica exposure. It is not necessary to edit or alter the OSHA Respirator Questionnaire. The questionnaire must be completed and reviewed at the time of placement and should be repeated at least every two years, independent of the individu­al’s years of potential exposure.

2. Review of specific silica exposure history: The worker’s comprehensive occupational and health history should be reviewed at the time of placement. In addition, the history obtained should specifically address weight loss, fever, and loss of appetite.5

3. Repeat review: Review of specific silica exposure history and review of silicosis symptoms not cov­ered by the OSHA respirator questionnaire should be repeated at the same interval as for the OSHA respirator questionnaire, at least every two years.

4. Physical examination: A baseline physical exami­nation should be completed at the time of place­ment with emphasis on the respiratory system. The physical examination should be repeated:

a. if respiratory symptoms develop;

b. if interval pulmonary function tests or respira­tor or silica questionnaires indicate the need for further evaluation;

c. every 5 years for individuals who have had less than 20 years of exposure;

d. every 2 years for individuals with 20 or more years of exposure;

e. at the time of exit from work with exposure to crystalline silica.

5. Spirometry: Pulmonary function decline is seen in silica-exposed individuals even before there are radiographic findings. Pulmonary function studies need to be obtained:

a. at placement;

b. if respiratory symptoms develop;

c. if interval histories indicate a need for further evaluation;

d. every 5 years for individuals with less than 20 years of exposure;

e. every 2 years for individuals with 20 or more years of exposure.

6. Chest x-ray: Chest x-rays should be obtained at the time of placement. Repeat chest x-rays would be appropriate every 5 years for individuals with less than 20 years of exposure and every 2 years for individuals with 20 or more years of exposure. If questionnaires, pulmonary function tests or symptoms indicate, then the chest x-ray should be repeated. There should also be a repeat chest x-ray at the time of exit.

7. Surveillance for tuberculosis: Individuals with known silicosis and those who have had 10 or more years of exposure to crystalline silica should have annual PPD skin test for tuberculosis. In this popu­lation 10 mm of in duration is considered positive.

Non-compliance with current standards is very com­mon among miners, construction workers, foundry workers, sand blasters, and the stone products indus­tries. This is where the occupational health professionals need to recognize their very important role in educat­ing employers and workers about the risks of silica exposure. We need to take on the duty of oversight to assure that the appropriate environmental monitoring is done, the medical surveillance program is in place, and employers and workers comply with what we know will