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Original article
Personal exposure to ultrafine particles from PVC welding and concrete work during tunnel rehabilitation
  1. Rikke Bramming Jørgensen1,
  2. Morten Buhagen2,3,
  3. Solveig Føreland2,4
  1. 1Department of Industrial Economics and Technology Management, Norwegian University of Science and Technology, Trondheim, Norway
  2. 2Department of Occupational Medicine, St. Olav's Hospital, Trondheim University Hospital, Trondheim, Norway
  3. 3Department of Public Health and General Practice, Norwegian University of Science and Technology, Trondheim, Norway
  4. 4Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, Trondheim, Norway
  1. Correspondence to Dr Rikke Bramming Jørgensen, Department of Industrial Economics and Technology Management, Norwegian University of Science and Technology, Trondheim N-7491, Norway; rikke.jorgensen{at}iot.ntnu.no

Abstract

Objectives To investigate the exposure to number concentration of ultrafine particles and the size distribution in the breathing zone of workers during rehabilitation of a subsea tunnel.

Methods Personal exposure was measured using a TSI 3091 Fast Mobility Particle Sizer (FMPS), measuring the number concentration of submicrometre particles (including ultrafine particles) and the particle size distribution in the size range 5.6–560 nm. The measurements were performed in the breathing zone of the operators by the use of a conductive silicone tubing. Working tasks studied were operation of the slipforming machine, operations related to finishing the verge, and welding the PVC membrane. In addition, background levels were measured.

Results Arithmetic mean values of ultrafine particles were in the range 6.26×105–3.34×106. Vertical PVC welding gave the highest exposure. Horizontal welding was the work task with the highest maximum peak exposure, 8.1×107 particles/cm3. Background concentrations of 4.0×104–3.1×105 were found in the tunnel. The mobility diameter at peak particle concentration varied between 10.8 nm during horizontal PVC welding and during breaks and 60.4 nm while finishing the verge.

Conclusions PVC welding in a vertical position resulted in very high exposure of the worker to ultrafine particles compared to other types of work tasks. In evaluations of worker exposure to ultrafine particles, it seems important to distinguish between personal samples taken in the breathing zone of the worker and more stationary work area measurements. There is a need for a portable particle-sizing instrument for measurements of ultrafine particles in working environments.

  • ultrafine particles
  • FMPS
  • particle size distribution
  • personal exposure
  • PVC welding

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Introduction

Tunnel construction workers are exposed to particulate and gaseous air contaminants, and respiratory diseases are known to be frequent among these workers.1 During the past few years, the industry has focused on exposure reduction, but recent studies show that the levels that tunnel construction workers are exposed to still appear to have a detrimental impact on lung function.2 Known contaminants are α-quartz, oil mist, oil vapours, organic carbon, elemental carbon, nitrogen dioxide, ammonia and thoracic, total and respirable dust.3–5

It has been suggested that exposure to ultrafine particles can explain some of the adverse health effects of exposure to particulate matter. However, exposure to ultrafine particles is not evaluated in the tunnel construction industry. Tunnel rehabilitation work normally involves little or no excavation, and in that respect it differs from tunnel construction work. However, many of the tasks performed during the finishing stages of tunnel construction will be similar to the work carried out by tunnel rehabilitation workers. Exposure to ultrafine particles is documented in other industries and processes relevant to tunnel construction or rehabilitation, including welding,6–8 the rubber manufacturing industry,9 asphalt work,10 paving and related road construction operations,11 industrial plants,12 as well as other types of exposure such as cooking13–16 and surgical smoke exposure.17 ,18

It is not known which agents are causing the observed lung function changes in tunnel workers. Ultrafine particles contribute just barely to the mass of particles in the air. However, the number concentration of ultrafine particles may be very high, especially close to the production site, even though the mass concentration is low. The small size of ultrafine particles facilitates uptake into cells and transcytosis across epithelial and endothelial cells into the blood and lymph circulation to reach potentially sensitive organs.19 Ultrafine particles have a larger specific surface area compared to larger particles, and the importance of surface area becomes evident when considering that surface atoms or molecules play a dominant role in determining bulk properties. Increased surface reactivity predicts that ultrafine particles exhibit greater biological activity per given mass compared with other particles.20 Epidemiological studies have shown a strong association between ultrafine particles in air pollution and adverse pulmonary and cardiovascular health effects.21 ,22

To understand the effect of the exposure to ultrafine particles, the measurements need to be relevant to the personal exposure that individuals experience, which means that the measurements need to be performed in the breathing zone of the operator. For other measurements than ultrafine particles, this is normally the case, but the advanced measurement equipment that allows measurements of the concentration of ultrafine particles and the size distribution of the particles is relatively large; moreover, the equipment needs power, and it is not designed for personal exposure monitoring. The measurements are generally performed as close as possible to the job activities without disturbing the ongoing work, which often means that it is done in the range of 1–3 m from the job activities. Table 1 presents a summary of the studies performed. Personal exposure, measured by sampling in the breathing zone, has been measured only in a few studies. These studies concern exposure to cooking fumes,13 ,16 exposure levels for airport employees,30 surgical smoke exposure,17 ,18 paving and road construction workers11 and welding.6 ,34 An overview of the results is presented in table 2. As seen from the tables, the knowledge about personal exposure to ultrafine particles in the industry is limited.

Table 1

Research studies of exposure of ultrafine particles in working environment—with stationary sampling methods

Table 2

Personal samples—ultrafine particles—working environment

The objective of this study was to examine the exposure to number concentration of ultrafine particles and the size distribution in the breathing zone of workers during rehabilitation of a subsea tunnel. The selected working tasks studied were operating the slipforming machine, operators finishing the verge and operators welding the PVC membrane, and the results were compared to the background levels. The measurements of ultrafine particles are part of a larger study on exposure to dust and exhaust-related agents. Further results obtained in the study are published separately.

Materials and methods

Tunnel characteristics

A 5.1 km long subsea road tunnel on the western coast of Norway has been rehabilitated due to excess leakage of seawater into the tunnel. Part of the rehabilitation work involved building new verges and making the tunnel waterproof by mounting a PVC membrane. The rehabilitation work was performed during night-time while traffic passed in convoys. During the daytime and on Saturdays, the tunnel was open for normal traffic with reduced speed limit, from 70 km/h to 50 km/h.

Tunnel rehabilitation work tasks

Measurements were performed in connection to three different job tasks: operating the slipforming machine, finishing the verge and welding the PVC membrane (horizontal and vertical welding), in addition to measurements of background levels. A slipforming machine (Wirtgen SP 250) with an open platform on top was used to build the new verge in the tunnel. One person operated the slipforming machine standing on the platform, and two persons did the finishing of the verge using hand floats. The finishing work was performed on the newly formed verge, so the people carrying out the task followed right behind the slipforming machine. The PVC membrane came in 21 m long and 5.4 or 6 m wide pieces that were mounted onto a steel framework. The seams were welded together using a hot air gun. The horizontal seam was first spot-welded; then an adhesive was applied between the two membranes before the whole seam was welded using a hot air gun and a roller. No spot-welding or adhesive was applied on the vertical seam; rather, it was welded using a hot air gun and a roller.

Instrument and measurements

The personal-level exposure to ultrafine particles and submicrometre particles were measured using a TSI 3091 Fast Mobility Particle Sizer (FMPS) (TSI Incorporated, Shoreview, USA). The FMPS measures the number concentration of submicrometre particles (including ultrafine particles) and the particle-size distribution in the size range 5.6–560 nm.

It uses two consecutive unipolar corona chargers of opposite polarity to obtain a predictable charge distribution. The instrument continuously draws an aerosol sample into the inlet. After passing a 1 µm cyclone, the aerosol passes through the charging region where it receives a predictable charge. Net positive charged particles are then introduced to the measurement region near the centre of a high-voltage electrode column and transported down the column via HEPA-filtered sheath air. A positive voltage applied to the electrode creates an electric field that repels the particles outwards according to their electrical mobility. Charged particles strike the respective electrometers and transfer their charge. A particle with high electrical mobility strikes the electrometer near the top, whereas a particle with lower electrical mobility strikes an electrometer lower in the stack. This arrangement, using highly sensitive electrometers, allows for concentration measurements of multiple particle sizes simultaneously. The time resolution of the measurements is 1 s.

The FMPS operates with a sampling flow rate of 10 L/min and was coupled to a diluter with a dilution rate of 100:1 (DIL 550, TOPAS GmbH, Germany). The measurements were performed in the breathing zone of the personnel by the use of 9.51 m flexible conductive silicone tubing.

Background measurements were performed in periods when no work was performed in the tunnel. The only pollution source was the traffic passing in convoys; one convoy every 15 min, each convoy involving only a few cars. The background measurements were performed by the use of the same 9.51 m flexible conductive silicone tubing as all other measurements.

The lowest possible background concentration is measured when no work was performed, and no traffic is passing; this is called ‘Background—low range (break)’.

Data analysis

The FMPS measures the particle number concentration in the size range 5.6–560 nm. All measurements were performed using the instrument software FMPS Software V.3.1.1. The number concentration of ultrafine particles was calculated in Microsoft Excel as the sum of particles of the first 20 channels, resulting in the concentration of particle range 5.6–100.0 nm.

The results were calculated as AM: arithmetic mean; GM: geometric mean; GSD: geometric SD; maximum; minimum and IQR, using Excel.

To make the results comparable to results of other types of instruments, the particle size distribution is reported as normalised concentration (dNdlogDp): Embedded Image where, dN, particle concentration; Dp, midpoint particle diameter; Dpu, upper channel diameter; Dpl, lower channel diameter.

Results

The summary of exposure to ultrafine particles is presented in table 3, which presents AM, SD, GM GSD for each working operation together with the maximum and minimum values for each measurement series, IQR and the number of measurements. AM for PVC welding in the vertical position showed the highest concentration, with 3.34×106 particles/cm3 and maximum peak concentrations at 6.4×106 particles/cm3. PVC welding in the horizontal position gave the highest maximum peak values. The concrete worker was exposed to concentrations between 8.66×105 particles/cm3 during work on finishing the verge and 1.02×106 particles/cm3 while operating the slipforming machine. Both results are given as AM. Background concentrations are as low as 4.0×104–3.1×105 particles/cm3.

Table 3

Statistics for the different job tasks

Figure 1 shows the particle size distribution of the different job tasks given as normalised concentration (dNdlogDp). The percentage of ultrafine particles can be determined from the particle size distribution from the FMPS. Table 4 shows the percentage of the ultrafine particles and the dominating mode, together with the normalised concentration at the dominating mode. Horizontal PVC welding produced very small particles with a maximum of the 10.8 nm-sized particles; vertical PVC welding produced slightly larger particles (34 nm), while work on finishing the verge produced particles with a maximum of 52.2–60.4 nm particles.

Table 4

Summary of particle size distribution analysis of different job tasks

Figure 1

Particle size distribution at four different job tasks (A) welding of PVC membrane horizontally, (B) welding of PVC membrane vertically (C) finishing the verge (D) operator of the slipforming machine.

Discussion

The results of this study show that the worker performing PVC welding was exposed to a very high concentration of ultrafine particles. AM for PVC welding in the vertical position gave the highest concentration, with an AM of 3.34×106 particles/cm3 and maximum peak concentrations at 6.4×106 particles/cm3. Horizontal welding resulted in the highest maximum peak concentration, but as seen from the IQR values there are only a few very high peaks, and hence a lower overall AM value. The concrete worker was exposed to concentrations between 8.66×105 particles/cm3 during work on finishing the verge and 1.02×106 particles/cm3 while operating the slipforming machine. Both results are given as AM. Background concentrations are as low as 4.0×104–3.1×105 particles/cm3.

The results are in line with the results from Wake, who found plastic welding to be the type of industry where the highest maximum concentrations were found among industries using or handling ultrafine powders, with a factor of 10 higher concentrations of ultrafine particles compared to other types of work.30 Wake reported only the total concentrations measured by an SMPS instrument, which is a particle instrument with a broad size range (16.5–805 nm). The size distribution of the results in this study shows that the particles produced by PVC welding is mainly in the ultrafine fraction (97–99%). This high percentage of ultrafine particles in welding might also relate to the study by Wake, with the consequence that the SMPS results are comparable to our results, and that the maximum concentration of 3.77×106 particles/cm3 reported by Wake could be compared directly with our results. The results from Wake confirm the very high exposure experienced by workers performing PVC welding.

The two work types finishing the verge and operating the slipforming machine are both examples of concrete work. The study by Elihn showed concrete work exposure with median values of 1.2–2.2×104 particles/cm3, with the highest peak values reaching 4.42×105 particles/cm3;12 this is a factor of 10 lower concentrations than were found in the present study. The difference is probably explained partly by the fact that Elihn's measurements were taken in the work area, not in the breathing zone and partly due to concrete work close to a machine releasing diesel exhaust in this study.

The higher exposure experienced by the operator of the slipforming machine compared to the operators finishing the verge might be due to the lack of a closed cabin and the position of the exhaust pipe close to the operator of the slipforming machine's head. The difference seen between horizontal and vertical PVC welding is probably due to differences in the techniques used. The vertical welding was more or less continuous, while during horizontal welding tasks such as gluing and moving the rack were performed as well.

Traffic passed in convoys during the rehabilitation work. This meant that the workers were exposed to ultrafine particles from traffic in addition to the ultrafine particles produced during work. The results show a relatively low background exposure compared to working periods; as the mean value representing the background concentration, we found 3.1×105 particles/cm3, but we found values as low as 4×104 during the period measured. For comparison, Hitchins et al35 reported particle exposures between 2×104 and 8.5×104 particles/cm3 depending on the distance to the road and the wind speed. Rehabilitation work in a tunnel while the traffic passes by in convoys results in noticeable exposures for the workers even during periods when the worker is not performing his or her own work tasks.

Short-term exposure to ultrafine particles at average levels of 1.2–1.5×105 particles/cm3 induced a variety of changes in cardiovascular parameters for health volunteers in recent clinical studies.36 ,37 A recent study of mortality examining the effect of occupational exposure for construction workers showed that occupational exposure to particulate air pollution, especially diesel exhaust, increases the risk for ischaemic heart disease.38 It might be further investigated if exposure to ultrafine particles at high levels may represent a potential additional risk for IHD to the construction workers.

The particle size distribution in the measurements performed shows that the dominating mode of particles emitted from PVC welding changes from 10 nm during horizontal welding to 34 nm during vertical welding. The explanation might be the different techniques used, as explained above. Brand et al found the dominating mode to be 16 nm for TIG welding and 11 nm for RSE welding, while all welding processes included in this study by Brand et al6 showed domination modes above 100 nm. As an example, metal-active gas (MAG) welding produces particles with a diameter of 108 nm. Gomes et al8 found 137.4 nm for MAG welding of carbon steel and 49.5 for friction stir welding (FSW) of aluminium, and point out that the welding material influences the concentration of ultrafine particles and the particle size distribution. During concrete work, the dominating modes are 52.2 and 60 nm, but as seen in figure 1 the size distribution is quite similar for the two types of job. No comparable size distributions are found for concrete work, but some results from other industries are 70 nm (asphalt work) and 46.1 nm (iron foundry). Background measurements showed dominating modes of 10.8/19.1 nm, which correspond to the results from traffic-generated particle studies, where peaks between 15 nm and 50 nm are found.39

Long-term intermittent exposure to ultrafine particles with a diameter <180 nm has been shown to enhance the chronic genesis of atherosclerosis in mice at a number concentration of 5.6×105 particles/cm3.40 These are particle concentrations comparable to those found in our study.

Two different types of exposure descriptions for assessment of ultrafine exposure are found in the literature. Most studies are performed as area measurements, while only a few are performed as personal measurements, as illustrated in tables 1 and 2. Comparing different studies is difficult, since no basis for comparison exists. Table 3 reports AM, GM, IQR and maximum peak, while other studies report, for instance, AM and maximum peak, GM or mean average. A comparison of tables 1 and 2 shows that large differences exist between types of work and between stationary and personal measurements. In order to increase the knowledge about personal exposure, it might be recommended to study the personal exposure in the breathing zone—it is, however, a challenge that appropriate equipment is of limited availability. The CPC3007 and the Ptrak are small and semiportable instruments; they measure a larger size range and not exactly the ultrafine fraction; however, Nanotracer is almost the same. The FMPS, SMPS and ELPI instruments are scientific instruments, well established for measurements of particle size distributions, but are large, power-demanding and require use of silicone tubing.

Limitations

Personal sampling of exposure to ultrafine particles is complicated, a fact that is also illustrated by the limited number of studies that have been performed (table 2). No portable equipment for measurements of the exact ultrafine fraction with the possibility of measuring the particle size distribution exists. In this study, a quite large piece of equipment was used; it is power-demanding, and it is not possible to move the instrument during use. The workers performed tasks that involved movement, and the solution was to use long flexible silicon tubing and relatively short measurement periods. The use of an almost 10 m long silicon tubing causes a risk of particle deposition in the tube as pointed out by Tsai41 and Kumar et al.42 Ideally, particle deposition in the tube should be taken into account, but experimental studies have difficulties in determining how large the particle loss is, and for which particle sizes. Tsai41 report that there is a maximum of 30% loss for particles with the size 8 nm by using a 8.4 m tubing, and that low particle loss was observed for particles greater than 40 nm, while Kumar et al report modest particle losses for particles below 20 nm.42 No recalculations are performed according to the particle deposition in the tubing in this study; since recalculation itself might introduce an undesired risk of introduction of a source of error. In order to keep the particle deposition as low as possible, the tubing was kept as straight as possible, and the bends as gradual as possible. The same length of tube was used for worker measurements and background measurements, so the results within the study are comparable. The reported exposure values could be regarded as underestimated, when regarding particle deposition, and this further emphasises the conclusion of the study.

Koponen divides the factory into near-field and far-field, and studies the exposure to ultrafine particles in both situations. They define the near-field as the area 0.5 m from the powder-pouring openings of the mixing tanks, which should be the most polluted operation, and the far-field as the area 5–15 m from the near-field instruments in areas where there was at least 2 m distance to the area of activity; and they place the instruments there in order to measure both places.43 The results show that personal level exposure was higher than what was measured from the near-field, and Koponen43 concludes that ‘to better understand the activities leading to worker's personal exposure, workers should be equipped with real-time particle monitors so that the exposure can be linked to different work tasks’.

Conclusion

The FMPS has been successfully applied to assess the particle size distribution of submicrometre particles during rehabilitation work in a subsea tunnel.

The mobility diameter at peak particle concentration varied between 10.8 nm during horizontal PVC welding and during breaks and 60.4 nm during the work on finishing the verge. The percentage of ultrafine particles varied between 83% for work on finishing the verge and 99% during vertical PVC welding.

PVC welding in the vertical position resulted in very high exposure to ultrafine particles for the workers compared to other types of work tasks. The maximum peak in this study was 8.1×107 particles/cm3.

There are large differences between personal samples of ultrafine particles in the breathing zone of the worker and more stationary work area measurements. In evaluations of worker exposure, it seems to be important to distinguish between the two types of measurements. The use of FMPS was successful but demanding. There is a need for a portable particle-sizing instrument for measurements of ultrafine particles in order to increase our knowledge about personal exposure to ultrafine particles within occupational hygiene and medicine.

Acknowledgments

The study was partly founded by The Norwegian Public Roads Administration. The authors wish to thank the workers for participating in the study. This project could not have been accomplished without the positive attitude and the valuable and necessary contributions from the workers carrying the sampling equipment.

References

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Footnotes

  • Contributors Only the authors contributed to the paper. RBJ, MB and SF participated in planning, conducting and reporting the work.

  • Funding The study was partly funded by The Norwegian Public Roads Administration.

  • Competing interests None declared.

  • Provenance and peer review Commissioned; internally peer reviewed.