The tank that exploded contained remnants of sulphuric gasoline (coker gasoline), 50 m3 of alkalinised sludge containing sodium thiolates (mercaptans), 200 m3 of treated wastewater and 15 m3 of concentrated hydrochloric acid. The smoke content was not analysed, but thiols and other sulphides were probably oxidised, forming sulphuric dioxide (SO2) during the fire. Air measurements in the industrial harbour two to three weeks after the accident showed low levels of mercaptans
. Sulphuric compounds from the tanks were probably also spread to the soil and water in the industrial area.
Clean-up operations started soon after the accident. Contaminated soil was collected and stored in big bags, but was not removed from the harbour area until 2010, three years later.
In an area west, north and east of the accident site within a distance of six kilometres there are individual and small clusters of houses spread within the whole area mainly along the roads. This area is separated by fields, hills and woodland from houses further away. South and south west of the harbour is the fiord. Except for their neighbourhood to the industrial harbour the inhabitants in this area, which belong to two municipalities, live in a rural environment with low road traffic. The average distance from the accident site to the residential addresses for the inhabitants is about 3.7 kilometres (range 1–6 km).We anticipated that the highest environmental chemical exposure from the accident had been within this area, and all the inhabitants here were defined as exposed to the pollution from the accident. We also anticipated that people living 1–3 kilometres from the accident site had been more exposed to pollution from the accident than those living 3–6 kilometres away. The control group was selected from inhabitants living in two separate areas to the east and north east of the industrial harbour in the same two municipalities as the exposure area, but an average 28 kilometres away (range >20-30 km). We anticipated low accident related exposure in the control areas, but otherwise the residence environment was considered to be similar to the area where the exposed population lived, with small clusters of houses in a rural environment.
Study design and population
This cross sectional study was carried out 18 months after the explosion, from November 2008 to March 2009. Information about residence was obtained from the National Population Register.
All inhabitants from 18 years old and upwards living within six kilometres (average distance 3.7 kilometres) of the accident site at the time of the incident were defined as exposed (N = 308). A random sample of inhabitants living more than 20 kilometres away (average distance 28 kilometres) in the same two municipalities, were defined as controls (N = 316). These controls had the same age and gender distributions as the exposed.
The population was informed about the study at a public meeting, through the local press, television channels and posters in public buildings. A letter inviting recipients to a health examination in the local municipality, a questionnaire and up to two reminders were sent by mail to each selected person.
Lists of workers employed in the industrial harbour at the time of the accident were obtained from the local employers. Inhabitants in the control group who worked in the industrial harbour were excluded from the study population.
A total of 402 individuals participated, 223 in the exposed group (115 men, response rate 70%, and 108 women, response rate 75%), and 179 in the control group (99 men, response rate 51% and 80 women, response rate 65%). In the exposed group, 59 men and 55 women lived within three kilometres, and 56 men and 53 women lived from three to six kilometres from the explosion site. The non-responder group comprised mostly of younger adults, especially younger men.
Questions about symptoms from lower airways included (yes/no): Usually morning cough, daily cough, cough at least three months a year, cough with phlegm, cough with phlegm at least three months a year, dyspnoea while walking on flat ground, dyspnoea while walking uphill compared to others, ever had episodes of wheezing from chest, were taken from the ATS-DLD-78A questionnaire
Questions about current symptoms from upper airways: Blocked nose, rhinorrhoea, irritated nose and sore throat, were taken from the Wasserfallen et al. validated questionnaire
. Participants were to report the degree of present symptoms on a five-point scale (0–4).
Spirometry was performed in accordance with the ATS/ERS standardisation recommendation from 2005
, but accepting 200 mL repeatability in accordance with the 1994 recommendation
. A dry wedge spirometer, ‘Vitalograph Gold Standard plus’ (model 2160) was used for all measurements. Among men, 99 spirograms were found to be acceptable in the exposed group and 91 in the control group; for women, the figures were 94 and 71, respectively.
The chosen forced expiratory volume in one second (FEV1) and forced vital capacity (FVC) were compared with reference (predicted) values from a non-smoking, healthy, west-coast Norwegian population
. Spirometry was performed before and 15 minutes after inhaling 0.4 mg adrenergic beta2-agonist salbutamol from a Discus inhalator
. Airway obstruction was defined as FEV1% predicted <80% and an FEV1/FVC ratio <0.70, which is moderate and severe obstruction according to the GOLD criteria
Post-traumatic stress symptoms
To assess stress symptoms relating to the 2007 accident, questions were included from the Impact of Event Scale- Revised (IES-R)
. This scale is suggested to be useful when screening for signs of post-traumatic stress (PTSD)
. It contains 22 questions on different stress symptoms experienced during the past week relating to the 2007 accident. Answers were given on a five-point scale, from 0 (not at all) to 4 (extreme). The maximum obtainable impact score was 88, a high number representing a high symptom level. A cut-off value of 22 was used as this has been shown to be optimal for classification accuracy, sensitivity and specificity of PTSD
Population characteristics and covariates
The participants were asked (yes /no): Have you had an infection in the preceding month, and have you cat or dog, moisture damage or carpeted floors in your home. They were also asked about years of education after lower secondary school, whether they were working, were on sick leave or rehabilitation, in receipt of disability pension, retirement pension or were students.
Subjects in employment were asked to state their occupation and industry using free text. Information about current occupation was coded as a three-digit code in accordance with the Norwegian version of the International Standard Classification of Occupation (ISCO −88) by two skilled researchers
. The individual’s occupation was then linked to a general population job-exposure matrix (JEM) which classifies each occupation into none, low or high exposure according to occupational exposure to mineral dust, biological dust or fumes/gases
. In the present study we classified these three exposure groups into two where the original none and low exposure groups were merged.
Information was also obtained about years of daily smoking, previous smoking (yes/no), and cigarettes per day for current smokers. Smoking was defined as present versus never/previous smoking.
A blood sample was taken by vein puncture, and the blood was coagulated in tubes at room temperature and centrifuged for serum within two hours. Serum was analysed at the laboratory at Haukeland University Hospital the next day using Phadiatop® based on the Immuno-CAP-FEIA system (Phadia AB, Uppsala, Sweden). Subjects with positive Phadiatop® (specific IgE toward one or more of the following airway allergens: Dermatophagoides pteronyssinus, Cladosporium herbarum, cat, dog, horse, birch, timothy and mugwort) results were defined as having atopy.
The height and weight of participants were measured while wearing indoor clothing without shoes and jacket. Height was used as a covariate.
The described population characteristics and covariates were assessed because they may be associated with respiratory health or lung function.
Comparison of characteristics variables among exposed and controls were done by Pearson Chi-Square test, or Fisher exact test for small numbers in categorical variables. Continuous variables were compared by independent sample t-tests. However, when the continuous variables were not normally distributed, the Mann–Whitney U test was used.
The relationship between categorical airway symptoms and exposure groups was analysed by logistic regression, adjusting for smoking (present versus previous or never), present occupational exposure (high versus low), Phadiatop® (positive versus negative), infection in the preceding month (yes versus no), age (continuous scale) and impact score (≥22 versus <22). The answers to the upper airway symptoms were dichotomised; answers 0 and 1 were defined as no symptom and 2 to 4 as symptom present. We categorized the answers to these questions, as the underlying variables are naturally categorical (like symptoms from lower airways)
. Regression analyses with adjustments were not performed if the number were less than six individuals in any of the exposure groups. The relationship between continuous values as spirometry results and exposure was analysed by linear regression models, adjusting for smoking, occupational exposure, Phadiatop®, infection in the preceding month, age and height.
Most comparisons were performed for both genders and for men and women separately. When analysing men and women together, adjustment for gender (women versus men) was done.
In order to investigate a possible exposure-response of air polluting airway effect on the population within the 6 kilometre distance from the site, we made post-hoc calculations by analysing the airway effects separately for those living within three kilometres and those living 3–6 kilometres of the site, both compared to the control group. In total, 59 men and 55 women lived within three kilometres (average distance 26 kilometres), and 56 men and 53 women lived from three to six kilometres from the explosion site (average distance 4.7 kilometres).
If there were missing data points such as if a participant did not fill in a question in the questionnaire, this question was not used for that person in analyses. An exception for omitting a missed data point was if it seemed logical not doing so out of the context for the question. If a person answered “no” to each of the questions “have you usually morning cough”, “daily cough” or cough with phlegm”, and did not answer the question “have you cough more than three months a year”, then we anticipated that the answer also should be “no” here, and we used the answer “no” in the analyse for that question.
The comparative tests were two-sided, and p-values below 0.05 were considered significant. SPSS 18.0 was used for statistical analyses.
The study was approved by the Regional Committee for Medical Ethics of Western Norway and Norwegian Social Science Data Services.