Study population
Based on the directory of factories and manufacturers from the Ministry of Economic Affairs of the Republic of China, we selected enterprises in the metal surface treatment or electroplating industries in municipalities and counties in Taiwan that employed 300 or more employees and were willing to participate in this survey. In this study, we selected 4 acidic solution factories willing to collaborate since July to November, 2016. The factories were in Northern Taiwan, and the workers may have been exposed to different acidic solutions, including hydrochloric acid, sulfuric acid, and nitric acid. Annual reports of work environments were summarized in table 1. The total numbers of acid non-exposed and exposed workers joined the study were estimated to be 5,100 and 500.
We contacted the factory managers or supervisors, who in turn contacted and confirmed which study subjects were willing to participate. We then conducted the survey at the time of a routine health check at each factory. Study subjects were workers aged 20 years or older who were enrolled at a 1:1 ratio into exposure and control groups that were defined by work history information. The exposed groups comprised workers engaged in acid mist work, while controls were subjects conducting similar tasks but who were not exposed to acids (e.g., other production line workers). All participants completed oral examinations and self-report questionnaire. A total of 309 subjects (157 in control 21 and 152 in exposed group) participated was enrolled in the further analysis (Figure 1). The study was reviewed and approved by the Institutional Review Board (IRB) of the Tri-Service General Hospital (TSGHIRB No.: 1-105-05-080) before the study began. Each subject signed the informed consent form at the time of enrollment.
Oral examination
Each enrolled subject underwent a comprehensive examination of their teeth and oral mucosa by a team of qualified and uniformed trained dentists to minimize the variation caused by different dentists. The detailed results were recorded in the subject’s medical records for subsequent data analysis. To reduce error, the dentists were unaware of each subject’s occupational exposure. A mouth mirror and a probe were used to record the decayed, missing, and filled teeth (DMFT) index based on WHO criteria[16]. The community periodontal index (CPI) and loss of attachment (LA) index were measured for assessment of periodontal status, by using a mouth mirror and a specifically designed periodontal probe[17]. The measured variables included tooth erosion (Keels-Coffield clinical severity scale[18]) (Supplemental table 1), tooth abrasion, tooth decay (DMFT), and periodontal disease (CPI and LA).
The DMFT corresponds to the average number of decayed, missing, and filled permanent teeth. The WHO CPI[19] and LA[20] levels were used to assess periodontitis (Hu-Friedy PCP11.5B CC SE probe 3.5-5.5-8.5-11.5). Periodontitis was defined as a CPI or LA value greater than or equal to 1 (Supplement table 2). The index teeth numbers were 11, 16, 17, 26, 27, 31, 36, 37, 46 and 47. A CPI was used at the gingival margin as a reference to measure the periodontal pocket. During the measurement, the probe was aligned with the long axis of the tooth and the total extent of the pocket was explored. The results were recorded as follows: 0: the 1st marking of probe remained completely visible, without bleeding after probing - healthy gum; 1: the 1st marking of probe remained completely visible, with bleeding after probing - gingivitis; 2: the 1st marking of probe remained completely visible, with dental calculus (including supra-or subgingival calculus); 3: the 2nd marking of probe remained completely visible - shallow pocket; and 4: the 3rd marking of probe remained completely visible - deep pocket. An LA was used at the cemento-enamel junction (CEJ) as a reference to measure the periodontal pocket. During the measurement, the probe was aligned with the long axis of the tooth and the total extent of the pocket was explored. The results were recorded as follows: 0: CEJ was within the first marking; 1: CEJ reached the second marking; 2: CEJ reached the third marking; 3: CEJ reached the fourth marking; and 4: CEJ reached beyond all markings (Supplemental table 1).
Covariables assessment
Demographic characteristics in the present study were obtained from participants by self-structured questionnaire (supplemental questionnaire). Educational level was divided into studying less than 12 years and more than 12 years. Acidic foods were defined as foods where the pH value was ≤ 6[21]. For dietary habits with acid foods, a type of acidic food was only considered when the subject consumed that acidic food more than once per week. Owing to the absence of data regarding dietary habits with acidic foods, we pooled each type of acidic foods and categorized into four groups (Q1 to Q4). We collected information such as cigarette smoking, alcoholic drinking, chewing areca, teeth brushing, mouthwash use, dental floss use, mouth breathing, and halitosis from self-reported questionnaire that participants answered yes or no.
Statistical analysis
IBM SPSS Statistic version 20 was used for statistical analysis. For descriptive analyses, the t-test, one-way analysis of variance, and chi-square test were performed for comparative analyses of the demographics. Moreover, the frequency, and the severity of tooth erosion, DMFT, CPI and LA scores were analyzed. Both t-tests and chi-square tests were performed to investigate the relationship between demographic characteristics and oral health. Multivariable adjustment was done by applying extended model. Model 1: adjusted for sex, age, worked years and education level; Model 2: adjusted for model 1+ mouthwash use, dental floss use, tooth brushing, mask use, smoking, drinking, and chewing areca. Model 3: adjusted for model 2+ dietary habits with acidic foods. Moreover, multivariate logistic regression analysis was used to investigate the association of occupational acid exposure with tooth erosion, DMFT, CPI, and LA. A significance level of < 0.05 was adopted for this study.