Results from our study showed that strong inorganic acids, such as hydrochloric acid, nitric acid, or sulfuric acid, used in industrial processes have a positive association with LA. Emerging evidence point out LA is a kind of severe periodontal disease and a potential effect of exposure to acid mists . This result was similar with the findings of Finnish and Brazil study. The results from the Finnish study showed that among workers with occupational exposure to sulfuric acid, 36.9% developed periodontal pockets (periodontal pockets detected by LA); the ratio was merely 30.9% in the control group . In Brazil study, periodontal attachment loss was observed in workers who had acid mists exposure and did not use dental floss . Recent studies also 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. Although typical periodontal disease should have gingival bleeding and periodontal pockets, another indicator of periodontal disease, CPITN, can connect these two aspects. A previous study had used CPITN as an indicator for acid mists related periodontal change . The results were only showed significant difference in unadjusted model but not statistically differences after considering numerous covariates in the analysis. The reason could be explained by good oral hygiene and mask use in our acid exposed group.
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 [2, 25]. 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 [3, 23, 25,26,27,28,29,30], 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 used the mean DMFT score as a marker. However, 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 do not show). The reason might be due to related young adult. Therefore, history of diabetes was not considered as a confounder for CPI. In the present study, we recruited voluntary participants instead of randomized sample population. Selection bias might be occurred due to the recruitment strategy in the present study. In addition, socioeconomic status, which might be the potential confounding factor, was unavailable from the survey. We adjust educational level, which is associated with economic and social outcomes , for the association between acid mist exposure and dental illness.