Gas and dust exposure in underground construction is associated with signs of airway inflammation

ABSTRACT

Exposure to gases and dust may induce airway inflammation. It was hypothesized that heavy construction workers who had been exposed to dust and gases in underground construction work for 1 yr, would have early signs of upper and lower airway inflammation, as compared to outdoor workers.

A study group comprising 29 nonsmoking underground concrete workers (mean+- sd age 44+-12 yrs), and a reference group of 26 outdoor concrete workers (39+-12 yrs) were examined by acoustic rhinometry, nasal and exhaled nitric oxide spirometry and a questionnaire on respiratory symptoms. Exposure measurements were carried out.

The underground workers had higher exposure to total and respirable dust, α‐quartz and nitrogen dioxide than the references (p<0.001). The occurrence of respiratory symptoms was higher in the underground workers than in the references (p<0.05). Exhaled nitric oxide (NO) (geometric mean±sem) was higher in the underground workers than in the references (8.4+-1.09 versus 5.6+-1.07 parts per billion (ppb), p=0.001), whereas spirometric values were comparable. The underground workers had smaller nasal cross-sectional area and volume than the references, and more pronounced increases after decongestion (p<0.001).

To conclude the exposure in underground construction may cause nasal mucosal swelling and increased levels of exhaled nitric oxide, indicating signs of upper and lower airway inflammation.

MATERIAL AND METHODS

Tunnel site selection and characteristics

A tunnel site in Oslo, Norway, was selected. The excavation work was finished, and the study was performed during on-going concrete work. The volume of the excavated tunnel was 124,000 m3. It had a local one-way ventilation system and the airflow into the tunnel area was ∼1800 m3.min−1. The machinery operated inside the tunnel was diesel powered.

Exposure

Exposure to dust and gases was determined by means of personal sampling. Each person measured two or more agents for one or two days. Total dust was collected on acryl copolymer membrane filters (Versapore 800) with pore size 0.8 μm in 25 mm aerosol filter cassettes (Gelman Sciences, Ann Arbor, USA) with a sampling flow rate of 2.0 L.min−1. Respirable dust was collected on 37 mm cellulose acetate filters with pore size 0.8 μm by using a cyclone separator (Casella T13026/2, London, UK) with a sampling flow rate of 2.2 L.min−1. The sampling time varied 5–7 h. The particle mass was analysed with a microbalance (Sartorius AG, Goettingen, Germany). The determination of α‐quartz in the respirable fraction was analysed by X‐ray diffraction 11. Gas concentrations of NO2 were measured by direct reading instruments, electrochemical sensors with data-logging facility built into the instrument (Neotox-xl personal single-gas monitor, Neotronics Limited, Takeley, UK). A sampling rate of one reading every second minute was selected. The sensors were calibrated every third month with certified calibration gases.

Study populations

The study group was based on all male concrete workers (n=59), who had been performing finishing-work for a period of 1 yr after the excavation of the tunnel, but otherwise had no previous tunnel work experience. From this group, only nonsmokers (n=29) were invited to participate in the study. Reference subjects were recruited from three outdoor construction sites located in the vicinity of the tunnel site. All nonsmoking subjects (n=26) from the 55 outdoor concrete workers who had never worked in tunnels, were invited to the study. None of the subjects reported physician-diagnosed asthma, which was a criterion of exclusion from the study. All participants had to be free from respiratory infections for three weeks prior to testing. Nonsmokers were defined as never-smokers and former smokers (smoking cessation >12 months). Smokers were excluded in order to avoid the concomitant effects of tobacco-smoke pollutants on the respiratory system and because cigarette smokers are known to have decreased NO levels. The underground workers and the reference subjects performed the same job tasks, and had the same work schedule (10 h shifts with two breaks of 30 min each). The study was carried out between September and November 1998. The attendance rate was 100% for both the index group and the reference group. All subjects were tested during the working day at a hospital located 10 min from the work sites. The study was approved by the Data Inspectorate and the Regional Medical Ethics Board.

Questionnaire

A self-administered questionnaire applied in earlier Norwegian investigations 13, 14 and validated in a previous study, was used to assess the presence of airways symptoms. Questions included the occurrence of work-related sore throat, nasal congestion, cough with phlegm, chest tightness and wheeze. The questionnaire also asked about former smoking.

Immunoglobulin E measurements

Screening for atopic allergy was done with Phadiatop (Pharmacia Diagnostics AB, Uppsala, Sweden), a multiple radio allergo sorbent test (RAST) of immunoglobulin (Ig)E against nine common respiratory allergens (birch, timothy, mugwort, cladosporium herbarum, alternaria tenuis, dermatophagoides pteronyssinus, cat dander, dog epithelium, horse dander). Total IgE was measured by the UniCap method (Pharmacia Diagnostics AB, Uppsala, Sweden).
Acoustic rhinometry

Acoustic rhinometry was performed with the Rhin2100 (Rhino Metrics AS, Denmark) with the subject in the seated position and stabilization of the head, but without instrument fixation. Briefly, in this method acoustic signals generated in a tubular probe wave tube are conducted via a nasal adapter to the nasal cavity. The incident signal and its reflections from the nasal cavity are detected by a microphone within the sound wave tube. Resulting electrical signals are processed by analysing software to provide a graphic display of cross-sectional area-distance relationships and numeric descriptions of minimum cross-sectional areas and volumes between selected points in the nasal cavity. The following variables were recorded: the total (sum of unilateral) minimum cross-sectional areas (TMCA1, TMCA2) and volumes (TVOL1, TVOL2), measured at 1) the anterior 22 mm of the nasal cavity and 2) 22–52 mm from the nostril. Three independent traces for each nasal airway were recorded, and the mean values computed. Coefficients of variation (CV) were also recorded. TMCA2<=0.9 cm2 was considered a threshold value, predicative of subjective feeling of nasal obstruction. Measurements were performed before, and 15 minutes after, standardized application of a nasal spray containing xyclometazolin. The degree of mucosal swelling was estimated indirectly via the decongestive effect.

Nitric oxide measurements

NO was measured by a chemiluminescence analyser (LR 2000, Logan Research, Rochester, UK) adapted for on-line recording of NO concentration, as previously described 7. The sampling rate of the analyser was set to 250 mL.min−1 for all measurements. The analyser was calibrated daily using certified NO mixtures (100 parts per billion (ppb)) in nitrogen (BOC Special Gases, Surrey Research Park, Guildford, UK). Ambient NO was recorded daily. Exhaled and nasal NO measurements were performed in accordance with recommendations outlined in the European Respiratory Society's Task Force Report. Measurements of exhaled NO were made by slow exhalation (20–30 s) from total lung capacity through a Teflon mouthpiece, against a mild resistance (target mouth pressure of 4–5 cmH2O) to avoid nasal NO contamination. End-expiratory NO values were measured at the plateau level of the last part of the exhalation curve. Nasal NO was measured with a Teflon tube inserted into one of the nares, while the subject held breath, and the value of the last plateau part of the trace recorded. For both exhaled and nasal measurements, three technically acceptable measurements were obtained, and the mean of the two closest measurements was reported.

Spirometric measurements

Spirometry was performed using a pneumotachograph (Vitalograph, Birmingham, UK) which was calibrated daily by a 1 L syringe. The measurements were performed in accordance with the guidelines recommended by the American Thoracic Society. Recorded variables were forced vital capacity (FVC), forced expiratory volume in one second (FEV1), and FEV1/FVC×100 (FEV1%). The lung function variables were expressed in absolute values and as percentage of predicted, using the reference values of the European Coal and Steel Community (ECSC).

Statistical methods

The relationship between respiratory symptoms and the covariates occupational group and age was investigated by means of logistic regression. The covariate years employed in the same job was not included in the model due to high correlation (>0.8) with the covariate age. Atopy and former smoking were controlled for, but did not have any influence on the models. The relationship between acoustic rhinometry data prior to nasal decongestion and the covariates occupational group and age were investigated by means of analysis of variance (ANOVA). Since age had no influence on the statistical model, unadjusted data are presented and summarized for each occupational group. Changes in acoustic rhinometry after nasal decongestion were evaluated using ANOVA with occupational group and acoustic rhinometry data prior to nasal decongestion as covariates in the model. Exhaled and nasal NO data were analysed using the same ANOVA model as for the acoustic rhinometry data prior to nasal decongestion. Age had no influence on the model and unadjusted data are presented. Values for exhaled NO were log transformed. The relationship between lung function data and the covariates occupational group and age were investigated by means of ANOVA. The exposure data were best described by log-normal distributions and were log-transformed before statistical analyses.

RESULTS

Exposure characterization

Table 1 shows the geometric mean exposure levels by occupational group. The underground workers had significantly higher exposure to total- and respirable dust than the outdoor workers. They were also exposed to significantly higher levels of α‐quartz and NO2. The highest 8‐h time-weighted averages were: total dust=19.4 mg.m−3, respirable dust=4.4 mg.m−3 and α‐quartz=0.16 mg.m−3. The underground workers were periodically exposed to high concentrations of NO2 (peak value 7.4 ppm (ceiling value 2 ppm, Norway 1998)). NO2 concentrations outdoors were not detectable with the method used.

Clinical findings and symptoms

The underground workers were somewhat older than the reference subjects (mean± sd age 44+-12 versus 39+-2 yrs). The two groups were comparable with respect to years of employment (20+-9 versus 17+-12 yrs), height (178+-6 versus 178+-6 cm), atopy (n=5 versus n=6) and former smoking (n=5 versus n=6). Work-related upper airways symptoms were more pronounced in the underground workers (table 2). They also reported higher occurrence of symptoms from the lower airways. Both productive cough and chest tightness and wheeze occurred more frequently in the underground workers than in the reference subjects (table 2).

Acoustic rhinometry

Prior to decongestion, the underground workers had significantly lower absolute values of TVOL2, TMCA1 and TMCA2 than the outdoor workers (table 3). The increases in TMCA2 and TVOL2 after nasal decongestion were significantly more pronounced in the underground workers (table 3). There was no significant difference in TVOL1 between the two groups. TMCA2=0.9 cm2 was correlated to a subjective feeling of nasal congestion (Pearson correlation=0.4, p=0.001). The repeatability of the measurements was high (mean CV=3% for TMCA2 and 2% for TVOL2).

Nasal and exhaled NO

Nasal NO levels did not differ between the underground workers and the outdoor workers (arithmetic mean±sem) 882+-42 versus 827+-54 ppb. Workers reporting nasal congestion had significantly higher nasal NO levels than workers without the complaint (910+-46 versus 779+-45 ppb, p=0.04).

The underground workers had significantly higher levels of exhaled NO than the outdoor workers (8.4+-1.1 versus 5.6+-1.1 ppb, p=0.003) (fig. 1). The exhaled NO levels in underground workers complaining of having chest tightness and wheeze (n=11) were significantly higher than in workers without the complaint (9.6+-1.2 versus 6.3+-1.1 ppb, p=0.004).

Spirometry

The underground workers did not differ significantly from the reference subjects with respect to spirometric values (FVC 102+-2 versus 103+-5% pred, and FEV1 94±2 versus 100+-3% pred). Only three of the 11 underground workers who reported chest tightness and wheezing had FEV1/FVC ratio <0.7.
