Health hazard evaluation report: HETA-2003-0275-2926, U.S. Department of Interior, Denver, CO.
In June 2003, the U.S. Department of Interior (DOI) asked NIOSH to evaluate exposure to airborne crystalline silica among rock drilling workers at Rocky Mountain National Park (RMNP) in north-central Colorado. Although employees had not reported any symptoms related to rock drilling, DOI management wanted documentation of worker exposure along with recommendations on the use of respiratory protection during the use of two types of portable gas-powered rock drills. The park employs 45 trail workers to build new hiking trails and to maintain 360 miles of existing trails. Holes are drilled into large rocks so they can be split and used as steps for steep sections of trail. Crews of five or six workers per project use either a 75-pound drill or a 30-pound drill for splitting rocks. The larger drill forces compressed air through the drill shank as a means of keeping the drill hole free of dust. Water can not be used as a dust suppression method because it causes the drill bit to bind in the rock. The smaller drill does not have an air-flushing mechanism and water may be applied to the drill holes. The large drill can drill holes about four times faster than the small drill. Workers wear powered air-purifying respirators when using the large rock drill, since few other control options are available in remote areas. Exposure to airborne total dust, respirable dust, respirable crystalline silica (quartz and crystobalite), and carbon monoxide (CO) was evaluated for five workers during three days of sampling with maximum wind conditions of three miles per hour. Two bulk samples of rock dust contained 15% and 23% quartz. No crystobalite was found in any of the air or bulk samples. When workers used the large drill, maximum 8-hour time-weighted average (TWA) air concentrations were as follows: airborne total dust, 4.8 milligrams per cubic meter (mg/m3 ); respirable dust, 0.63 mg/m3 ; and respirable quartz, 130 micrograms per cubic meter (microg/m3 ). These results indicate that overexposure to quartz occurs if workers use the large drill without respiratory protection. All other measured contaminants were below recommended evaluation criteria. Using the small drill, 8-hour TWA exposure to total dust ranged up to 0.18 mg/m3 , respirable dust ranged up to 0.12 mg/m3 , and exposure to respirable quartz was non-detectable (< 30 microg/m3 ) during both wet and dry drilling. Low exposure to CO (up to 9 parts per million, 8-hour TWA) was found during the use of both drills. Overexposure to crystalline silica occurs when the large air-flushing, gas-powered rock drill is used on quartz-containing rock. Recommendations are provided to minimize exposure through personal hygiene, training, respiratory protection, and medical monitoring.