Study design and setting
An institutional-based comparative cross-sectional study was conducted in April 2018 among flour mill and soft drink factory workers in Hawassa city of Southern Ethiopia. In Hawassa city, there are ten flour processing factories (4 medium scales and 6 small scales) and one soft drink factory.
Study population
One hundred ninety-six flour mill workers worked for at least one year in the factory were included in this study. In addition, we included two hundred ten soft drinks factory workers as a comparison group. We performed a lung function test only for 108 randomly selected workers (54 from the flour mill factory and 54 from the soft drink factory) from 406 study participants because of limited resources.
Inclusion and exclusion criteria
We included all workers above 18 years of age who had direct involvement in production and working in the flour mill and soft drink factory for one year and above. Workers who had heart failure, tuberculosis, drug addicts, cigarette smokers, emphysema, recent surgery of the thorax, abdomen, undergone vigorous exercise and any acute illness were excluded from the lung function test.
Sampling procedures
First, the work process was stratified by production units in both flour mill and soft drink factories. Then, systematic sampling techniques applied to draw the required sample size. The workers’ roster was used as a sampling frame. First subject selected randomly from each department, then we selected other subjects every 2nd interval by taking roster of the workers in each department.
Variable measurement
Outcome variables
Chronic respiratory symptoms: are defined as the development of one or more of the respiratory symptom(s) of a chronic cough, chronic cough with sputum, chronic breathlessness, chronic wheezing, chronic chest illness which lasts at least three months in one year [20, 21].
Lung function reduction: Forced vital capacity (FVC); the maximum volume of air that can be breathed out forcefully and rapidly following a maximum inspiration, Forced expiratory volume in 1 second (FEV1) (the volume exhaled during the first second of the FVC maneuver) and FEV1/FVC; the percentage of the FVC expired in the first second of maximal forced expiration following full inspiration [22].
Exposure variables
The socio-demographic factors of the workers; age (20–29 years, 30–39 years, 40–49 years, 50–59 years and ≥ 60 years), sex (male, female), educational status (illiterate, primary education, secondary education, certificate and above) and monthly income (20 US$-70 US$ and ≥ 70 US$) were included.
The behavioral factors of workers (like a smoking habit); current smokers (workers who were smoking at the time of the study or a person who smoke cigarettes every day or some days) and every smoker: a worker who has smoked at least one hundred cigarettes during his/her life, which includes current smokers [23]. The work-related factors included the work experience in the factory (1–5 years, 6–9 and ≥ 10 years), working hours per day (8 h or greater than 8 h) and working departments; Cleaning (the first milling steps involve manually separating wheat from seeds and other grains, Mixing (a mix process (to combine additives and raw materials) was done manually to mixers, and this activity generates a lot of flour dust which is inhaled by the workers, Packaging (Packing of the flour was done manually whereby the workers fill the flour to the bags and this activity generates a lot of flour dust) and loading (carrying of flour from the packing areas to the stores or trucks).
Previous exposure history; workers experience in the dusty environment before the current working position. Previous medical history (like the history of Asthma, Chronic bronchitis, Lung cancer and tuberculosis, emphysema, recent surgery of the thorax, abdomen, undergone vigorous exercise and any acute illness) that could be developed before and confirmed by physicians.
Ethical approval
We conducted the study after having an ethical clearance from the Institutional Review Board of the College of Health Sciences of Addis Ababa University. Before performing measurements, we obtained verbal and written consent from each study participant and participants informed that they have full rights to refuse and discontinue taking part at any point in the study. The study participants with lung function impairments were advised and linked to a health facility.
Data collection procedure
Interviews
Data were collected by using questionnaires adopted from British Medical Research Council (BMRC) [24]. The components of the questionnaires were socio-demographic variables, work-related variables, common chronic respiratory symptoms variables, behavioral factors of workers and past medical history. The questionnaire translated to Amharic retranslated to back English to check its consistency with the original one. Before the interview: a brief explanation given to the participant about the purpose of the study and administered (face to face) for selected flour mill and soft drink factory workers.
Anthropometric measurements
Weight of study participants were measured by using a standardized electronic weighing machine, with the subjects standing and wearing light clothes and height of the subjects measured with the stadiometer with portable field survey scales. Body mass index (BMI) calculated by using Ndd Medical technologies’ software.
Spirometry
Ndd Easy on-PC spirometer was used for determining lung function parameters (FVC, FEV1 and FEV1/FVC) among both flour mill and soft drink factory workers according to the American thoracic society recommendation [25]. Before performing the procedure, the subjects had instructed to practice deep inspiration and complete forceful expiration. We performed the spirometer test before work time. By putting a nose clip to prevent air leak through the nose, the subject was initially breathing for a few breaths normally, followed by deep inhalation and forceful expiration of the air fast and forcefully.
We performed all measurements in the sitting position. While doing this maneuver, flow and volume curves inspected on the screen for detecting whether subjects displayed enough effort during inspiration and expiration. Repeatability of FEV1 and FVC parameters were checked and three acceptable maneuvers with repeatable results were taken and we recorded the highest reading of these. Predicted values for lung function tests were not obtained due to lack of reference equation for the Ethiopian population. Trained and certified health professionals selected from different hospitals performed lung function tests. Standard operative procedures (SOPs) were used to improve quality of lung function test. In addition, the volume was measured by using 3 l syringe at the beginning of the test.
Statistical analysis
Collected data were organized and entered Epi data version 3.1 and we did cleaning to avoid missing values, outliers, and other inconsistencies. For data cleaning, frequency, sort, and list were used. Cleaned data exported to SPSS version 24 for analysis. Descriptive statistics were used to summarize data. Prevalence odds ratio with 95% CI was used to compare the prevalence of chronic respiratory symptoms of a flour mill with soft drink factory workers. Logistic regression analysis used to identify whether exposure variables are significantly associated with outcome variables or not. Thus, variables with p < 0.2 were included in the multivariable analysis by adjusting confounding variables; age, smoking habit, income, educational status, past dust exposure, working hours per day. The P-value of < 0.05 was considered as statistically significant. An independent t-test was used for continuous data; to compare the lung function test between flour mill and soft drink factory workers.