Oral diseases are generally not immediately life-threatening and can be easily overlooked. Poor oral health is often associated with individual behaviors, but sometimes the effect of environmental or occupational factors are overlooked. Strong inorganic acids, such as hydrochloric acid, nitric acid, and sulfuric acid, are essential materials used in industrial processes, and the output or use of these minerals is often regarded as an important indicator for the development of a nation’s related industrials. Recent studies showed that acidic solutions in the work environment increase the risk of oral soft tissue (oral mucosa and periodontal tissue) diseases, such as mucosal ulcers, which can increase the risk of periodontal disease, oral fibrosis and stomatitis, leading to periodontal pocket induction and loss of attachment [5, 15, 28]. Studies on the relationship between acidic solution work environments and soft tissue disease showed that age, work history, and oral hygiene habits may further increase the risk of soft tissue disease in workers exposed to acid mist . Periodontal disease is a common and most important oral soft tissue diseases that affect oral health. Periodontal disease is characterized by periodontal tissue inflammation, including the gums, alveolar bone, and the periodontal ligament. The gums are affected first. They become inflamed and swollen, which leads to periodontal pocket induction between the teeth and gums. As inflammation progresses, periodontal pockets become bigger and deeper, which destroy alveolar bone and eventually causes tooth loss.
In addition to analyzing the effects of occupational acid mist exposure on oral health, we also analyzed several other factors that may affect oral health, including personal oral hygiene (the frequency of brushing per day, use of mouthwash and dental floss, and mouth breathing), personal health (halitosis, history of gastroesophageal reflux, and history of gastric ulcer), poor personal oral habits (smoking, drinking, chewing areca, frequency of consuming acidic foods), and mask use. After controlling for these potential interfering factors, the results showed that acid exposure and age increased the risk of severe periodontal disease (periodontal pockets detected by LA). These results were consistent with the findings of a Finnish study, which included fewer interfering factors. The results from the Finnish study showed that among workers with occupational exposure to sulfuric acid, 36.9% developed periodontal pockets; the ratio was only 30.9% in the control group. Moreover, the periodontal pocket incidence in the exposure group increased significantly with age . Interestingly, halitosis was related with periodontal disease (CI 1.58 - 20.81, p = 0.01) and the use of dental floss reduced the risk of periodontal disease (CI 0.09 - 0.88, p = 0.03); however, these two factors were not risk factors for loss of attachment. A study of workers in metal processing plants in Brazil showed that after controlling for interfering factors, such as age, drinking, and smoking, the odds ratio for periodontal disease was 1.77 between workers with more than six years of exposure and those with no exposure, but these results applied only to workers who did not use dental floss . The Finnish study, on the other hand, showed no significant correlation between periodontal disease and exposure . These results showed that for workers with acid exposure, oral hygiene helped to prevent mild periodontal disease and had no significant prophylactic role for more severe periodontal diseases, such as loss of attachment, suggesting that acid exposure may interact with other oral factors that directly or indirectly result in severe periodontal problems. Therefore, we infer that acidic solutions can cause oral soft tissue lesions by one or more of the following mechanisms: 1) acidic solutions directly irritate soft oral tissue, such as the gums and periodontal tissue, and can directly disrupt cell function and arrangement of the soft tissue ; 2) acidic solutions may suppress the immune-protective components of saliva, thus indirectly affecting gingival or periodontal immunity, resulting in persistent gingival or periodontal inflammation, and aggravating periodontal disease; and 3) acidic solutions may damage the ability of saliva to balance the pH, resulting in an acidic oral environment, which, together with poor personal oral hygiene, smoking, drinking, and chewing areca, may cause bacteria to grow, thereby damaging the gums or periodontal tissue . On the other hand, some researchers investigated periodontal tissue health and found that occupational acid exposure was not significantly related to periodontal disease [5, 21]. As a result, more long-term follow-up studies are needed to clarify the relationship between different occupational acid exposures and oral soft tissue damage.
Previous studies showed that besides periodontal disease, occupational exposure to acid mist and acid solution may cause tooth damage [6, 15-21], especially tooth erosion . In 2010, a Japanese study showed that the mean prevalence rate of tooth erosion among battery factory workers was 22.5%, which was also proportional to work history. In a report by Chikte et al , clinical examination showed that among electroplating workers, 60% had toothache and sensitivity, 76% had varying degrees of loss of tooth structure, and 25% had occupational tooth loss . Petersen et al  reported that due to exposure to sulfuric acid mist, 56% of battery factory workers had tooth thinning and tapering, 29% reported tooth shortening, and 31% had tooth erosion; however, this study did not confirm the effect of occupational acid exposure on tooth erosion. In 1984, Gamble et al  showed a strong correlation between exposure to sulfuric acid mist and tooth erosion; tooth damage occurred as early as 4 months after a mean sulfuric acid exposure of 0.23 mg/m3. In this study, the mean sulfuric acid exposure in the factories included in the survey was less than 0.066 mg/m3, which may be related to more advanced engineering control of occupational acid mist exposure, the awareness of oral hygiene, and the use of protective equipment, resulting in a great reduction in occupational hazards. As a result, this study showed no significant correlation between acid exposure and tooth erosion.
In the evaluation of acidic exposure related dental caries, we found the mean DMFT score was 9.29±6.41 in control group and 6.65±5.57 in exposed group (p<0.001). A mean decayed tooth (untreated caries teeth) was 1.26±2.01 in control group and 1.68±2.39 in exposed group (p=0.097) (data not show). Compare with national oral survey, the Taiwan adult and elder oral health survey found a mean DMFT score was 11.51±6.22 and a mean decayed teeth (D) was 1.74±2.90 in the 18-34 years age group . As a result, dental caries was not correlated with acidic exposure in our study population. The same results were also found in the UK , Japan  and Brazil  caries experience studies in acid workers.
Another interesting point in our study was the use of masks did not significantly reduce the risks. Personal protective equipment is a worker’s last line of defense against workplace hazards, especially when all other controls set up to minimize risk and protect the worker have been exhausted. However, the efficiency of personal protective equipment is significantly reduced if it is worn incorrectly or if it does an inadequate selection. Employees need to be adequately educated the potential hazards and trained in how to wear protective equipment in workplace.
The limitation of our study included a cross-sectional study design and reporting bias of self-reported questionnaire. Diabetes would be an important factor for CPI. However, there were only 5 diabetes patients in our study population (data not show). The reason might be due to related young adult. Therefore, history of diabetes was not considered as a confounder for CPI.