Association between overtime work hours and preventive dental visits among Japanese workers

Background This study aimed to examine the association between overtime work and the frequency of preventive dental visits among workers in Japan. Methods A self-administered questionnaire was completed by 14,847 daytime-workers. We used a logistic regression model stratified by sex and age and adjusted for marital status, occupation, education, and oral status to investigate the association between overtime work hours and the frequency of preventive dental visits. Results In total 1037 men (9.3%) and 511 women (13.9%) attended quarterly preventive dental visits, and 2672 men (23.9%) and 1165 women (31.8%) attended annual preventive dental visits. Overtime work was statistically significantly associated with quarterly preventive dental visits among men aged 50–59 years, with adjusted odds ratios (aOR) and 95% confidence intervals (CI) of 0.73 (0.56–0.95), 0.75 (0.54–1.04), and 0.55 (0.34–0.90) for < 20, 20–39, and ≥40 h overtime/month, respectively. No such trends were observed for men aged < 50 years and women of all ages. Overtime work of < 20, 20–40, and ≥40 h overtime/month was statistically significantly associated with annual preventive dental visits among men aged 40–49 years (aOR [95%CI]: 0.76 [0.61–0.95], 0.84 [0.65–1.09], and 0.72 [0.51–1.00], respectively) and 50–59 years (aOR [95%CI]: 0.75 [0.61–0.91], 0.76 [0.59–0.97], and 0.63 [0.45–0.88], respectively). No such trends were observed in men < 40 years and women of all ages. Conclusions Our study revealed associations between overtime and preventive dental visits among male workers aged in their 40s and 50s. Supplementary Information The online version contains supplementary material available at 10.1186/s12889-020-10107-7.

Preventive dental visits to maintain good oral condition (e.g., check-ups, professional teeth cleaning, and oral hygiene advice) has beneficial effects for oral health, and poor oral hygiene is a major risk factor for periodontitis and caries. Professional teeth cleaning combined with oral hygiene advice results in a reduction in plaque and gingival bleeding [15]. In addition, dental visits reduce the incidence of periodontitis [16]. Women and older individuals are more likely to attend preventive dental visits [17]. Highly educated people also have more frequent dental visits for check-ups than poorly educated people [18]. Research suggests that overtime workers have fewer opportunities to visit hospital for general medical treatment than those that do not work overtime [19]. However, if opportunities for preventive dental visits decrease, the morbidity associated with these diseases may increase. In addition, an increase in periodontitis may influence the increase of other diseases such as coronary heart disease.
A previous cross-sectional study examined the association between working hours and use of preventive health services [20]. Individuals working long hours (> 60 per week) were significantly less likely to attend dental check-ups and cancer screening (e.g., mammograms) than those working shorter hours. Factors contributing to these results were time barriers to making appointments for screening visits and time barriers to keeping any appointments made. Another study revealed that 71% of participants reported delaying or skipping preventive dental examinations because of lack of time to schedule and attend appointments [14]. However, it is difficult to apply those results more generally because the sample size in that study was small and all participants were medical doctors.
The impact of visits to a dental clinic for preventive purposes (rather than for dental examinations) has not been examined. Therefore, we conducted the present study to examine the association between overtime work and preventive dental visits among men and women in different age groups. Preventive dental visits refer to visits to a dental clinic for preventive purposes, rather than for a set dental check-up.

Methods
This cross-sectional study included participants from six companies in Japan (five manufacturing companies and one information and communications company). Participating companies did not conduct dental examinations. A self-administered questionnaire was disseminated to 32,026 workers across the six companies between July and October 2017. Participants received an explanation about the study and were informed that completion of the self-administered questionnaire was voluntary. The number of participants (response rate) from each of the six companies was 5728 (81.4%), 4083 (71.0%), 3943 (43.3%), 2302 (72.7%), 709 (11.7%), and 418 (46.5%). This gave a total of 17,183 participants (overall response rate: 53.7%).
The questionnaire collected information on personal characteristics, such as educational background, working conditions (working pattern [daytime, shift work, night shift and semi-night shift], mean overtime hours per month), preventive dental visits, and oral status (see Additional file 1). For the purpose of this study, 1674 shift workers (1580 shift work, 33 night shift, 28 seminight shift, and 33 with missing data) were excluded to eliminate the effect of shift work on the frequency of preventive dental visits. Workers aged 60 years and over (n=662) were also excluded from the study population because working conditions in Japan change significantly for those over age 60 years. Therefore, 14,847 workers who provided valid responses were included in the present analysis.
This study used a web-based survey. The study design was explained to all employees and employers via email, the company's intranet homepage, or the company's occupational health and safety committee. Employees could freely choose whether to participate in this study. Employees' responses to the questionnaire were not disclosed to their employers. The research protocol was approved by the Ethics Committee of Medical Research, University of Occupational and Environmental Health, Japan (H26-026).

Outcome: frequency of preventive dental visits
A Likert scale question with five levels was used to assess the frequency of preventive dental visits. Participants were asked "Are you currently visiting a dental clinic for prevention?" Response options were: "more than once every 3 months," "once every 6 months," "once a year," "sometimes," and "never." A previous study reported that dental visits at an interval of 3 months [15,21] or more than once per year [16] prevent periodontitis. We set two outcomes (binary variables). The first outcome was "more than once every 3 months" (yes or no), and this was classified as "quarterly preventive dental visits." The second outcome was annual visits for prevention (yes or no), which included "more than once every 3 months," "once every 6 months," and "once a year." This outcome was categorized as "annual preventive dental visits."

Covariates
Age was considered a continuous variable and categorized as ≤29, 30-39, 40-49, and 50-59 years. The categories for occupation (clerk, sales, research and development, engineer, production line and engineer, others) were taken from the human resource data of the participating companies. Marital status was classified into four groups: married, unmarried (single), unmarried (living with family and relatives), and divorced/bereaved. Responses were categorized as married, single/unmarried, and divorced/bereaved. Educational background and oral health status are thought to affect the dental visit behavior [18]. Educational background was categorized as "junior high school or high school," "junior college, technical school, or high professional school," "college," or "postgraduate." To understand oral health status, we asked participants: "During the past month, have you had any dental problems (such as toothache)?" (yes or no). We defined this variable as "dental problems."

Statistical analysis
We calculated the proportions of participants who made quarterly preventive dental visits and annual preventive dental visits for each subgroup (age, occupation, marital status, education, dental problems, and overtime work hours per month) stratified by sex. Chi-square tests were performed to evaluate associations in each category.
Logistic regression analysis was used to examine the association between overtime work hours and preventive dental visits, stratified by sex and age because the frequency of regular dental visits is high for women and older age groups in Japan [17]. "Quarterly preventive dental visits" and "annual preventive dental visits" were set as the outcome variables. "Overtime work hours" was set as an exposure variable. Adjusted odds ratios (aOR) and 95% confidence intervals (CI) were calculated, with adjustment for factors that may be associated with preventive dental visits (occupation, marital status, educational background, and dental problems) [18]. Workers with dental problems may be more likely to attend preventive dental visits than workers without dental problems because they may take preventive measures when visiting a dentist for treatment. Therefore, we performed an additional sensitivity analysis involving workers without dental problems. Statistical significance was set at p< 0.05. All analyses were conducted using IBM SPSS 25.0 software (SPSS Inc., Chicago, Illinois) and Stata 16.0 software (StataCorp. Texas, USA).

Results
Overall, 11,179 men (75.3%) and 3668 women (24.7%) responded to the questionnaire ( Table 1). The age distribution (≤29, 30-39, 40-49, and 50-59 years) was 8, 21, 37, and 34% in men and 20, 25, 35, and 20% in women, respectively. The majority of men were married (79%), whereas the number of single and married respondents was almost the same for women. Overall 6% of respondents (both men and women) had dental problems. Table 2 shows the association between preventive dental visits and demographic characteristics excluding those with missing data (n=2) for outcomes. In total, 1037 men (9.3%) and 511 women (13.9%) attended quarterly preventive dental visits, and 2672 men (23.9%) and 1165 women (31.8%) attended annual preventive dental visits. In any analysis of annual and quarterly preventive dental visits, the proportion of respondents attending preventive dental visits tended to increase as age increased for both men and women. In addition, the proportion of respondents attending preventive dental visits decreased as working hours increased. Men and women with dental problems tended to proactively visit a dental clinic more often. Table 3 shows the results of the association between preventive dental visits and overtime work hours, stratified by sex and age. Overtime work had a statistically significant association with quarterly preventive dental visits among men aged 50-59 years, with aORs (95%CI) of 0.73 (0.56-0.95, p=0.019), 0.75 (0.54-1.04, p=0.085), and 0.55 (0.34-0.90, p=0.017) for < 20, 20-40, and ≥40 h overtime/month, respectively (reference: men of the same age with no overtime). No such trends were observed in men aged < 50 years and women of all ages. Overtime work had a statistically significant association with annual preventive dental visits among men aged 40-49 years, with aORs (95%CI) of 0.76 (0.61-0.95, p= 0.017), 0.84 (0.65-1.09, p=0.194), and 0.72 (0.51-1.00, p< 0.05) for < 20, 20-40, and ≥40 h overtime/month, respectively (reference: men of the same age with no overtime). Similar results were observed for men aged 50-59 years, with aORs (95%CI) of 0.75 (0.61-0.91, p= 0.004), 0.76 (0.59-0.97, p=0.028), and 0.63 (0.45-0.88, p=0.007) for < 20, 20-40, and ≥40 h overtime/month, respectively (reference: men of the same age with no overtime). No such trends were observed in men aged < 40 years and women of all ages. Table 4 shows the results of the sensitivity analysis among workers without dental problems. The results were the same in both the sex and age categories.

Discussion
Our study revealed associations between overtime and preventive dental visits among male workers aged in their 40s and 50s. This association was more pronounced in people working longer hours; however, even a small amount of overtime work (< 20 h a month) statistically significantly reduced preventive dental visits.
Some occupational factors, such as overtime work [9] and shift work [22], have been reported to increase the morbidity of dental diseases (e.g., periodontal disease); however, the direct cause has not yet been established. Preventive dental visits reduce the risk for caries and periodontal disease and have an important role in oral hygiene. The results of this study suggested that overtime work may increase dental disease morbidity in workers because of a decrease in preventive dental visits.
Overtime work had significant associations with nonattendance of preventive dental visits among male workers aged in their 40s and 50s. However, men in their 20s and 30s did not show any statistically Overtime work hours: average number of overtime work hours in a month significant association between long working hours and preventive dental visits. As men get older, they may assume more managerial roles and think more about their responsibilities at work, which may result in putting off preventive dental visits. Among the respondents in this study, the proportion of male managers was 0.1% for those aged ≤29 years and 3.3% for those in their 30s, compared with 32.3% for those in their 40s and 43.9% for those in their 50s. Previous research has shown that workers delay or skip preventive dental examinations because of lack of time or not wanting to burden their coworkers by delegating additional work [14]. Previous studies have also shown that workers who have more frequent dental visits are more likely to be absent from work [23]. In addition, many workers do not want to take time away from their jobs, and may therefore stop making preventive dental visits to avoid delaying or not completing work. However, few studies have investigated the reasons for failure to attend preventive dental visits, and more research is needed on this topic. This study also showed that there was no statistically significant association between long working hours and preventive dental visits among women of all ages. The proportion of female workers in managerial positions in this study was considerably lower than that of males (0% for those ≤29 years, 2.3% for those in their 30s, 11.6% for those in  Table 3 Logistic regression analysis of preventive dental visits according to age aOR Adjusted odds ratio (adjusted for occupation, marital status, education, and dental problems) CI Confidence interval a : Unable to analyze because of small numbers in each group Table 4 Logistic regression analysis of preventive dental visits for workers without dental problems according to age Overtime work hours: mean overtime work hours per month in the last 6 months aOR: adjusted odds ratio (adjusted for occupation, marital status, and education) CI: confidence interval a : Unable to analyze because of small numbers in each group their 40s, and 10.4% for those in their 50s). However, the sample size of women in this study might not have been sufficient, and further studies with more women are needed. The younger generation had a lower percentage of people who received preventive dental visits, although the proportion increased with age ( Table 2). We also observed that women attended preventive dental visits more regularly than men. This was consistent with a previous study in the Japanese population, which reported that 6.5% of men and 9.0% of women attended 3monthly preventive dental visits, and 31.4% of men and 39.9% of women attended annual preventive dental visits [17]. Similar sex differences were observed in the present study (quarterly preventive dental visits: men 9.6%, women 14%; annual preventive dental visits: men 37.8%, women 47.8%).
The influence of overtime work hours on preventive dental visits in men was greater for annual visits, compared with quarterly visits. A previous study found oral health literacy was associated with the frequency of using regular dental care [24]. For workers attending quarterly dental visits, longer working hours did not appear to affect their dental clinic visits.
The results of this study suggested that even a relatively small amount of overtime work is likely to increase the risk for dental disease because it reduces the frequency of preventive dental visits. We believe that improving management of working hours and reducing the amount of overtime work are important factors for promoting preventive dental visits, which play a vital role in oral health. In addition to reducing overtime hours, it is important to consider other options, such as conducting dental examinations in workplace settings [25] or allowing workers to visit dental clinics during working hours, particularly younger workers and male workers aged over 40 years.
The main strengths of this study were the large sample size and the detailed analysis of Japanese people belonging to large companies. Our analyses were classified by the frequency of preventive dental visits and stratified by sex and age and adjusted for many covariates. This study also had some limitations. First, we were unable to examine causal relationships because this was a crosssectional study. It is difficult to assume a relationship whereby preventive dental visits affect overtime. Future longitudinal analyses using prospective cohort designs should be conducted. Second, our study used a selfadministered questionnaire to collect data on overtime and frequency of preventive dental visits, and recall bias might be present. Consideration should be given to obtaining objective data on overtime in further studies, such as obtaining company personnel data. Regarding preventive dental visits, it is difficult to distinguish between dental visits for treatment and preventive dental visits in objective data for receipts in Japan. Therefore, it is necessary to verify the validity of preventive dental visits by interviewing some respondents and asking them about their specific behavior and the purpose of their dental visits. The third limitation is the possibility of selection bias because the data were from employees of six large companies. The effects of overtime work may differ in our study population from those in the general population because background factors were evened out.

Conclusion
Our study revealed an association between overtime and preventive dental visits among male workers aged in their 40s and 50s. In addition to reduction of overtime work, access to oral health measures in the workplace will help improve oral health in for people who work full-time.