Climate can have a major impact on symptoms of AD. Many published data show that cold and dry weather increase the prevalence and risk of flares in patients with AD [6–10]. An ecological analysis was conducted using information on long-term climatic conditions in the different study areas from the World Weather Guide [6]. Among latitude, altitude, average outdoor temperature, and relative outdoor humidity, AD symptoms correlate positively with latitude and negatively with annual outdoor temperature, but correlations were not observed with any of the other factors. In a cross-sectional population-based survey of schoolchildren from ten Spanish centers in three different climatic regions, AD was positively associated with precipitation and humidity and was negatively associated with temperature and the number of sunny hours [7]. In a study of Taiwanese schoolchildren, no associations were found for the highest monthly means of temperature, whereas the annual means and the lowest monthly means of temperature were negatively related to flexural eczema but only in girls [8]. These results show that AD is significantly dependent on meteorological conditions. Studies that have researched flare factors in established AD support this notion, as lower outdoor temperatures, especially in combination with skin irritants, can contribute to the worsening of disease, whereas indoor climate seems less important [9]. However, the relationship between outdoor climate and disease flares is complex, with some children reporting worsening in the summer and others in winter, and the effects of outdoor temperature, ultraviolet (UV) light, and humidity are likely to interact. Engebretsen KA et al. reviewed the literature regarding the effect of environmental humidity and temperature on skin barrier function and dermatitis and conclude that low humidity and low temperatures lead to a general decrease in skin barrier function and increased susceptibility toward mechanical stress [10].

In Japan, yearly average temperature and humidity differ from region to region. The national yearly average temperature and humidity are 15.8 °C and 69%, respectively. The yearly average temperature and humidity of Hokkaido, Tokyo, and Okinawa are 9.8 °C/69%, 16.9 °C/61%, and 23.1 °C/74%, respectively. A cross-sectional study of the prevalence of AD conducted by the Japanese Ministry of Health, Labour and Welfare actually confirmed that the lowest prevalence of AD was found in Okinawa, where the highest temperature occurred. Hayashi et al. reported that the prevalence of AD in Gifu, with a temperate climate (15.8 °C/64%), was significantly higher than that in Itoman (in Okinawa), with a subtropical climate [11]. Significantly lower rates of current AD in the Ogasawara Islands (23.3 °C/78%), which are located away from mainland Japan and belong to a subtropical area, were also observed compared to previous reports from Japan [12]. In our population-based survey of children aged 5 years and younger on Ishigaki Island, Okinawa, Japan (Kyushu University Ishigaki Atopic Dermatitis Study: KIDS), the AD prevalence on Ishigaki Island was apparently lower than that reported by a research team of the Japanese Ministry of Health, Labour and Welfare [13]. They reported that the national mean prevalence of AD was 12.8% in individuals aged 4 months, 9.8% in those aged 18 months, 13.2% in those aged 3 years, 11.8% in those aged 6–7 years, 10.6% in those aged 12–13 years, and 8.2% in those aged 18 years. The AD prevalence on Ishigaki Island (6.3%) was approximately half that of children of corresponding ages in mainland Japan. In addition, although five types of loss-of-function mutations in the filaggrin (FLG) gene isolated in previous Japanese FLG mutation studies were identified, the frequency of FLG loss-of-function mutation in children of the KIDS cohort was not significantly different between the AD and non-AD groups (7.9% and 6.1%, respectively, p = 0.174), suggesting that FLG loss-of-function mutations are not always a predisposing factor for AD prevalence [14]. There are many possible explanations for this, such as different pathogens, dietary habits, or flora. However, we believe that the high temperature (24.4 °C) and humidity (73.1%) of Ishigaki Island might reduce the incidence of AD, presumably reducing the onset of AD in those who have FLG mutations or a defect in enzymes that digest filaggrin. This study definitively showed that the involvement of the interaction of environmental and genetic factors in AD onset differs from region to region. Extremes of climate might suppress the effects of genetic factors.

Although it has been generally accepted that seasonal aggravation of skin symptoms is a basic feature of AD, recent studies have suggested a decrease in the seasonal dependence of the dermatosis in European countries [15]. In Japan, the difference in climate in each of the four seasons is fairly clear-cut. Uenishi et al. examined the incidence of seasonal deterioration of AD in Japanese subjects. Of 682 patients aged 3–30 years with AD, 452 (66%) showed a seasonal aggravation of skin symptoms, and 230 (34%) experienced perennial deterioration. The overall incidences of exacerbation in spring, summer, autumn, and winter were 25%, 19%, 11%, and 36%, respectively [16]. In 1967, Masuda reported that the incidences of deterioration in spring, summer, autumn, and winter were 22%, 38%, 13%, and 34%, respectively [17]. Considering these results makes it evident that a real decrease in the incidence of seasonal aggravation of AD has occurred in Japan and that the incidence of summer deterioration has greatly decreased. The authors speculated that an important cause of the decrease in summer deterioration of AD may be a change in attitudes regarding the use of soap for bathing. In the 1960s, dermatologists in Japan prohibited patients with AD from using soap. As a result, the bodies of these patients were covered with sweat, scales, crusts, and debris of medicaments. Subsequently, however, several studies have demonstrated that such unclean skin is accompanied by severe pruritus, especially during the summer months when it is hot and humid in Japan [18, 19]. Therefore, most dermatologists now recommend that patients with AD use skin cleansing agent (i.e., soaps, detergent, cleanser) while bathing to keep both involved and uninvolved skin areas clean. Instead of the decrease in seasonal deterioration, perennial deterioration of the symptoms of AD has increased. Although it is not clear what factors are responsible for the increase in the occurrence of perennial worsening, the increase could be attributable to changes in environmental factors. For example, housing in Japan has generally changed from houses with open wooden structures to airtight homes made of concrete and synthetic materials. As a result, many patients with AD are perennially exposed to house dust and molds, which may provoke aggravation of the disease. In another study performed to investigate the relationship between residential environments and AD, 1378 elementary school children were examined by dermatological specialists in October 1994 (first series) and April 1995 (second series). Half of the children with AD showed symptoms in only one of the two seasons, either the autumn or spring, so the drifting of symptoms of AD occurred largely by season. Multiple logistic regression analysis showed that more children who were diagnosed positive for AD twice (AD(+,+)) lived in damp and moldy homes than did children who were diagnosed negative for AD twice (AD(−,−)). Thus, damp and moldy houses may be risk factors for the occurrence of AD [20].

In a study that evaluated the relationship between month of birth and prevalence of AD, individuals born in autumn showed the highest (7.5%), and those born in spring showed the lowest (5.5%), prevalence of AD [21]. These findings lead us to speculate that the climate to which one is exposed in early infancy affects the condition of the skin and that those born in autumn have dry skin in early infancy, which may ultimately result in a higher prevalence of AD among young schoolchildren. In fact, a recent study showed that skin barrier function measured through transepidermal water loss (TEWL) of the forehead during the first week of life is associated with the development of AD [22].

Bathing is an important behavior for keeping the body clean and is one of the habits of Japanese daily life. Bathing styles involve a full bath (body immersed in water in bathtub) or whole-body bathing (shower bath). Japanese individuals are known for their “specific” bathing habits. People in Japan usually take a bath every day, and a full bath (with or without shower) is the most common type of home bathing in Japan. Many Japanese individuals like high water temperatures, and some take baths of a long duration. Most people use “skin cleansing agent (i.e., soaps, detergent, cleanser),” and some, particularly elderly individuals, enjoy scrubbing their skin when they wash and dry their body. For many Japanese people, bathing is not only for cleaning the body but also for relieving fatigue and attaining a relaxed state of mind.

In a recent report about bathing habits among 189 Japanese healthy people, 128 participants (67.7%) took ≥7 baths per week. The most common water temperature was 40–41 °C, reported by 70 participants (40.2%). The mean water temperature was 40.14 ± 1.14 °C. The most common bath duration was 10–15 min, reported by 58 participants (30.9%). The mean bathing duration was 11.95 ± 9.03 min. In terms of water level, 167 participants (83.3%) took a full bath [30]. Nishioka et al. previously reported the bathing habits of Japanese people with various skin diseases. About 80% of the participants took a bath every day, and the most common bath duration was 10–20 min. Over 80% took a full bath (with or without shower). Most of them used soap or detergent, and 85% used a towel when washing the body [31]. Hattori et al. also reported that about 85% of patients who have various skin diseases use implements such as towels or gauze, and 56.4% scrub their skin when they wash their body [32]. These data remind us of traditional bathing habits in Japanese daily life. Another report regarding bathing habits in Okinawa found that 36% of people bathe more than twice a day in the summer season. Interestingly, 81% of these individuals take only a shower bath [33].

Although most dermatologists agree that the skin of patients with AD should be kept clean, there is no unanimity of opinion or good evidence regarding bathing technique or proper use of soap or detergent.