Determinants of Respirable Quartz Exposure Concentrations Across Occupations in Denmark, 2018

ABSTRACT

Background

High concentrations of respirable quartz have been reported from workers in construction, foundries, and quarries. Current exposure concentrations in prevalent but presumably lower exposed occupations have been less examined. We aimed to quantify current exposure concentrations of respirable dust and quartz across prevalent occupations and to identify determinants of respirable quartz exposure across these occupations.

Methods

One hundred and eighty-nine full-shift personal samples of respirable dust of workers within 11 occupations in Denmark were sampled during 2018. Respirable dust was determined gravimetrically and analysed for quartz content with infrared spectrometry. Determinants for respirable quartz exposure, i.e. use of power tools, outdoor or indoor location, and percentage of quartz in respirable dust, were analysed in linear mixed effect models.

Results

The overall geometric means (geometric standard deviations) for respirable dust and quartz were 216 µg m−3 (4.42) and 16 µg m−3 (4.07), respectively. The highest quartz concentrations were observed among stone cutters and carvers [93 µg m−3 (3.47)], and metal melters and casters [61 µg m−3 (1.71)]. Use of power tools increased exposure concentrations of quartz by a factor of 3.5. Occupations explained 27%, companies within occupations 28%, and differences between workers within companies within occupations 14% of the variability in quartz concentrations. Thirty percent was due to day-to-day variability in exposure concentrations. In total, 19% of the variation in quartz concentration could be explained by type of tool, indoor/outdoor location, and percentage of quartz in respirable dust.

Conclusion

Current exposure concentrations are generally low, but some occupations in this study had average exposure concentrations to respirable quartz above the ACGIH threshold limit value of 25 µg m−3. Preventive measures to lower excess risk of quartz-related diseases among these workers are still needed. In terms of preventive strategies, use of power tools and quartz content of used materials were identified as main determinants of exposure. Lowering of exposures will be most efficient when focussed on these major determinants, e.g. tool dust control with water, dust extraction, and use of low quartz content materials.

MATERIALS AND METHODS

Companies and participants

Based on the prevalence of occupations in Denmark with expected quartz exposure (BGIA, 2008; Peters et al., 2011; IARC, 2012), we identified companies employing construction-, metal-, and concrete workers and farmers. Occupations were classified based on the four-digit level of the Danish version of the International Standard Classification of Occupations, ISCO-88 (ILO, 2004). Industry was classified at two-digit level of the European classification of industries, NACE vers.2 (The European Parliament and Council of the European Union, 2006) (Table 1, Supplementary Table S1). We prioritized inclusion of companies of different sizes, and when feasible companies with employees from more than one relevant occupation located in the eastern part of Jutland. A total of 38 companies were approached; 15 large companies with more than 100 employees and 23 small companies with less than 100 employees. In total 24 companies (63%) agreed to participate of which 5 employed workers from more than one relevant occupation. Sixty percent of the large companies and 65% of the small companies accepted the invitation. Farmers were contacted through a farmers’ trade associations; however, no farmers were recruited.

Managers at the worksites were instructed to select up to eight employees with work tasks representative for the targeted occupations.

Sampling and analytical method

On the measurement day, participants filled in a questionnaire about primary task, tools, or construction machines used, whether their work location was indoor or outdoor, and use of a respirator. We conducted full-shift measurements; however, pumps were turned off during breaks lasting more than 15 min. Measurements with sampling time below 4 h were excluded. All companies were asked to participate in a second measuring round. If they agreed, repeated measurements were carried out on study participants who remained at the worksite. All measurements were carried out by the same technician between April and December 2018.

Respirable dust was collected on 25-mm PVC filters using a conductive plastic sampler with a respirable dust cyclone (SKC LTD conductive plastic cyclone) connected to SKC AirChek XR5000 portable pump (SKC Inc., Eighty-Four, PA) calibrated at a flow rate of 2.2 l/min. The cassette was attached to the upper part of the participant’s chest within the breathing zone.

Respirable dust was determined gravimetrically. Filters were conditioned for a minimum of 24 h (22°C, 45% relative humidity) before weighing using a Mettler UMT2 analytical scale (Mettler-Toledo Ltd, Greifensee, Switzerland) with 0.1-mg precision. One field blank was included per visit (n = 45). The lower limit of detection (LOD) for respirable dust was calculated as three times the Standard deviation (SD) of the weight changes of the field blanks, corresponding to a concentration of 24 µg m−3, when assuming 8-h measurements.

Quartz was determined by Fourier transform infrared spectrometry, in accordance with MDHS 101/2 (HSE, 2014). The analytical level of quantification for quartz was 10 µg, assuming 8-h measurements correspond to a concentration of 9 µg m−3.

Statistical analysis

Respirable dust and respirable quartz concentrations were log normally distributed, assuming values below LOD followed the same distribution. Hence, statistical analyses were performed using log-transformed values. We used mixed effects Tobit models (metobit, Stata) for interval censored data. All left censored values (values below LOD) were assumed to be in an interval between (-∞) and the LOD (Hughes, 1999; StataCorp., 2019).

In the applied mixed effect models, worker, company, and occupation were included as random effects, and tool, location, and percentage of quartz in respirable dust as fixed effects. β-coefficients are displayed as Exp β with 95% confidence intervals (CI). Geometric standard deviation factor (GSD) was calculated as exp(√σ2wY+σ2bY), where σ2wY= within-worker variance and σ2bY = between-worker variance. The occupational exposure limit (OEL) in Denmark and several European countries is 100 µg m−3 (The European Parliament and Council of the European Union, 2017), and we calculated the exceedance fraction above as P[Z<ln(OEL)−ln(GM)ln(GSD)].

If an occupational group was represented by less than 10 persons on ISCO-88 major group 4 level, it was merged with similar occupations on the corresponding ISCO-88 major group 3 level. Tool was categorized into no tool, hand tools, power tools, and operating construction machines. Location was dichotomized into primarily working inside or outside (Table 1, Supplementary Table S1). The percentage of quartz in respirable dust was imputed for quartz measurements below LOD (38%). For the majority of missing values, we used the median of the percentage of quartz from other workers doing the same job at the same company. For the remaining seven missing values, where none of the co-workers had detectable values of quartz, we used the median percentage from all workers in the same job and company using the estimated LOD value of quartz.

All analyses were carried out using Stata, version 16 and 17.

RESULTS

We performed 194 measurements on 143 participants. One measurement was lost during transportation and four with a sampling time of less than 4 h did not fulfil our inclusion criteria and were excluded, leaving 189 measurements from 140 participants for further analyses. The median sampling time was 428 min, with an interquartile range of 367–456 min. Repeated measurements were available for 35% of the participants, with a median duration between the two measurements of 91 days, interquartile range 84–125 days.

All together 15% of workers (21 participants) reported use of respirators at some point during the day, with missing information from 5% of the measurements. For nine participants using respirators, quartz concentrations were below the LOD, and the range in concentrations among workers using respirators was from 11 to 1083 µg m−3. The majority (68%) of the respirator users reported using power tools, 45% were employed in a large company (<100 employees), and 55% in a smaller company.

Five percent of the respirable dust measurements and 38% of the quartz measurements were below LOD (Table 1). Thirteen percent of all measurements were above the OEL. Measured quartz concentrations ranged from values <LOD to 1083 µg m−3. Stone cutters and carvers were the only occupation having measurements with concentrations above 200 µg m−3 and had a probability of exceedance of 48%. Construction workers (bricklayers, stonemasons, and other building frame workers), mineral or stone processing-plant operators, and metal melters and casters had lower quartz concentrations and probability of exceedance between 14 and 18% (Table 1).

The geometric mean, GM (geometric standard deviation, GSD) for respirable dust exposure concentration was 216 µg m−3 (4.42). Highest exposure concentrations were found among demolition workers and scaffolding fitters (included in the building frame workers category), with a GM of 741 µg m−3 (2.83), metal melters and casters with a GM of 719 µg m−3 (1.97), and blacksmiths with a GM of 718 µg m−3 (2.50) (Table 1).

The GM (GSD) of overall quartz exposure concentration was 16 µg m−3 (4.07). Highest concentrations were observed among stonecutters and carvers, GM of 93 µg m−3 (3.47), and metal melters and casters, GM of 61 µg m−3 (1.71) (Table 1). Percentage of quartz in respirable dust varied from 6 to 30% across occupations. Highest percentage were seen among bricklayers and stonemasons (Table 1).

Use of hand or power tools compared with no tools increased quartz exposure concentrations, e.g. use of power tools resulted in a 3.5 times higher exposure [exp(β) = 3.46 (1.66–7.21)] (Table 2). The quartz content was also an important determinant, with 3 percentage increase in quartz exposure concentration for each percent increase in quartz content in respirable dust.

Of the total variance, occupations explained 27%, companies within occupations 29%, and workers within a company within an occupation 14% of the variability in quartz concentrations. Thirty percent was due to day-to-day variability in quartz concentrations. Including tool and location as fixed effects into the model explained 13% of the total variability, primarily decreasing the variability between workers within companies and occupations (35% explained). When percentage of quartz in respirable dust was added, the fixed effects explained 19% of the total variability, 38% of the variability between occupations, 14% between companies within occupation, and 29% between workers within companies and occupations (Table 3).
