Total and respirable dust exposures among carpenters and demolition workers during indoor work in Denmark

ABSTRACT

Background

Within the construction industry the risk of lung disorders depends on the specific professions probably due to variations in the levels of dust exposure, and with dust levels depending on the work task and job function. We do not know the extent of exposure in the different professions or the variation between the different work tasks. The purpose of this study was therefore to assess if there were differences in dust exposure between carpenters and demolition workers who were expected to have low and high dust exposure, respectively.

Methods

Through interviews of key persons in the construction industry the most common work tasks were selected, and the concentration of dust during these tasks (indoors) were measured by personal sampling varying between 4 and 6 h of a working day. In total 38 measurements of total dust, and 25 of respirable dust on seven different work tasks were carried out for carpenters and 20 measurements of total dust, 11 of respirable dust and 11 of respirable crystalline silica dust on four different works tasks for demolition workers. Dust measurements were tested for differences using linear regression, t-test and one-way ANOVA.

Results

For carpenters the geometric mean for all the measurements of total dust was 1.26 mg/m3 (geometric standard deviation 2.90) and the respirable dust was 0.27 mg/m3 (geometric standard deviation 2.13). For demolition workers the geometric mean of total dust for all the measurements was 22.3 mg/m3 (geometric standard deviation 11.6) and the respirable dust was 1.06 mg/m3 (geometric standard deviation 5.64).

The mean difference between total dust for demolition workers and carpenters was 11.4 (95 % confidence interval 3.46–37.1) mg/m3. The mean difference between respirable dust for demolition workers and carpenters was 3.90 (95 % confidence interval 1.13–13.5) mg/m3.

Dust exposure varied depending on work task for both professions. The dustiest work occurred during demolition, especially when it was done manually.

Only few workers used personal respiratory protection and only while performing the dustiest work.

Conclusions

This study confirmed that the exposure to dust and especially total dust was much higher for demolition workers compared to carpenters.

METHODS

Measurements of exposure

Interviews of key persons from the trade union in the construction sector were performed to identify the most common work tasks. The work tasks were categorized as installation of gypsum, ceilings, floorings, windows and doors, insulation, and ‘other work tasks’ for carpenters. For demolition workers the work tasks were categorized as manual demolition, mechanical demolition, waste management and ‘other work tasks’.

The measurements were only carried out on indoor work tasks because of the outdoor measurement-uncertainties due to changing wind conditions. The working environment council of the construction industry participated in finding construction companies performing the requested tasks in two areas of Denmark (Copenhagen and Aarhus). Among companies accepting to participate we selected companies of different sizes and different work places.

A total of 11 companies (and 11 workplaces) were selected for carpenters. Measurements of total dust (TD) were carried out on 38 carpenters (1 measurement per worker) and of respirable dust (RD) on 17 carpenters (25 measurements) (1–2 measurements per worker).

Measurements of TD, RD and respirable crystalline silica dust (RCS) were carried out in 5 companies (5 workplaces) for demolition workers. Measurements of TD were carried out on 16 demolition workers. For seven workers RD was measured at the same time.

The measurements were made after identification of the most relevant work tasks, included measurements for each work task. Furthermore the measurements were conducted over an entire working day (excluding pauses) or as long as the task was done. Workplaces were not selected by level of dust (worst cases) but chosen if the work task was carried out.

The measurements were spread throughout the year: 11 % (winter), 22 % (spring), 30 % (summer), and 34 % (autumn).

In connection with the dust measurements it was registered if the workers used personal respiratory protective equipment, the use of local exhaust or other dust-reducing measures.

Performed measurements corresponded to a total of 255 working hours for carpenters and 113 working hours for demolition workers (pauses excluded).

Work place monitoring of airborne dust

The concentration of TD was measured by personal sampling during 4 to 6 h of a working day. Sampling was performed using 37 mm Millipore filter cassettes mounted with 0.8 μm Mixed Cellulose Ester Membrane filters (fa. Millipore AAWOP) mounted in closed face Millipore field monitors with a 5.6-mm inlet at 1.9 L/min (inlet velocity = 1.25 m/s) (SKS Inc. Pennsylvania, U.S). The field monitor was placed in the breathing zone just below the collarbone. The inlet pointed downward. Flow rates were adjusted and controlled in the field before and after sampling by Porter Flow Meters (Porter Air Flow meters, model F65, measurement range 0.5-3 L/min.) calibrated against certified primary references. Air velocity in inlet was adjusted to 1.25 m/s according to Danish legislation. Each series of sampling were controlled and corrected towards two blank field samples. Conditioning of filters were performed prior and after sampling under controlled climatic conditions (temperature 22–28 °C and humidity 40–52 % RH). We used sampling pumps SKC 224 and SKC SideKick.

RD was sampled with modified Higgins and Dewell cyclones. The 50 % aerodynamic diameter cut-point for collection efficiency was 5 μm, and the volumetric air sampling rate 2 L/min according to the Danish Working Environment Authority Guidelines. The respirable fraction was collected on 37-mm diameter 1-μm membrane filters.

The collected mass of dust on the filters was determined gravimetrically. The limit of detection, LOD was calculated as three times the standard deviation (SD) of the blanks (30 μg) and the relative standard deviation of the method was 10 %.

Respirable crystalline silica dust (RCS) was analysed according to National Institute of Occupational Safety and Health (NIOSH) method 7602 (modified, accredited). The limit of detection was 5 μg. Both sampling and gravimetric analysis were carried out by Eurofins Miljø A/S. (Eurofins Miljø are accredited by The Danish Accreditation Fund); DANAK and testing was performed in accordance with the national and international standards as approved by DANAK, (DANAK Reg. No. 168 (sampling) and Reg. No. 522 (gravimetry)).

Data analysis

Statistical analyses were conducted using SPSS software (IBM SPSS Statistics, version 22, IBM Corp. 2013). We used a logarithmic scale for the graphic presentation because the distribution was skewed with a long right trail, and normalized by log-transformation. For measurements below the lowest limit of detection and above maximum quantification the detection limit/the maximum quantification values were used in the calculations. These values account for 4.8 and 1.9 % of the measurements, respectively.

The exposure levels were described by arithmetic means (AM), geometric means (GM), and geometric standard deviations (GSD) for each occupation and for each task.

The average exposure over an eight hour time period (normal work shift), 8-h TWA was calculated as: 8-h TWA = ∑ i = 1 n CiTi/8 h, where Ci = concentration during the ith interval, and Ti= duration of the ith interval.

Histograms, QQ-plots, and tests of skewness showed lognormal distribution and the dust concentrations were therefore lognormal transformed before statistical analysis. Differences in TD and RD between the two occupations, between work task measurements within the two occupations and across occupations were tested using t-test, one-way ANOVA and linear regression. Log(TD) and log(RD) were used in the models. Correlations between TD and RD for the two professions were tested (Spearman’s correlation).

The relationship between dust concentration and occupation was also investigated using linear regression analysis with carpenters used as reference category and adjusted for seasonal variations. Season was in the analysis divided in winter/spring and summer/autumn with summer/autumn used as reference category. Analyses were made on log-transformed data.
RESULTS

Carpenters

The GM of all the measurement of TD was 1.26 mg/m3 ranging from minimum 0.08 mg/m3 when installing material of iron to maximum 8.40 mg/m3 when stiffening of beams. Within the work task installation of gypsum the concentration of TD varied between 1.40 mg/m3 and 7.00 mg/m3 (Table 1). The main reasons for dust exposure among carpenters was especially use of hand-held high-speed tools, grinding, lack of local exhaust ventilation, lack of cleaning during a work task, lack of cleaning before the next occupation started their work and dust exposure from other occupations who worked at the same time. None of the carpenters used airway protection during the work.

The calculated 8-h-TWA for TD was 1.07 mg/m3 for all the measurements. None of the calculated 8-h-TWA for the individual measurements (0.04 to 4.27 mg/m3) exceeded the Occupational Exposure Limit (OEL) of 10 mg/m3 for TD.

The GM of RD was 0.27 mg/m3 ranging from no detectable RD during wood work to a maximum of 1.50 mg/m3 when installing gypsum on walls. The calculated 8-h-TWA for RD was 0.16 mg/m3 for all the measurements. None of the calculated 8-h-TWA for the individual measurements (0.01 to 0.33 mg/m3) exceeded the OEL of 5 mg/m3 for RD.

Figure 1a shows the dust concentrations for TD and RD for carpenters divided in different work task. The dust concentrations differed comparing the work tasks for carpenters (F = 2.39, df = 37, p =0.05) for TD, and (F = 5.47, df = 24, p = 0.003) for RD (Fig. 2a). The difference between the work tasks for RD disappeared when the job tasks: ‘installing gypsum’ and ‘other work tasks’ (stiffening of beams) were excluded from the analysis (F = 0.48, df = 14, p = 0.70).

TD was moderate correlated with RD (Spearman’s correlation r s = 0.64), Fig. 2.

Demolition workers

The measurements showed GM of all the measurements of TD of 22.3 mg/m3 ranging between 0.30 mg/m3 for installation of scaffolding and support of ceilings and > 460 mg/m3 for manual demolition (Table 2). In general, mechanical demolition showed lower dust concentrations (4.4 mg/m3) compared to manual demolition (177 mg/m3). The concentrations were lowest during mechanical demolition, good ventilation, and/or use of water. During manual demolition using high-speed tools, dry-cutting, waste management and working indoors without sufficient ventilation the exposure was high. Waste management led to a high exposure to TD, RD and RCS. Demolition workers only used respiratory protection during manual demolition.

The calculated 8-h-TWA for TD was 17.8 mg/m3 for all the measurements. For 40 % of the measurements the calculated 8-h-TWA (0.06 to >86 mg/m3) exceeded the OEL of 10 mg/m3 for TD.

In total 11 measurements of RD and RCS were made. The measurements showed GM for RD concentrations of 1.06 mg/m3 (0.10–10 mg/m3), and for RCS of 0.12 mg/m3 [<0.01 (no detectable crystalline silica) to 0.92 mg/m3]. The lowest concentrations were shown for mechanical demolition and the highest for manual demolition.

The calculated 8-h-TWA for RD was 1.40 mg/m3 for all the measurements. Only one of the calculated 8-h-TWA for the individual measurements (management of waste) (range 0.06–5.02 mg/m3) exceeded the OEL of 5 mg/m3 for RD.

The calculated 8-h-TWA for RCS was 0.08 mg/m3 for all the measurements. In total 45 % of the calculated 8-h-TWA for the individual measurements (<0.02 to 0.24 mg/m3) exceeded the OEL of 0.1 mg/m3 for RCS.

Figure 1b shows the dust concentrations for TD and RD for demolition workers divided in different work task. When comparing the four work tasks differences in dust concentrations were found for TD (F = 18.9, df = 19, p < 0.001), but not for RD (F = 3.16, df = 10, p = 0.10). The difference between the work tasks for TD disappeared when the job tasks: ‘manual demolition’ and waste management were excluded from the analysis (F = 4.03, df = 10, p = 0.09).

TD was moderate correlated with RD (Spearman’s correlation r s = 0.69), Fig. 2.

Demolition workers compared to carpenters

The mean difference between TD for demolition workers and carpenters was 11.4 (95 % CI 3.46–37.1) mg/m3 (t = 4.26, df =56, p < 0.001). The mean difference between RD for demolition workers and carpenters was 3.90 (95 % CI 1.13–13.5) mg/m3 (t = 2.40, df =34, p = 0.03).

In linear regression models, TD was exp(2.43) = 11.4 (95 % CI 4.58–28.2) in demolition workers compared to carpenters unadjusted and exp(2.20) = 9.03 (95 % CI 4.00–20.3) adjusted for season. TD was exp(1.71) = 5.53 (95 % CI 2.39–12.7) in winter/spring compared to summer/autumn. Written in equation: logTD = ‐ 4.12 + 2.20 * occupation + 1.71 * season.

RD was exp(1.36) = 3.90 (95 % CI 1.65–9.23) for demolition workers compared to carpenters unadjusted and exp(1.24) = 3.46 (95 % CI 1.52–7.83) adjusted for season. RD was exp(1.08) = 2.94 (95 % CI 1.13–2.03) in winter/spring compared to summer/autumn.

Written in equation: logRD = ‐ 3.79 + 1.24 * occupation + 1.08 * season.
