The aim of this longitudinal epidemiological study was to investigate possible associations between occupational exposure, respiratory symptoms and change in lung function among employees at a mineral fertiliser production plant.
Material and methods
Lung function indices (forced vital capacity (FVC), the forced expiratory volume in 1 second (FEV1) and lung diffusing capacity (DLCO)) were measured in 2007 and 2010 among employees at a fertiliser production plant. In 2007, 349 persons participated (86% of those eligible). Of the 283 participants remaining at work three years later, 6% did not participate in the second phase of lung function measurement. In addition to those participating in 2007, 34 additional employees participated in 2010, for a total of 383 participants. The employees consented to spirometry test and diffusing capacity test and answered a respiratory symptoms questionnaire during each testing session. In 2007, rhinometry was performed at the same time as the lung function testing. In 2010, the subjects at the plant answered a questionnaire on work history with an emphasis on the three-year follow-up period. The employees were classified in job groups by production department according to their principal affiliation during follow-up. Study participants tested only once were grouped in the department they served on the day of lung function testing. Employees in the administration or working in average less than 2 hours/week in the production were assigned to the job-group “Other”.
The exposure assessment was performed in 2007-08 in all the departments at the plant; ammonia, nitric acid, compound fertiliser, and calcium nitrate departments, as well as a shipping area. A total of 178 inhalable and 179 thoracic personal aerosol mass fraction samples were collected from randomly chosen workers (N=141), whereof 23% of the workers participated more than once. Masses of inhalable and thoracic aerosol fractions were measured gravimetrically. Water-soluble and water-insoluble aerosol fractions were analysed for the major constituents, Ca, K, Mg, and P. Concentrations of F-, NO2-, NO3-, PO43-and SO42- in the water-soluble aerosol fraction and leachates from impregnated gas filter-pads were analysed. NH3, CO, and NO2 were measured using direct-reading electrochemical sensors.
In addition, a sub study on exposure assessment using video exposure monitoring strategy was conducted during a week in May 2009 to identify short-term peak episodes.
An adjusted, statistically significant decline in FEV1 of 18 mL/year during the followup was found for the total group, but no significant decline in FVC. The workers in the nitric acid department had a statistically significant decline in FEV1, but the absolute decline was of the same magnitude as for those in the Ammonia and Compound fertiliser A departments. DLCO showed a statistically significant decline of 0.068 mmol/min/kPa/year for the total group. The prevalence of selected self-reported respiratory symptoms; morning cough, cough with phlegm, cough with phlegm >3 months/yr, and wheezing, varied between 6.5 to 26.2%, with only morning cough showing a statistical significant increase from 8.0% in 2007 to 13.7% in 2010. No association was found between respiratory symptoms and the decline in lung function indices, and borderline significant correlation was found between nasal patency and FEV1% predicted. The median inhalable and thoracic aerosol mass concentration exposure levels were 1.1 mg/m3 (min-max: <0.93 - 45) and 0.21 mg/m3 (min-max: <0.085 - 11), respectively. The highest median aerosol mass concentrations were found in the compound fertiliser departments with median inhalable mass air concentration of 3.0 mg/m3 in Compound fertiliser C and median thoracic mass air concentration of 0.78 mg/m3 in Compound fertiliser A. The median air concentrations of CO, NH3, and NO2 in all departments were predominantly below the limit of detection (2 ppm, 3 ppm, 0.2 ppm, respectively). However, some short-term peak episodes of NH3 and NO2 were detected, e.g. when performing tasks like cleaning and sampling for quality control.
An observed adjusted decline in lung function indices was found in this three-year follow-up of workers at a nitrate fertiliser plant. The prevalence of morning cough increased in the follow-up period, but no association between respiratory symptoms and decline in lung function indices was found. A borderline correlation was found between nasal patency and FEV1% predicted. The exposure levels for aerosols and gases were generally low with many measurements below the limit of detection. No plausible exposure related explanation for the overall lung function decline during follow-up was found.
List of papers. The papers are removed from the thesis due to publisher restrictions.
Paper I Kristin H Hovland, Yngvar Thomassen, Nils Petter Skaugset, Knut Skyberg, Marit Skogstad, Berit Bakke: Characterisation of occupational exposure to air contaminants in a nitrate fertiliser production plant Journal of Environmental Monitoring (now Environmental Science: Processes & Impacts) 2012;14:2092-9 doi:10.1039/c2em30370a
Paper II Kristin H Hovland, Marit Skogstad, Berit Bakke, Øivind Skare, Knut Skyberg: Longitudinal lung function decline among workers in a nitrate fertiliser production plant International Journal of Occupational and Environmental Health, 2013;19;119-26 doi:10.1179/2049396713Y.0000000025
Paper III Kristin H Hovland, Marit Skogstad, Berit Bakke, Øivind Skare, Knut Skyberg: Longitudinal decline in pulmonary diffusing capacity among nitrate fertiliser workers Occupational Medicine, 2014;64;181-187, first published online February 10, 2014 doi:10.1093/occmed/kqt174