Objective: In the late 1950s and 1960s, Japan recognized that it had a shortage of dentists and that they were unevenly distributed. To solve these problems, four national and eight private dental schools were established, leading to a significant increase in the number of dental students in the 1970s and 1980s. The purpose of this study was to investigate the effects of this increased supply on the geographic distribution of dentists in Japan. Method: We determined the number of dentists and the population in each of Japan's 3,252 municipalities. The ratio of the number of dentists to the population of an area was assessed using Gini coefficients calculated from Lorenz curves. Results: From 1980 to 2000, the average number of dentists per 100,000 persons in Japan increased from 44 to 70. The Lorenz curve plotted for 1980‐1990 appeared as a nearly diagonal line, with the Gini coefficient decreasing from 0.310 to 0.263. The Gini coefficient in the year 2000 was 0.255, indicating only a slight improvement in 10 years from 1990 to 2000. Conclusions: The results suggest that the geographical distribution of dentists in Japan is influenced by municipalities' population size. While the number of dentists in municipalities with populations of less than 5,000 increased during the years from 1980 to 2000, 25.9 percent of these municipalities still had no dentists at the end of this period. This is an important issue that warrants prompt corrective action.

Dental erosion, and specifically its symptoms, has long been studied in Japan as an occupational dental disease. However, in recent years, few studies have investigated the development of this disease or labor hygiene management aimed at its prevention. As a result, interest in dental erosion is comparatively low, even among dental professionals. Our investigation at a lead storage battery factory in 1991 found that the work environmental sulfuric acid density was above the tolerable range (1.0mg/m 3 ) and that longterm workers had dental erosion. Therefore, workers handling sulfuric acid were given an oral examination and rates of dental erosion by tooth type, rates of erosion by number of working years and rates of erosion by sulfuric acid density in the work environment investigated. Where dental erosion was diagnosed, degree of erosion was identified according to a diagnostic criterion. No development of dental erosion was detected in the maxillary teeth, and erosion was concentrated in the anterior mandibular teeth. Its prevalence was as high as 20%. Rates of dental erosion rose precipitously after 10 working years. The percentages of workers with dental erosion were 42.9% for 10-14 years, 57.1% for 15-19 years and 66.7% for over 20 years with 22.5% for total number of workers. The percentages of workers with dental erosion rose in proportion to work environmental sulfuric acid density: 17.9% at 0.5-1.0, 25.0% at 1.0-4.0 and 50.0% at 4.0-8.0mg/m 3 . This suggests that it is necessary to evaluate not only years of exposure to sulfuric acid but also sulfuric acid density in the air in factory workers.

The purpose of this study was to determine the risk factors for root surface caries in the elderly and to evaluate the factors associated with gingival recession, one of the main risk factors for root surface caries. A total of 153 elderly people (35 men, 118 women) aged between 60 and 94 years ‫5.7ע5.37(‬ years) were surveyed. All participants were relatively healthy elderly who did not need special care in their daily lives. The survey was conducted in Chiba prefecture, Japan, and oral examinations and a questionnaire with face-to-face interviews were also carried out. Correlation analysis revealed that number of present teeth (pϽ0.001), gingival recession (pϽ0.001), bleeding on probing (pϽ0.001) and presence or absence of dentures (pϽ0.05) were significantly correlated with number of root surface caries. Stepwise multiple linear regression analysis for root surface caries revealed that the risk factors for increasing numbers of teeth with root surface caries were number of teeth with gingival recession (pϽ0.0001), bleeding on probing (p‫)7100.0ס‬ and self-reported dry mouth (p‫.)4540.0ס‬ Sex (pϽ0.05), number of present teeth (pϽ0.001), bleeding on probing (pϽ0.01), the presence or absence of systemic disease (pϽ0.01), dentures (pϽ0.01), drinking alcohol (pϽ0.01) and smoking (pϽ0.01) were significantly correlated with amount of gingival recession by correlation analysis. Moreover, the risk factors for increasing number of teeth with gingival recession were living in an institution (p‫,)4420.0ס‬ number of present teeth (pϽ0.0001) and smoking (p‫,)7300.0ס‬ as determined by stepwise multiple linear regression analysis for gingival recession.

Postgraduate clinical training for dentists has been mandatory in Japan since 2006. Hirata et al. reported that the geographic distribution of postgraduate dental trainees by prefecture in 2006 was worse than that of practicing dentists. This suggests that the postgraduate clinical training system could intensify the problem of distribution of dentists. In this study, therefore, we reviewed the geographic distribution of postgraduate dental trainees and practicing dentists between 2006 and 2010 in detail by city, ward, town and village by using the Lorenz curve and Gini coefficient. The results showed that while there was no significant worsening of geographic distribution of postgraduate dental trainees, the distribution of practicing dentists continued to deteriorate. A number of reasons may explain these findings: the clinical training system is based on a one-year employment contract, and dentists subsequently relocate as driven by the market; and geographic distribution among cities, towns and villages has worsened as a result of the merger of municipalities. The geographic distribution of practicing dentists is expected to deteriorate further if the number of dentists takes a downward turn in the future. Therefore, it is necessary to continuously review the distribution of postgraduate dental trainees.

To investigate dental erosion in employees working with sulfuric acid at a lead storage battery manufacturing plant and level of personal exposure to sulfuric ions, we measured sulfuric ion concentrations in the mouth rinse of those employees. We also measured exposure levels from air samples obtained from 2 employees from the same plant who did not work with sulfuric acid using a portable air sampler. At the same time, we collected and compared their mouth rinses with those from other employees. More specifically, we measured and compared sulfuric ion, calcium, and magnesium concentrations, along with pH levels from the mouth rinse of these two groups. Positive correlations were found between sulfuric ion and calcium concentrations (r‫,16.0ס‬ pϽ0.005), calcium and magnesium concentrations (r‫,16.0ס‬ pϽ0.005), Ca/Mg and calcium concentrations (r‫,46.0ס‬ pϽ0.005), and sulfuric ion and magnesium concentrations (r‫,55.0ס‬ pϽ0.005). Negative correlations were found between sulfuric ion concentrations and pH levels ‫,13.0מס‪(r‬‬ pϽ0.01), and magnesium concentrations and pH levels ‫,23.0מס‪(r‬‬ pϽ0.01). This suggests that mouth rinse from employees working with sulfuric acid could function as an indicator of sulfuric ion concentration in the work environment. Furthermore, this could lead to the development of a more accurate indicator of individual exposure.