Occupational Exposure to Crystalline Silica During Tuckpointing and the Use of Engineering Controls

ABSTRACT

In August 1996, the Occupational Safety and Health Administration (OSHA) implemented a nationwide Special Emphasis Program (SEP) for establishments in construction, general industry, and maritime to ensure worker protection from over-exposure to crystalline silica dust. Aerosols of particulates can be deposited in the lungs and cause tissue damage and disease. Inhalation of silica containing dust has been associated with silicosis and other debilitating diseases. The National Institute for Occupational Safety and Health (NIOSH) estimates that more than one million workers are at risk for developing silicosis. Under the SEP, OSHA inspections target establishments where there are known cases of silicosis or over-exposures to crystalline silica are most likely.

During an inspection under the SEP, OSHA looks for elements of a silicosis prevention program which may be effective at preventing crystalline silica over-exposures. Many of these elements are required by specific OSHA standards when an over-exposure to crystalline silica exists. If OSHA determines that the employer has an effective program, the inspection may be terminated.

Exposures to crystalline silica in the construction industry are widespread. This report documents exposures to crystalline silica and the effectiveness of dust collection equipment during tuck-pointing operations. Tuckpointing involves grinding mortar from the joint between the bricks. During this process, respirable crystalline silica concentrations are often well above permissible limits. The depth of the cut is usually specified by an architect; it is typically three-quarters of an inch. New mortar is then mixed and applied to the joint. The grinding portion of the operation generates the majority of the respirable dust. In addition to respirable dust containing silica, there may be metal dust from the pigment used in the colored mortar or coating on the bricks.

At the request of two employers, a hygiene consultant from the Illinois OSHA On-Site Consultation Program conducted personal air monitoring during tuckpointing operations at four locations. Two to four employees at each work site were involved with tuckpointing operations. At three of the work sites, employees were working on scaffolds on high-rise buildings. The process involved two employees on a scaffold first grinding the mortar on a section of wall, then applying new mortar to that section. At the other work sites, employees did grinding work for three to five hours in the morning, and applied mortar for the remainder of the day. Employees who were monitored while using the dust collection equipment were not on scaffolds. Employees were occasionally observed to be using a leaf blower to blow dust out of the cracks and off the wall.

METHODS

At least two employees at each site were monitored for respirable crystalline silica exposure. Representative air monitoring was conducted for approximately five to seven hours. Although the sampling period did include break time, the monitoring equipment was removed during lunch and replaced after lunch. To demonstrate the use of dust collection equipment, representative air monitoring was conducted for one to two hours.

Personal samples were collected on preweighed 37-millimeter (mm) diameter, 5-micron (μm) pore size, low ash polyvinyl chloride (PVC) filters at 1.7 liters per minute (L/min) with a nylon cyclone. The samples were weighed after sampling to determine respirable dust concentration. The filters were then transferred to a dish, dissolved in tetrahydrofuran, redeposited on a silver membrane filter by suction filtration, and analyzed by x-Ray diffraction (XRD) to determine silica content based on OSHA ID-142. Samples were analyzed by the Wisconsin Occupational Health Laboratory. Results are expressed as milligrams per cubic meter of air (mg/m3). Silica content was compared to the respirable dust concentrations to determine percent silica in the sample. The result was compared to the OSHA permissible exposure limit (PEL) per 29 CFR 1926.55, Appendix A, Mineral Dusts Table which is calculated using the formula:

10 mg/m3  / (% silica) + 2

RESULTS AND DISCUSSION

Eight-hour time-weighted average (TWA) results ranged from 18 to 116 times above the OSHA Permissible Exposure Limit (PEL) with silica content ranging from 1.4 to 23 percent. All employees monitored were wearing approved respiratory protection for silica. However, those employees exposed greater than 10 times the OSHA PEL were wearing respiratory protection which did not provide adequate protection. In addition, neither company had developed a written respiratory protection program as required by OSHA or an effective silicosis prevention program.

Air monitoring, both with and without the use of dust collection equipment, was conducted during the grinding portion of tuckpointing operations. The dust collection equipment consisted of a shroud on the grinder and a hose attachment leading to a collection bag. This system captured dust as it was generated at the grinder. Air monitoring of two employees using the dust collection equipment showed a substantial decrease in exposure to respirable dust containing silica, 37 percent and 47 percent, respectively (Table II).

However, the employees using the dust collection equipment stated that it was heavy and difficult to use. In addition to the fact that they had not previously used the equipment, they found the handle for gripping the grinder and the hose attachment to be awkward. They were accustomed to being able to move the grinder forward and backward along the mortar joint. However, the grinder equipped with dust collection equipment would not allow this type of action and slowed their work progress. From an economic standpoint, they found this slowing of the work process unacceptable.

The manufacturer of the grinder with dust collection equipment agreed to modify the equipment; after rearranging the handle and the hose attachment, the equipment was easier to handle. The shroud on the grinder was also adjusted. Air monitoring during grinding subsequent to these modifications demonstrated an additional reduction in employee exposure to respirable dust containing silica for a total reduction of 94 percent (Table III).

This was the first time exposure monitoring indicating that exposures could be reduced to levels below 10 times the PEL. This finding is important because it allows an employee to use a negative-pressure half-mask respirator for protection. Respiratory protection with a higher protection factor is required when exposure exceeds 10 times the PEL. Visual observations during grinding with the dust collection equipment showed substantially less dust in the air. On some job sites, use of dust collection equipment would also decrease clean up time considerably, an economic advantage to the contractor.
