Pulmonary function, respiratory symptoms, and dust exposures among workers engaged in early manufacturing processes of tea: a cohort study

Tea is a popular beverage all over the world. Tea is made of the leaves and leaf buds of Camellia sinensis, and the manufacturing processes can be divided into two stages. The early stage includes withering, panning, rolling, ball-rolling, and drying (Figure 1), and the late stage includes sifting and blending, packing, and other processes before export and trade. Common teas can be classified into non-fermented tea (green tea, yellow tea, etc.), semi-fermented tea (Oolong tea, Paochong tea, etc.), and fully-fermented tea (black tea), but all are produced through a series of processes. The withering process includes solar and indoor withering, which allows the polyphenol catechins in leaves to be oxidized, a process called "fermentation." This process gives the tea its color and taste. Panning stops the fermentation and evaporates the water in leaves by the high temperature of the panning machine. Rolling rubs and destructs the surface tissues of leaves which results in liquid coating on the surface and makes the tea easy to brew. Ball-rolling involves rotating and pressing the dried leaves in a sack and turns leaves into strip and ball shapes, which requires considerable amount of labors. The final process is drying, in which water evaporates further. The tasks are different from those of the late-stage tea manufacturing, which might have dusts from fungal origins [1].

Occupational asthma has been described among workers involved in packaging and blending [2‐4]. Acute symptoms, including cough, chest tightness, and rhinorrhea, were observed by some studies in tea workers during their work, and some studies found higher prevalence of chronic respiratory symptoms among blending workers [5‐8]. Exposures to tea dusts could cause airway obstruction [9‐11], and higher prevalence of sensitization to tea was found in tea workers, especially herbal tea workers [11, 12], although the relations of tea sensitization status to respiratory symptoms and function were unclear [8, 11, 12]. There are, however, few, if any, studies on early-stage manufacturing.

The two stages of tea manufacturing have differences in work schedule, workplace, processes, and materials, which may lead to variations in the intensity, frequency, and contents of tea dust exposure. Therefore, workers engaged in different stages might suffer from different respiratory effects. We conducted a study to assess respiratory symptoms, pulmonary function, and tea dust exposure of workers engaged in early-stage tea manufacturing and to identify contributing factors of respiratory effects.

Of the 135 candidates, 93 tea workers and 33 office workers participated in this study, yielding a participation rate of 93.3%. However, two tea workers and one office worker were excluded due to existing diseases. The final study population of 91 tea workers and 32 office workers (the comparison group) had similar age and smoking habits, indicating successful matching (Table 1). The two groups also had similar height, weight, and distribution of race, but tea workers drank tea more frequently. The comparison group had higher education levels because of the requirements for obtaining the jobs.

According to the task, we divided the tea workers into four subgroups: withering (W), withering and panning (WP), ball-rolling (B), and withering and panning plus ball-rolling (WPB). The subgroups were similar in terms of height, weight, education level, race, and smoking and tea-drinking habits, but the B subgroup were younger than the others due to the requirement of more extensive labor (Table 2). We found that ball-rolling had the highest inhalable dust level in most of the factories and panning had the highest respirable dust level in all the factories (Table 3). Withering had the lowest levels of both dusts. Ball-rolling also had the highest coarse fraction (defined as inhalable dusts minus respirable dusts), which represents exposures from nose to tracheobronchial tract.

In comparison with office workers, tea workers had higher prevalence of most chronic symptoms (Figure 2). The four subgroups of tea workers had different baseline prevalence of chronic phlegm, chronic bronchitis, and dyspnea, even after adjusting for age, and the B subgroup had the highest prevalence of most chronic symptoms (Figure 3). In tea workers, the most common work-related symptoms were rhinorrhea/sneezing, throat irritation, and cough, and workers engaged in different processes had different prevalence of all the work-related symptoms except for dyspnea. Among the processes, ball-rolling was associated with the highest prevalence of all work-related symptoms (Figure 4).

Tea workers and the comparison group had similar baseline pulmonary function tests (Table 1). In tea workers, the B subgroup had the lowest FEV₁/FVC and MMF expressed as% of the predicted value--MMF (%pred) (Table 2). In comparison with office workers, the B subgroup still had lower FEV₁/FVC (86.94% vs. 90.60%) and MMF (%pred) (108.32% vs. 123.44%). Five participants, all were tea workers, had abnormal baseline pulmonary function defined as having an FVC or FEV₁ smaller than 80% of the predicted value or an FEV₁/FVC smaller than 75%. All of them had been tea workers for more than 10 years and were engaged in ball-rolling or mixed processes, and most of them had multiple chronic symptoms. After adjusting for age, height, weight, work duration, smoking and tea-drinking, the work task (as defined by the subgroups) was an independent predictor of both reduced FEV₁/FVC and MMF (Table 4).

According to the mean inhalable and respiratory dust levels and work task, we calculated every tea worker's dust exposure levels, and the level of coarse fraction was still an independent predictor of both reduced FEV₁/FVC and MMF. Fifty-eight tea workers participated in the during-work assessments, including 39 withering workers and 19 ball-rolling workers. We observed declines in FVC, FEV1, and FEV1/FVC, but not MMF. (Table 5) Compared to withering workers, ball-rolling workers had greater changes in pulmonary function.

The amount and size of dusts are two important factors of the respiratory effects of tea dusts. In a study on the sorting and grinding of dried fruits and teas, the total dusts ranged from 8.3 to 24.9 mg/m³ with a mean of 16.8 mg/m³, and the respirable fraction ranged from 1.0 to 6.4 mg/m³ with a mean of 3.2 mg/m³ [11]. However, area sampling assesses the distribution of dusts in the environmental, which does not necessarily reflect the worker's true exposure. We used personal sampling and found that panning and withering had higher respirable fractions (0.78/2.03 and 0.21/1.01, respectively) than ball-rolling (0.34/3.27), although ball-rolling had the most inhalable dusts. These results indicate that panning and withering produce smaller tea dusts, and ball-rolling generates larger dusts. Inhalable dusts can reach the respiratory tract anywhere from the nose to alveoli and cause effects throughout the entire respiratory system, while respirable dusts, with smaller particle sizes, can enter the alveolar region of the lung and cause fibrosis or pneumoconiosis that may be represented by changes in FVC [19]. The coarse fraction represents exposures from the nasal cavity to tracheobronchial tract, and ball-rolling has the highest exposure level. This may be related to respiratory tract obstruction that can be represented by changes in FEV₁ and MMF.

In our study, tea workers and the comparison group had similar prevalence and pattern of atopy to common aeroallergens, and a previous study on common tea packers observed the same [8]. We found only one case allergic to tea, which is also compatible to findings in that previous study [8], but some other studies on workers processing fruit and herbal teas reported higher prevalence [11, 12]. This may be due to the differences in allergenicity between common tea and other types of teas. Because of the low prevalence of sensitization to tea, the respiratory effects of tea dust exposure are more likely to represent an irritation response, rather than an allergic reaction.

In our study, tea workers had higher prevalence of almost all chronic symptoms than the comparison group, and in further analyses, we found the B subgroup had higher prevalence of all symptoms than the other tea workers. Whereas dust levels in our study were lower, we observed prevalence of chronic and work-related symptoms similar to those reported by previous studies [8, 10, 11], except for chronic phlegm, which might be due to the high prevalence of smoking and low prevalence of respiratory protecting equipment use (less than 2%) in our study. In addition to higher tea dust levels, ball-rolling is more labor-intensive, which may increase the ventilation rate and dust intake and thus increase the symptoms [20]. This might be the reason why the B subgroup had higher prevalence of chronic and work-related symptoms.

Although tea workers and the comparison group had similar baseline pulmonary function tests, the B subgroup still had worse pulmonary functions than the other tea workers and the comparison group. This is compatible to the results of tea dust assessment and reported respiratory symptoms. The abnormal pulmonary function tests we observed were mainly obstructive changes, similar to those observed in previous studies [9, 11, 12]. However, among our cases, two had combined obstructive and restrictive impairment. Therefore, further studies should be conducted to evaluate if long-term high-level tea dust exposure can lead to restrictive changes. Among tea workers, significant declines during work were observed in FVC, FEV₁, and FEV1/FVC, but not MMF. MMF is a sensitive parameter for early detection of smaller airways obstruction but has larger variations even using standard testing procedure [10, 21], which may affect the results of comparisons. Withering was also associated with during-work declines in pulmonary function, which might be due to the fact that some workers had performed ball-rolling or panning before the tests.

The multiple regression analyses showed that age and height were independent predictors of pulmonary function, which is a well documented fact [1, 21]. It should be noted that job task also appeared to be an independent predictor of FEV₁/FVC. The coarse fraction was an independent predictor of both FEV₁/FVC and MMF, which was compatible with the effects of dusts on the tracheobronchial tracts. Furthermore, the number of months worked each year had positive associations with all the parameters of pulmonary function and was an independent predictor of MMF. This might be attributable to the fact that healthier worker can work longer, a phenomenon known as "healthy worker effect". Whereas smoking is associated with declines in pulmonary function, it was not an independent predictor of pulmonary function in our study. The classification of smoking habits in our study was simple, which might lead to remarkable variations within each category as reflected by the relatively large standard errors of the regression coefficients. The healthy worker effect might also contribute to this observation. Further studies with larger numbers of participants that allow stratification of smoking habit in greater details are warranted to better evaluate the effect of smoking. In the analysis of the changes in pulmonary function during work, workers engaged in withering also had declines in FVC and FEV₁, which might be due to the fact that some workers had performed panning or rolling before the test. MMF is an early parameter of small airway obstruction and usually has large variations [10, 21], which might be the main reason why MMF was not a sensitive outcome in our study.

The current study showed that exposure to tea dusts in the early processes of tea manufacturing, especially ball-rolling, could lead to the development of respiratory symptoms and impairment of pulmonary function. Therefore, proper preventive or control measures should be adopted [19, 21‐23]. Due to need of dry environment and manufacturing equipment, local exhaust ventilation and the wetting method are not suitable for this industry, and therefore effective general exhaust ventilation is required. In addition, personal respiratory protection such as simple disposable dust masks should be applied. Workers may also benefit from health surveillance and educational programs. In fact, Taiwan mandates annual health examinations, including pulmonary function tests and chest X-ray, for workers with exposure to dusts. Although smoking did not appear to be a significant predictor for respiratory symptoms and pulmonary function in our study, its effects to lung by themselves and by enhancing the toxicities of other occupational hazards with interference of pulmonary defense mechanisms are well documented [23, 24]. Therefore, anti-smoking campaigns are still desirable in the tea industry.

There are few studies on the respiratory effects of different processes in the early-stage tea manufacturing. We find that tea workers engaged in early manufacturing processes of tea are at risk of developing respiratory symptoms and pulmonary function impairment, which might be related to dust exposures. Whereas further studies with longitudinal follow-up designs and larger samples are warranted to confirm our findings, our study results have called for attention to the exposure to dusts among workers engaged in early manufacturing processes of tea.