Crystalline Silica Respiratory Health Effects
National Industrial Sand Association
It is essential to provide a safe and healthful workplace for workers exposed to crystalline silica. The Occupational Safety and Health Administration (OSHA) regulations and its OSHA Hazard Communication Standard, right-to-know laws, and other applicable federal, state, and local laws and regulations on crystalline silica should be strictly followed.
Each workplace is different; only you (the users, including customers and workers) can know and implement the appropriate controls, protections, policies, and procedures to protect those exposed to crystalline silica. It is imperative that you protect, warn, and train all persons exposed to crystalline silica concerning its hazards. This brochure contains general information only, and is intended to help refresh awareness of some workplace protection issues for those who mine, process, sell, distribute, or use crystalline silica. Respiratory health effects information concerning crystalline silica should be communicated to all who are exposed to crystalline silica in the workplace.
Silica is a compound of the elements silicon and oxygen. It is not chemically combined with other elements. When the basic structure of the molecule is a pattern that is repeated and symmetrical, the silica is considered to be "crystalline." Silica is considered to be amorphous if the molecule lacks crystalline structure. In this brochure, the term silica is used to mean crystalline silica (or crystalline silica's most common form, quartz).
Silica occurs virtually everywhere on the earth's surface. Crystalline silica is in most of the rocks found in the earth's crust and in gravels, sands and soils. Many minerals or rocks of commercial value contain varying amounts of crystalline silica. Silica can be a predominant constituent or be present only as a minor accessory mineral with minerals of commercial value. Such rocks and minerals include:
Common uses of sand and construction material containing crystalline silica include:
||Brick and Tile
|Gypsum Wall Board
Industrial sand, also called silica sand, contains a very high percentage of silica, in the form of quartz, and is distinguished by its high degree of purity. Industrial sand deposits are thus less common than deposits mined for construction sand and gravel. Industrial sand is derived from hard rock quartzite formations and in unconsolidated beds of silica sand.
Industrial sand must meet stringent quality requirements since it is used as the principal ingredient in the manufacture of glass, and in foundry cores and molds for metal castings. Industrial sand also is an ingredient in paints, refractory products and specialty fillers. It is used in water filtration, abrasives, for enhancing production of oil and gas, and in specialty construction applications.
Industrial sand also satisfies recreational needs, such as golf courses, tennis courts and ball fields. It is used in municipal water filtration, in residential pool filters and sand boxes. Because of the ubiquitous occurrence and many commercial uses of silica, potential exposure to silica dust is widespread.
Occupational exposure to crystalline silica occurs by breathing silica-containing dusts present in many industries, such as: mining and quarrying; steel, iron, and other metal foundries; abrasive blasting; construction; glass and ceramics; paint and pigments; and, quarrying and crushing stone. OSHA estimates that one million workers in the U.S. are exposed annually to silica-containing dusts.
Inhalation of excessive amounts of silica-containing dusts is a serious health concern. This brochure generally describes the following: a) respiratory health effects of breathing silica- containing dusts, b) the permissible exposure limits adopted by OSHA and the recommended exposure limit of the National Institute for Occupational Safety and Health (NIOSH), and c) some control measures you should consider concerning safety in the workplace while using crystalline silica.
A commonly known health hazard which has been associated historically with the inhalation of silica-containing dusts is silicosis. Silicosis is a fibrotic lung disease (development of scar tissue in the lungs) which can be progressive and disabling; it can lead to death. It is the primary health risk from breathing silica and the oldest known occupational disease.
To pose a danger to the lungs, a dust particle must be respirable. A dust particle is considered respirable if it is smaller than 10 micrometers, about four-ten thousandths of an inch. Dust particles that are respirable are capable of being inhaled into the conducting airways and gas exchange regions of the lungs. Dust particles larger than 10 micrometers are not capable of penetrating the defense mechanisms of the lung to produce injury to the important lower regions of the lung where oxygen transfer takes place.
There are three different types of silicosis. Chronic silicosis may result from prolonged inhalation of excessive levels of respirable silica dust, and may take many years of exposure to develop. A second type, accelerated silicosis, may occur in a relatively shorter period of time from the inhalation of intense excessive levels of respirable silica dust. Acute silicosis, the third type, develops rapidly and has been reported in occupations such as sand blasting and drilling through silica-containing rock. Cases of acute silicosis and complicated cases of chronic silicosis and accelerated silicosis can be fatal.
The number of cases of silicosis can be reduced by implementing measures to reduce exposure to silica-containing dusts. Such measures include engineering controls, improved work practices, training programs, and respiratory protection programs.
SILICA AND CANCER RISK
In 1997, a working group of the International Agency for Research on Cancer (IARC) published a monograph classifying inhaled crystalline silica from occupational sources as carcinogenic to humans, and categorized it as an IARC Group 1 agent. This category is used only when IARC finds there is sufficient evidence of carcinogenicity in humans. The human studies reviewed by the working group included inhalation resulting from workplace exposures. In making the overall evaluation, the IARC working group noted that carcinogenicity was not detected in all industrial circumstances studied, and may be dependent on inherent characteristics of the crystalline silica or on external factors affecting its biological activity or distribution of its polymorphs.
The 1997 publication followed by ten years IARC's 1987 classification of crystalline silica as Group 2A. The 2A category is used when IARC finds there is limited evidence of carcinogenicity in humans and sufficient evidence of carcinogenicity in experimental animals, and results in the agent being considered to be "probably carcinogenic to humans." The 1997 monograph was based on a working group action that was taken with a divided vote, indicating that there remains scientific controversy over whether breathing crystalline silica dust increases the risk of developing lung cancer in humans.
In 1992, the National Toxicology Program (NTP) of the U.S. Department of Health and Human Services published its "Sixth Annual Report on Carcinogens," which listed "silica, crystalline (respirable)" among the "substances or groups of substances ... which may reasonably be anticipated to be carcinogens." The report describes its criteria for determining substances "reasonably anticipated to be carcinogens." This classification has been retained in the NTP's "Seventh Annual Report on Carcinogens."
OSHA established Permissible Exposure Limits (PELs) for many substances, including airborne crystalline silica. The OSHA PEL for crystalline silica in general industry is listed in the "Code of Federal Regulations," 29 CFR 1910.1000, "Air Contaminants," under Table Z-3, "Mineral Dusts." It is a time-weighted average amount that cannot be legally exceeded for an 8-hour shift during a 40- hour week. OSHA has published general industry PELs for three different forms of crystalline silica. Cristobalite and tridymite are forms of crystalline silica, less abundant than quartz, that have lower PELs than quartz. OSHA's PELs for respirable crystalline silica, expressed in milligrams of respirable dust per cubic meter of air (mg/m3), are as follows:
|Permissible Exposure Limits
|Silica, Crystalline (Respirable) Quartz:
Cristobalite: Use 1/2 the value calculated from the above mass formula for quartz.
Tridymite: Use 1/2 the value calculated from the above mass formula for quartz.
NIOSH recommends exposure limits to OSHA; OSHA is responsible for establishing health and safety regulations. NIOSH recommendations are not mandatory exposure limits. In its criteria document for "Occupational Exposure to Crystalline Silica," published in 1974, NIOSH recommended an exposure limit for all forms of crystalline silica of 0.05 mg/m3 for a 10-hour day, 40-hour work week, for the prevention of silicosis. It recommends that silica sand should be prohibited as an abrasive substance in abrasive blast cleaning operations. In 1988, NIOSH recommended that crystalline silica be considered a potential occupational carcinogen.
Safety and health programs, policies, and procedures should be implemented and enforced to control silica hazards in the workplace. These programs, policies, and procedures must be designed to fit the specific needs of the workplace.
In order to improve safety in the use of industrial sand, exposure to airborne silica-containing dusts should be kept below the exposure limit. A program to protect the respiratory health of workers who use crystalline silica-containing materials should include:
- Warning and training workers concerning hazards;
- Crystalline silica dust sampling;
- Engineering controls;
- Good housekeeping; and
- Medical surveillance of workers focusing on respiratory health
If engineering controls or administrative procedures cannot keep the respirable silica dust level below the exposure limit, then respiratory protective equipment is necessary. The respiratory protection program should be custom-designed for the workplace by a qualified industrial hygienist following a full assessment of workplace conditions. The program must also include compliance with the OSHA Hazard Communication Standard, other OSHA regulations, and applicable right-to-know and other federal, state, local laws and regulations.
WORKPLACE DUST SAMPLING FOR CRYSTALLINE SILICA
A workplace in which silica-containing materials are used should maintain a program of periodic crystalline silica airborne dust sampling. Air sampling is conducted to determine a worker's exposure to respirable size particles of crystalline silica. Sampling and analysis of respirable crystalline silica are conducted in accordance with NIOSH Method 7500 for "silica, crystalline respirable."
Consultation with an industrial hygienist can help determine appropriate places for workplace sampling, sampling procedures, and sampling frequency. This sampling program should take two forms: 1) workers should be sampled for their personal exposure to respirable crystalline silica in the course of their jobs, and 2) each area of the workplace with silica exposure should be sampled to determine the level of airborne crystalline silica. Dust sampling should be an ongoing program. Higher exposure measurements indicate that more frequent sampling is required to make sure proper control measures are effective in keeping silica levels below the exposure limit.
ENGINEERING CONTROLS, ADMINISTRATIVE CONTROLS, AND HOUSEKEEPING
Engineering, administrative, and housekeeping controls need to be used to reduce exposure if a worker is exposed to crystalline silica at a level above the exposure limit. Engineering controls consist of the design and installation of new or modified equipment to reduce airborne silica. Industrial ventilation, the most widely used engineering control, involves capturing the dust at points of generation using airflow and removing it through a system of ductwork, air-cleaning devices and fans. Administrative controls, such as limiting a worker's time and presence in areas that have higher silica exposure, can reduce a worker's average exposure to silica-containing dusts. Good housekeeping techniques are important and include dustless methods of cleaning such as washing down surfaces or vacuuming the workplace.
RESPIRATORY MEDICAL SURVEILLANCE PROGRAMS
A respiratory medical surveillance program can monitor the health of workers. Such surveillance programs should be designed by a physician experienced in occupational or pulmonary medicine. A program should include, at a minimum, the following:
- Complete work and respiratory medical history;
- Respiratory symptom questionnaire;
- Periodic chest x-ray, interpreted by a physician certified by NIOSH as a B reader with demonstrated proficiency in the classification of silicosis; and
- Evaluation by a physician with special attention to the lungs
The worker should be informed of the results of the examination and, if indicated, be provided with advice regarding continued exposure to crystalline silica. Comprehensive guidelines for a respiratory medical surveillance program can be found in "Surveillance for Respiratory Hazards in the Occupational Setting" by the American Thoracic Society, which is available from local chapters of the American Lung Association.
RESPIRATORY PROTECTIVE EQUIPMENT
If any work area exceeds the crystalline silica exposure limit, appropriate respiratory protective equipment should be worn by anyone entering that area. There are several types of respirators providing varying levels of protection against dust. OSHA regulations for respiratory protective equipment can be found in "Code of Federal Regulations," 29 CFR 1910.134. Additional guidance can be found in OSHA publication No. 3079, "Respiratory Protection" and in "American National Standard Practices for Respiratory Protection," ANSI Z88.2 (refer to latest edition).
OSHA HAZARD COMMUNICATION STANDARD
Every employer has the obligation to warn, protect, and train workers about workplace hazards and to provide a safe work environment. The OSHA Hazard Communication Standard, 29 CFR 1910.1200, contains specific requirements for employers to warn and train employees concerning workplace hazards. The IARC classification of crystalline silica as "carcinogenic to humans" and the NTP classification of "reasonably anticipated to be a carcinogen," affect specific compliance requirements of the Hazard Communication Standard. To assist in compliance with this standard, use the Material Safety Data Sheets provided by suppliers of silica-containing materials. Also, many state and local governments have right-to-know laws that require employer compliance.
One should not smoke, regardless of whether he or she is exposed to crystalline silica. It is well known that smoking causes lung cancer, emphysema, bronchitis and chronic obstructive pulmonary disease. Smoking also causes diseases of organ systems other than the lung and interferes with effective performance and clearance of particles by the lungs.
OTHER HEALTH EFFECTS
There are many papers in the medical literature which report an association between silica exposure and scleroderma, an autoimmune disorder. Medical literature also suggests that silica exposure is associated with an increased risk for kidney disease. You are advised to consult with a physician, or refer to appropriate medical textbooks, for further information on the relationship between silica exposure and these health effects.
ADDITIONAL SOURCES OF INFORMATION
There are many publications on crystalline silica, silicosis, the silica and cancer issue, exposure limits, medical surveillance, and workplace protection. Some of those publications which provide more detailed information are available from:
1. American Society for Testing and Materials, 100 Barr Harbor Drive, West Conshohocken, PA 19428, (610) 832-9500.
- Standard Practice for Health Requirements Relating to Occupational Exposure to Quartz Dust, ASTM E1132, 1993 (Revised)
2. American Thoracic Society, 1740 Broadway, New York, NY 10019- 4374, (212) 315-8700.
- American Thoracic Society Adverse Effects of Crystalline Silica Exposure, American Journal of Respiratory and Critical Care Medicine, Vol. 155, No. 3, February 1997, pp. 761-768
- American Thoracic Society Standardization of Spirometry - 1994 Update, American Journal of Respiratory and Critical Care, Medicine, Vol. 152, No. 3, September 1995, pp. 1107-1136
3. National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161, (703) 487-4650.
- Criteria for a Recommended Standard - Occupational Exposure to Crystalline Silica, HEW Publication No. (NIOSH) 75-120, NTIS Acquisition No. PB-246-697
4. World Health Organization, 49 Sheridan Avenue, Albany, NY 12210, (518) 436-9686.
- IARC Monographs on the Evaluation of Carcinogenic Risk of Chemicals to Humans - Silica and Some Silicates, Vol. 42, 1987
- IARC Monographs on the Evaluation of Carcinogenic Risk of Chemicals to Humans - Silica, Silicates, Coal Dust and Organic Fibers, Vol. 68, 1997
- IARC Monographs on the Evaluation of Carcinogenic Risk to Humans - Overall Evaluations of Carcinogenicity: An Updating of IARC Monographs, Vols. 1-42, Sup. 7, 1987
5. Oxford University Press, 2001 Evans Road, Cary, NC 27513-2009, 1-(800) 451-7556.
- IARC Occupational Exposure to Silica and Cancer Risk, IARC Scientific Publication No. 97 (ISBN 9283211979), 1989
6. U.S. Department of Labor, Attn: OSHA Publications Office, P.O. Box 37535, Washington, D.C. 20013-7535, (202) 219-4667 or Fax: (202) 219-9266.
- Occupational Safety and Health Administration Directives Pertaining to 29 CFR Sections 1910.94 - 1910.120. OSHA Instruction CPL 2-2.7, October 30, 1978. (Amended by CPL 2-2.7, CH-1, June 3, 1985.) Subject: Crystalline Silica
- Chemical Hazard Communication, OSHA Publication No. 3084, 1995 (Revised)
- Respiratory Protection, OSHA Publication No. 3079, 1997 (Reprinted)
7. American National Standards Institute, 11 West 42nd Street, 13th Floor, New York, NY 10036, (212) 642-4900.
- Practices for Respiratory Protection, ANSI Z.88.2
8. Superintendent of Documents, U.S. Government Printing Office, P. O. Box 371954, Pittsburgh, PA 15250-7954, (202) 512-1800.
- Code of Federal Regulations. 29 CFR 1910.1000, Air Contaminants. (See Table Z-3 for permissible exposure limits for silica)
- Code of Federal Regulations. 29 CFR 1910.1200, Hazard Communication
- Code of Federal Regulations. 29 CFR 1910.134, Respiratory Protection
9. Office on Smoking and Health, National Center for Chronic Disease Prevention and Health Promotion, Mail Stop K-50, Center for Disease Control and Prevention, 4770 Buford Highway, NE, Atlanta, GA 30341-3724, (770) 488-5707.
- The Health Consequences of Smoking - Cancer and Chronic Lung Disease in the Workplace: A Report of the Surgeon General, 1985
10. National Toxicology Program, P. O. Box 12233, Mail Drop E1-02, Research Triangle, NC 27709, (919) 541- 3419.
- Seventh Annual Report on Carcinogens, 1994 - Summary, U. S. Department of Health and Human Services, National Toxicology Program, 1994
11. National Institute for Occupational Safety and Health Publications Dissemination, 4676 Columbia Parkway, Cincinnati, OH 45226, 1-(800) 35-NIOSH.
- NIOSH Alert: Request for Assistance in Preventing Silicosis and Deaths from Sandblasting, Publication No. 92-102, 1992
- NIOSH Alert: Request for Assistance in Preventing Silicosis and Deaths in Rock Drillers, Publication No. 92-107, 1992
- NIOSH Alert: Request for Assistance in Preventing Silicosis and Deaths in Construction Workers, Publication No. 96-112, 1996
DISCLAIMERThe information in this brochure is not intended to comprehensively cover all the topics mentioned, or all areas of concern regarding crystalline silica in the workplace. Users, customers, workers, and readers are advised to consult professionals and experts concerning all matters regarding crystalline silica in each specific workplace. Neither the National Industrial Sand Association (NISA) nor any of its member companies intend or are undertaking, by the distribution of this brochure or otherwise, to give medical, engineering, or other specific advice on crystalline silica in your workplaces. NISA and its member companies disclaim all liability relating to any disease, death, or injury resulting from the purchase, resale, or use of crystalline silica.