Oxidative toxic stress in workers occupationally exposed to ceramic dust: A study in a ceramic manufacturing industry

Abstract

BACKGROUND: Exposure to compounds used in ceramic industries appears to be associated with induction of oxidative toxic stress. Objective: This cross sectional study was undertaken to assess the oxidative toxic stress parameters associated with occupational exposure to ceramic dust.

METHODS: Forty ceramic-exposed workers from a ceramic manufacturing industry and 40 unexposed referent subjects were studied. A questionnaire containing information regarding demographic variables, occupational history, history of any chronic disease, antioxidant consumption, and use of therapeutic drugs was administrated to them. Oxidative toxic stress biomarkers including lipid peroxidation (LPO), total antioxidant power (TAP), levels of total Thiol groups (TTG) and catalase (CAT) activity were measured.

RESULTS: Significant increments in blood LPO levels, CAT activity and concomitant lower TAP were observed in ceramic exposed workers in comparison to referent group. No statistically significant difference was noted between the means of TTG levels between the groups.

CONCLUSIONS: Findings of the study indicate that occupational exposure to ceramic dust induces oxidative toxic stress. Supplementation of workers with antioxidants may have beneficial effects on oxidative damages in ceramic industries.

Keywords

Workers total antioxidant power oxidative stress

1 Introduction

In tile and ceramic industries, workers are commonly exposed to airborne dusts such as crystalline silica, quartz, tridymite, mica, and kaolin via inhalation [1]. It is believed that continuous exposure to excessive levels of airborne silica can results in silicosis, a progressive, disabling, and incurable occupational disease [2]. The association between silica dust exposure and high incidence of radiological signs of silicosis in ceramic exposed workers is well-known and the risk of silicosis is the same as that of mine workers [3]. In ceramic industry, according to the nature and type of processes workers are exposed to different concentrations of crystalline silica. Higher silica dust levels occur during handling of damaged pieces, molding, and after firing, when higher levels of cristobalite are formed [4].

In ceramic industries, workers are exposed to airborne dusts including beryllium, silica, inorganic lead, lime and aluminum. Inhalation exposure to these chemicals can lead to production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) [5, 6].

ROS and RNS generation may results in orga-nelle’s function impairment and affects cell viability and trigger cell death [7]. Several possible sources of ROS are proposed following silica internalization and cell damage. After entering the body, hydrophilicity, binding, and absorption of silica particles in biological membranes are affected by forming hydrogen bonds of Silanol groups (SiOH) with oxygen and nitrogen molecules although, silanol groups are not considered as a free radical [8]. Free radicals can affect all types of macromolecules (carbohydrates, nucleic acids, lipids, and proteins and thus they are potentially toxic to lung tissue [9]. The present study was performed to determine the levels of cellular LPO, total antioxidant power (TAP), total thiol groups (TTG) and catalase (CAT) activity in workers occupationally exposed to ceramicdusts.

2 Materials and methods

2.1 Subjects and study design

In this study, 40 male employees working at a local ceramic manufacturing plant were studied. They were working in a 5 work shift schedule weekly, working 6 days a week in 8.5 h daily shifts starting at 7 : 30 AM and ending at 4 PM. Referent group consisted of 40 age- and sex-matched subjects with no history of exposure to ceramic dust. The participants were similar as far as living urban area, socio-economic and nutritional status were concerned. Before participating in the study, all participants signed an informed consent form. The university ethic committee approved the protocol of the study; and the study was performed in accordance with the Helsinki Declaration of 1964 as revised in 2000. A questionnaire containing information regarding demographic variables, smoking and dietary habits, complementary antioxidants consumption, the history of any chronic diseases and detailed occupational history at the ceramic plant and any other industries, if any, and use of therapeutic drugs was administrated to all participants. Furthermore, 5-milliliter heparinized blood samples were collected from the brachial veins of the participants. The plasma was separated and stored at –70°C until analysis.

2.2 Assay of LPO

Thiobarbituric acid (TBA) reagent was used to determine the LPO product expressed as the extent of malondialdehyde (MDA) production during an acid heating reaction. A calibration curve was prepared from standard solutions of tetraethoxypropane and it was used to obtain the levels of TBA+MDA adducts in the samples [10].

2.3 Assay of TAP

To measure TAP, ferric reducing ability of plasma (FRAP) method was applied. In this method, the ability of plasma in reducing Fe^3 + to Fe^2 + in the presence of 2,4,6 tripyridyl-s-tiazine (TPTZ) is measured. As a result of reaction between Fe^2 + and TPTZ a blue complex is formed which has a maximum absorbance at 593 nm [11].

2.4 Assay of TTG

To evaluate the plasma TTG, 2 thionitrobenzoic acid (DTNB), a reagent which reacts with thiol molecules and produce a yellow complex with a good absorbance at 412 nm, was used [12].

2.5 Assay of CAT activity

The primary principle of the reaction is the substrate (hydrogen peroxide) breakdown by catalase and measuring the absorbance decrement at 240 nm. This reaction requires the lowest levels of H₂O₂ (10 mM) and sodium phosphate buffer (50 mM, pH = 7.0). Change in the absorbance rate in the unit of time is considered as an index of the catalase activity which is measured as the amount of H₂O₂ disintegrated by catalase in 1 min and expressed as the unit of the enzyme in mL of saliva (units/ml) [13].

2.6 Statistical analysis

Data were analyzed using version 18.0 of SPSS on a personal computer. The statistical analysis was performed using the Student t-test and Chi-square or Fisher’s exact test. Mean and standard errors were determined for the studied parameters and compared by ANOVA test. A p value of less than 0.05 was considered statistically significant.

3 Results

Demographic characteristics, duration of employment and data on smoking are exhibited in Table 1. The average age and length of employment for exposed workers were 36.1±SD and 29.1±SD and for referent group were 29.1±SD and 4.63±0.53, respectively. Only 4 subjects in exposed group (10%) were smoker and none of referent subjects were smoker.

Table 2 shows the mean±SE (95% CI) of oxidative toxic stress (OTS) biomarkers in plasma samples of the groups. As shown, LPO levels were significantly higher in exposed workers than referent subjects (1.46±0.09 vs. 1.1±0.11 nmol/mL, P < 0.03). Likewise, significantly higher CAT activity was noted in exposed workers compared to referent group (10.3±2.4 vs. 4.8±0.7 U/mL, P < 0.03). In contrast, exposed workers had significantly lower TAP than referent subjects (5.50±0.16 vs. 8.07±0.22 umol/mL, P < 0.001). Furthermore, no statistically significant difference was observed in the means of TTG levels between the groups.

4 Discussion

The findings of this study showed that workers’ exposure to airborne ceramic dusts may increase CAT activity and LPO level and decrease TAP levels in comparison to those of an age- and sex-matched referent group. However, no statistically significant change was noted in the TTG levels. Therefore, exposure to airborne ceramic dusts may results in significant defects in body antioxidant defense which may lead to further consequences that have been already reported in animal and human studies [14]. Compounds used in ceramic industries including beryllium, silica, inorganic lead, lime and aluminum are known to have potentials for OTS induction and thus it is not surprising to see these symptoms among ceramic-exposed workers. There is evidence of liver and lung damages in ceramic workers [15, 16].ROS production continuously occurs during normal condition of metabolic processes in the body and interaction with environmental agents. If ROS generation is beyond the body’s antioxidant capacity, a condition known as OS will developed that may resulted in cell damage [17, 18].

LPO has been considered as one of molecular mechanisms of metals’ neurotoxicity that may cause some neurodegenerative diseases such as Parkinson and Alzheimer [19 –21]. In the present study,statistically significant higher levels of cellular LPO and lower antioxidant power and TAP levels were observed in exposed workers than referent subjects, a condition that does not have a good prognosis and there is a probability of further consequences leading to cellular injuries and pathological outcomes.

Workers’ exposure to airborne ceramic dust may increase the risk of end-stage renal disease and several multisystem autoimmune diseases [22]. The most common long-term health effect of silica dust exposure is pulmonary fibrosis and the resulting ROS and RNS generation [23]. Temporal association between silica-induced fibrosis and OTS as well as antioxidant effects on the fibrotic response has been used to assess the relationship between these conditions [16].

A link between silica-induced OTS and NF-κB and AP-1 transcription activation has been reported by some authors [24]. In vivo evidence showed silica-induced apoptosis throughout AP-1 activation after silica inhalation through ERK and p38 MAPK signaling pathways [25].

Silica exposure may lead to release of TNF-α andIL-1β which is mediated through phosphatidy-lcholine-specific phospholipase C regulated in a redox-dependent fashion [26, 27]. It can be concluded from these studies that the production of ROS and RNS following silica exposure is an important initiator for the fibrotic processes [28]. There is more pollution in industrials, therefore it is difficult to distinguish them. In conclusion, the findings of the present study indicated that ceramic-exposed workers may be at risk of reduced antioxidant capacity. Further studies are needed to determine the type of antioxidants that are affected in these workers. Additionally, the administration of antioxidants is recommended for ceramic-exposed workers.

Conflict of interest

The authors have no conflict of interest to report.

Footnotes

Acknowledgments

Funding through Yazd University of Medical Sciences, vice chancellor for research affairs, supported this study. We are grateful to the employers and employees of the ceramic manufacturing plant for their helpful cooperation.

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