Dust and gas exposure in tunnel construction work

Abstract

Personal exposures to dust and gases were measured among 189 underground construction workers who were divided into seven occupational groups performing similar tasks in similar working conditions: drill and blast crew; shaft-drilling crew; tunnel-boring machine crew; shotcreting operators; support workers; concrete workers; and electricians. Outdoor tunnel workers were included as a low-exposed reference group. 

The highest geometric mean (GM) exposures to total dust (6-7 mg/m3) and respirable dust (2-3 mg/m3) were found for the shotcreters, shaft drillers, and tunnel-boring machine workers. Shaft drillers and tunnel-boring machine workers also had the highest GM exposures to respirable alpha-quartz (0.3-0.4 mg/m3), which exceeded the Norwegian occupational exposure limit (OEL) of 0.1 mg/m3. 

Shaft drillers had the highest exposure to oil mists (GM=1.4 mg/m3), which was generated mainly from pneumatic drilling. For other groups, exposure to oil mist from diesel exhaust and spraying of oil onto concrete forms resulted in exposures of 0.1-0.5 mg/m3. Exposure to nitrogen dioxide was similar across all groups (GM=0.4-0.9 ppm), except for shaft drillers and tunnel-boring machine workers, who had lower exposures. High short-term exposures (>10 ppm), however, occurred when workers were passing through the blasting cloud.

MATERIALS AND METHODS

Site Selection and Characteristics

Fifteen Norwegian underground construction projects and one project in Italy with a Norwegian contractor were surveyed to assess the personal exposures of underground construction workers. In addition, concrete workers performing ironwork and carpentry work outside the tunnels were included in the study to serve as a reference group for the epidemiological study

The sites were selected because they were considered to be representative of projects in Norway. The projects built tunnels, rock caverns, and shafts. The excavated cross sections ranged from 13 to 340 m2. Tunnels were between 500 m and 2500 m long, rock caverns were about 100 m long, and the one shaft in the study was 200 m long at the time of the study. 

All of the rock caverns and tunnels had a one-way ventilation system. The distance from the end of the ventilation duct to the tunnel face was 40-60 m and the fan flow rate was typically between 1800 and 2500 m3/min. The shafts had no mechanical ventilation system. In all projects the machinery was diesel powered and the same types of machines were used.

Sampling Strategy

After walk-through surveys of the sites were conducted and in- formation on jobs and tasks was collected, workers were divided into groups performing similar tasks under similar working conditions. Occupational groups included in this study are described in detail in Table I.

A random sample of workers from each group was asked to participate in the study. Participation was voluntary. Exposures to dust and gases were determined by means of personal sampling, and two or more agents were measured for each person for at least two days. Workers were interviewed after sampling for their perception of the normalcy of the exposure conditions.

Under the labor agreements of the workers the work shift was 10 hours with two breaks of 30 min each. The sampling time was limited to 5-8 hours (unless otherwise noted) because of the limited battery capacity of the sampling equipment. High dust concentrations further increased power consumption. However, the sampling time was considered representative for the whole work shift because the sampling periods within a shift were selected randomly and tasks were often repeated during the day.

Sampling Methods and Analysis

Total dust and particulate polycyclic aromatic hydrocarbons (PAHs) were collected on acrylic copolymer membrane filters (Versapore 800, Gelman Sciences, Ann Arbor, Mich.), with a 0.8 m pore size, fitted in 25 mm closed-faced aerosol filter cassettes (Gelman Sciences) at a sampling flow rate of 2 L/min. The par- ticle mass was measured with a microbalance (Sartorius AG, MC 210 p, Goettingen, Germany), with a detection limit of 0.06 mg (0.063 mg/m3 based on 8 hours of sampling).

For collection of PAHs and other volatile organic compounds (VOCs) the empty space behind the filter was completely filled with an adsorbent, XAD-2 (SKC, Blandford Forum, UK). Total PAHs and VOCs were measured by gas chromatography (GC) with a flame ionization detector (FID). The detection limits of PAHs and VOCs were 0.2 g/m3 and 0.01 mg/m3, respectively, based on 8-hour sampling at a flow rate of 2 L/min. The method for sampling and determination of PAHs is described in detail elsewhere.

Respirable dust was collected on 37-mm cellulose acetate filters with a pore size of 0.8 m using a cyclone separator (Casella T13026/2, London, UK) at a sampling flow rate of 2.2 L/min. The particle mass was measured gravimetrically (with a detection limit of 0.06 mg) and the alpha-quartz content in the respirable dust sample was measured by X-ray diffraction using National Institute for Occupational Safety and Health (NIOSH) Method 7500.

Formaldehyde was collected on a filter impregnated with 2,4- dinitrophenylhydrazine (GMD 570 Formaldehyde passive dosimeter badge, GMD Systems) in a polypropylene housing. Formaldehyde was analyzed by high performance liquid chromatography with an ultraviolet detector, according to information provided by the producer of the dosimeters. The detection limit was 0.003
ppm based on an 8-hour sampling period.

Oil mist was collected on glass fiber filters (Whatman GF [A], Maidstone, England) with a backup filter of cellulose acetate with a pore size of 0.8 m in 37 mm closed-faced aerosol cassettes (Millipore Corp.). Oil vapor was collected on charcoal (SKC,Blandford Forum, UK).

The sampling flow rate for both components was 1.4 L/min, and the sampling period was 2-4 hours. Oil mist was measured by Fourier transform infrared spectroscopy after desorption with Freon 113. A standard solution of the oil that was the source of exposure was measured together with the sample.

Oil vapor was measured by GC-FID after desorption with carbon disulfide and with n-decan (Fluka Chemie AG, CH- 9470, Switzerland) as a standard. The detection limit for oil mist was 0.008 mg and for oil vapor it was 0.17 mg (0.05 mg/m3 and 1.0 mg/m3 for oil mist and oil vapor, respectively, based on 2- hour sampling).

Elemental carbon was analyzed as a marker of diesel exhaust. Samples were collected on quartz filters in 37 mm closed-faced standard aerosol cassettes with a sampling flow rate of 2.0 L/min. The filters were analyzed for elemental carbon according to NIOSH Method 5040 with a detection limit of 1.28 g (1.33 g/m3 based on 8-hour sampling).

Concentrations of carbon monoxide and nitrogen dioxide were measured with direct-reading electrochemical sensors with a data- logging facility built into the instrument (Neotox-xl personal single-gas monitor, Neotronics Limited, Takeley, UK).

An averaging period of one reading every 2 min was selected. The detection limit of nitrogen dioxide and carbon monoxide measurements was 0.2 ppm and 2 ppm, respectively.

Direct-reading diffusion tubes (Drager Aktiengesellschaft, Lubeck, Germany) were used to measure carbon dioxide and ammonia and had a detection limit of 63 ppm and 2.5 ppm, respectively, based on an 8-hour sampling period.

Quality Control

One field blank was taken to the field per day for every 10 particulate samples, with at least 1 blank per day. All blanks were analyzed and found to be below the limit of detection (0.06 mg). The quality control procedures for the gravimetric measurements also included measuring two weights (19.99 mg, SD=0.03 and 49.95 mg, SD=0.04), at the beginning of each weighing session. The weights were calibrated annually by the Norwegian Metrology and Accreditation Service. The laboratory that analyzed formaldehyde, PAH, alpha-quartz, and oil mist participated in interlaboratory proficiency testing programs.

The response factors of the electrochemical sensors were calibrated every third month by the supplier with calibration gases obtained from Bedford Scientific Ltd., UK (carbon monoxide) and Norsk Hydro, Rjukan, Norway (nitrogen dioxide).


Data Analysis

Using cumulative probability plots, the exposure data were found to be best described by lognormal distributions and were ln-transformed for the statistical analyses. Standard measures of central tendency and distributions (arithmetic and geometric means and geometric standard deviations) were calculated. 

A small percentage of measurements of nitrogen dioxide (n=5),volatile organic compounds (VOC) (n=5)  and respirable dust (n=3) were below the detection limit. The geometric mean exposure (GM) and the geometric standard deviation (GSD) were therefore estimated according to Perkins et al.(26) The estimated GMs were used to calculate estimated values below the detection limit:

The Kruskal-Wallis test was used to evaluate the differences in exposure levels among the job groups because of the heterogeneous variances across job groups. To increase independence of the data only the first valid measurement from each person was used in these tests. The Mann-Whitney test was used to evaluate the differences in exposure levels between underground construction workers and outdoor construction workers. Statistical analyses were carried out with SPSS 8.0 (SPSS Inc. Chicago, Ill.).

RESULTS

Measurements were carried out on the 16 work sites over a period of three years between June 1996 and July 1999. Two of the projects were associated with power plants, four with railway installations, seven with road construction, one with a sports center, and two with cleaning/purification plants (Table II).

All 189 underground construction workers and 20 outdoor concrete workers invited to participate in the exposure assessment did so. The numbers of measured underground construction workers were 52 drill and blast workers; 8 shaft-drilling workers; 11 TBM workers; 17 shotcrete operators; 12 support workers; 61 concrete workers; and 20 electricians. Most of the workers (77%) were monitored on more than one occasion.

Table III gives a summary of the exposure levels by agent. In addition, 47 samples were analyzed for PAH (25 workers), which were all below the detection limit (<0.2 g/m3).

A Kruskal-Wallis test between job groups showed statistical difference for all agents (p<0.01) except for nitrogen dioxide (p=0.6), formaldehyde (p>0.1), and elemental carbon (p>0.1).

The mean exposures showed a moderate variability in exposure levels across job groups (Tables IV-VI). A quarter of the geometric standard deviations from all agent- and job group combinations were greater than 3.0.

The highest geometric mean exposures to total dust (>6 mg/m3) and respirable dust (>=2 mg/m3) were found in shotcreters, shaft drillers, and TBM workers (Table IV). The geometric mean exposure of alpha-quartz varied from 0.010 mg/m3 (support workers) to 0.39 mg/m3 (TBM workers) (Table IV).  

Ten elemental carbon samples were collected at a single work site (a rock cavern). The geometric mean exposures of the drill and blast workers and the concrete workers were 340 and 100 g/m3, respectively (Table IV). 

The geometric mean exposures to nitrogen dioxide varied from 0.2 ppm (TBM workers) to 0.9 ppm (electricians) (Table V). However, the drill and blast workers were exposed to high peaks of nitrogen dioxide when passing through the blasting fumes during transportation of the blasted rock out of the tunnel.

The maximum observed peak value was 20 ppm among these workers for a 2-min averaging period, which was much higher than in the other groups of tunnel workers, for which a maximum of 7.4 ppm was observed (concrete workers) (Table V). 

In total, 18% of the measurements performed on the drill and blast workers showed exposure peaks >10 ppm. The geometric mean exposures of carbon monoxide varied from 2.9 ppm (shotcreting operators) to 10 ppm (support workers), and carbon dioxide varied from 690 ppm (support workers) to 1300 ppm (shaft drillers) (Table V).

The highest geometric mean exposures to oil mist (1.4 mg/m3) were found in shaft drillers (Table VI). Formaldehyde, oil vapor, and VOC levels were low for all workers (Table IV). The outdoor workers as a group had a statistically lower geometric mean exposure (p<0.01) to all measured exposures, except oil vapor (p=0.1) (Table VII), compared with underground construction workers (Table III).

Respirators generally were not worn by the workers during the work shift, except for workers performing the shotcrete technique and TBM excavation method, both of whom occasionally wore dust masks. One-fifth of the measurements were reported by the workers to have been taken in conditions that were worse than usual, and 5% of the time it was reported to have been better than usual. When the conditions were reported to be worse than usual, the most frequent explanation was that the ventilation system was not functioning.  
