What Is Crystalline Silica?
Crystalline silica, or silicon dioxide (SiO2), is the basic component of sand, quartz and granite rock. Crystalline refers to the orientation of the SiO2 molecules in a fixed pattern as opposed to a non-periodic, random molecular arrangement which is defined as amorphous (without shape). Examples of amorphous silica include opal, flint and diatomaceous earth. Three of the most common forms of crystalline silica encountered in industry are quartz, tridymite, and cristobalite.

Background
While worker exposure to crystalline silica can potentially occur in a number of industries, the most common path of exposure in the construction industry may be workers who are exposed to silica during abrasive blast cleaning. Abrasive blast cleaning is a surface preparation method which propels an abrasive by air pressure, centrifugal force, or water pressure against the surface to be cleaned. Dry abrasive blasting cleaning is the most dusty of the methods used for surface preparation and therefore results in the highest levels of worker exposure to airborne concentrations hazardous dust. Because of the difficulty in controlling exposures when dry abrasive blast cleaning, the risk of silicosis is high among workers exposed to abrasive blasting with silica-containing media. For this reason, the National Institute for Occupational Safety and Health (NIOSH) has recommended since 1974 that silica sand (or other substances containing more than 1% crystalline silica) be prohibited as abrasive blasting material.
In 1992, NIOSH published an alert entitled "Preventing Silicosis and Deaths from Sandblasting." The design of the NIOSH alert is to request assistance in preventing silicosis and deaths in workers exposed to respirable crystalline silica during sandblasting. In their publication, NIOSH details recommendations for reducing crystalline silica exposures in the workplace and describes eight case reports involving 99 cases of silicosis from exposure to crystalline silica from abrasive blasting during sandblasting.
Demonstrating the severity of the hazard, NIOSH indicated that an estimated 1 million U.S. workers are at risk of developing silicosis and that 100,000 of these workers are employed as sandblasters. The NIOSH alert further reports that of these 1 million workers, approximately 59,000 will develop silicosis.

Health Effects
Workers exposed to crystalline silica may develop silicosis, bronchitis,
fibrosis (scar tissue in the lungs), granulomatosis infections (such as tuberculosis), and lung cancer. The International Agency for Research on Cancer (IARC) lists crystalline silica in Group 2A, as probably carcinogenic to humans.
Silicosis is defined as a fibrotic lung disease characterized by the formation of small nodules on the lungs. Symptoms include shortness of breath, fever, and cyanosis (bluish skin). In advanced cases, respiratory impairment may develop.
Workers exposed to crystalline silica may develop one of three types of silicosis depending upon the particle size, dust concentration, and the duration of exposure.
Acute Silicosis: Exposure to high concentrations of crystalline silica over short periods of time result in acute silicosis. Symptoms may develop within a few weeks of exposure.
Accelerated Silicosis: Accelerated silicosis is the result of high exposures to crystalline silica and may develop 5 to 10 years after the initial exposure.
Chronic Silicosis: Chronic silicosis results from prolonged exposures to low concentrations of free crystalline silica and may not develop until after 10 or more years of exposure.

OSHA Standards
As opposed to some of the other hazardous substances which workers may be exposed to during abrasive blasting (asbestos, cadmium, lead), OSHA has not issued a specific performance standard for crystalline silica. Crystalline silica is regulated by a number of OSHA standards, all of which the employer must be aware of. These include, but are not limited to:

Permissible Exposure Limits (PEL)
The permissible exposure limits for mineral dusts including crystalline silica in the construction industry (29 CFR 1926.55) depends on the percent of silica in the sand. For example, if the crystalline silica content is 20%, then the PEL is 10 mppcf (millions of particulates per cubic feet of air).

Crystalline Silica Worker Protection Program
29 CFR 1926.55 tells us that to achieve compliance with the established OSHA permissible exposure limits, the employer must first implement engineering controls or administrative controls whenever feasible. When such controls are not feasible to achieve full compliance, protective equipment or other protective measures (such as respiratory protection) shall be used to keep employee exposure to air contaminants below the permissible exposure limit.
Examples of engineering control methods for crystalline silica include: dust suppression, substitution (use of another abrasive), and ventilation. When ventilation is used as a control measure, provisions must be made for regular, periodic inspections of the ventilating systems to insure functional operation.
Administrative control methods include job rotation. Job rotation results in a reduction in the total number of hours of exposure to a hazard a worker receives during a full eight-hour work shift.
Prior to establishing the draft Special Emphasis Program for Silicosis, in 1972 OSHA issued CPL 2-2.7: Crystalline Silica. This directive provides guidelines for Compliance Safety and Health Officers (CSHO) to follow in inspections, and where necessary, the issuance of citations, regarding exposure to silica in the workplace. Both CPL 2-2.7 and the NIOSH alert contain detailed information which should be consulted prior to establishing a crystalline silica health and safety program.
Some of the basic elements of a crystalline silica health and safety program are:

Air Monitoring
Employers are responsible for determining if their employees are exposed to silica at or above the permissible exposure limits. Personal air monitoring consists of sampling personal respirable dust samples from a worker's breathing zone. This need may be determined by:
€Knowledge of the presence of crystalline silica in the workplace
€Reason to suspect that airborne concentrations of crystalline silica exist
€Observations indicating exposure to crystalline silica such as
employee complaints or symptoms attributable to crystalline silica exposure
€Any change in production or processes which may result in an increase of airborne concentrations of crystalline silica

Personal Hygiene Facilities and Practices
€Adequate handwash facilities
€Lavatories maintained and provided with soap and towels
€Showers
€Clean/dirty change areas with provisions for storing clean clothing
€Separate eating/lunch facilities away from areas of exposure
€Food, drinks, tobacco products, and unapplied cosmetics are prohibited in work areas
€Work cloths are not to be cleaned by blowing or shaking, but shall be vacuumed before removal with a HEPA filtered vacuum.

Personal Protective Equipment
Personal protective clothing shall be provided by the employer and consist of appropriate protection for eyes and face, head and extremities, hand, foot, hearing, full body, and respiratory protection. Employers are to provide workers with disposable or washable work clothes at the worksite. Workers are to change into clean clothing before leaving the worksite.

Respiratory Protection
As indicated earlier, the use of respiratory protection for controlling worker exposure to crystalline silica is to be used only after all feasible engineering and administrative controls have been investigated and implemented. When such controls are not feasible to achieve full compliance, these control methods are to be supplemented with protective equipment or other protective measures (such as respiratory protection) to keep employee exposure to air contaminants below the permissible exposure limit.
When respirators are used, employers are required to establish a comprehensive respiratory protection program as outlined in 29 CFR 1910.134, 29 CFR 1926.103, and the NIOSH Guide to Respiratory Protection (NIOSH 1987a).

Medical Surveillance
The NIOSH crystalline silica alert recommends medical examinations be made available to employees who may be exposed to crystalline silica before job placement and at least every three months thereafter. The examination should be performed under the supervision of a licensed physician during normal working hours at no cost to employees. Medical examinations should include a complete medical and occupational history; annual chest x-ray; pulmonary function tests; and an annual evaluation for tuberculosis.

Employee Training
Employee Training is required by 29 CFR 1926.21 (Safety Training and Education) and 29 CFR 1926.59 (Hazard Communication). Training shall consist of:
€Instruction of each employee in the recognition and avoidance of unsafe conditions concerning crystalline silica
€Information concerning the potential physical and health hazards, and adverse health effects, of crystalline silica
€Information on personal hygiene and personal protective measures or equipment required
€Details of the employer's hazard communication and crystalline silica programs including information on labeling, and material safety data sheets
€Instruction on the employer's personal protective equipment and respiratory protection programs including selection, inspection, use and maintenance of respirators

Housekeeping
€All exposed surfaces shall be maintained free of the accumulation of silica dust
€Dry sweeping and the use of compressed air for cleaning surfaces is prohibited
€Vacuum surfaces and clothing using a vacuum equipped with High Efficiency Particulate Air Filter (HEPA)

In Conclusion
When silica sand is used as an abrasive for abrasive blast cleaning, worker exposure to crystalline silica is often high and difficult to control. However, worker protection is an achievable goal. In this article we addressed some of the health effects associated with exposure to crystalline silica and attempted to review the elements of an effective worker protection program for silica which includes: implementing feasible engineering and work practice controls; personal protective equipment (including respiratory protection); personal air monitoring; medical surveillance; hygiene facilities; housekeeping; and employee training.