Perceived dryness in houses and adverse health effects during winter in Japan

. In recent years, as houses have become more insulated and airtight, indoor environments in Japan have tended to become low-humidity in winter, making dryness a problem. The relationship between the perception of dryness indoors and low indoor humidity is unclear, though it has been suggested that other environmental factors may influence perception of dryness and adverse health effects. It is essential to clarify the association between the indoor environment and human responses related to the perception of dryness. A questionnaire survey regarding indoor thermal environment, air quality, living environment, resident characteristics, resident behaviour, and health-related symptoms was conducted throughout Japan in the winter of 2022. The paper first describes the findings of this survey on resident characteristics, building characteristics, indoor humidity sensation, operating patterns of humidifiers and other devices, and resident health problems related to indoor dryness. Next, multiple regression analysis was used to identify the factors influencing the impact of dryness on health problems at various sites.


Introduction
In recent years, houses have become more insulated and airtight, causing indoor environments to tend to have low -humidity during the winter in Japan. 'Dryness' can be classified into two categories: dryness as a physiological response such as dryness of the nose and eyes, and dryness as a psychological response. When rooms in houses are low in -humidity, health problems such as dry nose, dry throat, dry skin, and dry eyes are more likely to occur and people are more likely to catch colds. These distressing and unpleasant problems experienced by residents of low-humidity environments have been identified in previous studies. However, the relationship between the perception of dryness and low humidity in living spaces is still unclear, and it is also unclear whether the perception of dryness directly affects health. It has been pointed out that not only low humidity but also other environmental factors may influence the perception of dryness and health hazards. Takada (2013) [1] has shown through a questionnaire survey that high temperature and high air velocity are factors that, in addition to low humidity, produce a perception of dryness with regard to the eyes, nasal cavity, lips, and throat in winter. Wolkoff (2018) [2] has also pointed out that the hygroscopic effects of chemicals may be perceived to produce a dry sensation.
The effects of a low-humidity environment on residents' health is still being focused on in research. However, it is unclear under what conditions, * Jo Tamura: m24c003@akita-pu.ac.jp Table 1. Surveyed number of respondents. and in what parts of the body, the perception of dryness occurs, and in whether it this has any effect on health. Therefore, an internet-based questionnaire survey was conducted during the winter of 2022 in order to determine the actual effects related to dryness in the home on human health., and to clarify the relationship between the environmental factors in houses and the health effects caused by indoor dryness. First, this paper describes the housing information from the questionnaire survey. Then, the factors affecting health problems related to a sense of dryness are identified using multiple regression.

Outline of survey
An internet-based questionnaire survey was conducted. First, a preliminary survey was conducted to select households that met the target conditions for the main survey, which are listed as follows: (1) living at the same location for more than 20 years, (2) households living in 'detached houses' or 'apartment houses', and (3) not single-person households. Table 1 summarizes the number of distributed questionnaires and number of respondents (response ratio). The target area for the questionnaire survey was all of Japan (47 prefectures), and the required sample size was set at more than 2,500 households. The questionnaire survey was conducted over a period of five days, from January 13 to 17, 2022. The number of responses was 2,614 (response ratio of 67.5%). Table 2 lists the contents of the questionnaire. There were 80 questions in total. We asked questions about the attributes of the residents, the house characteristics, the indoor thermal environment and IAQ during winter, the behaviour of the residents in their living space during winter, and the health hazards in their living space during winter. For the house characteristics, the level of thermal insulation was determined from the combination of the number of glass panes and the material of the window frames. The ventilation system was assessed from the combination of the location of fans and the air supply inlet. Regarding winter health effects, respondents were asked about the presence or absence of winter symptoms in four parts of the body (eyes, nose, airways/throat, and skin), followed by a question asking whether they had ever been to the hospital for these symptoms, and a subsequent question asking what they thought the cause of the symptoms was. Figure 1 shows outcomes from the questionnaire survey for the main questions in each region.

Survey results
The ratio of male to female respondents was similar for each region. The response rate of older respondents was higher, and the response rate of individuals in their 20s and 30s was the smallest overall.
More than 50% of the respondents responded that they tended to feel stressed easily. For this question, 32.4% of residents reported 'Rather easily stressed' and a similar trend can be observed in each region.
The insulation performance of each dwelling was estimated by the combination of window glazing and window frames and evaluated in the question about 'Thermal insulation'. Thermal insulation can have a grade from 1 to 4, with higher grades indicating better insulation. More than 50% of the dwellings had 'Grade 2' insulation, and there are no dwellings of 'Grade 1' insulation south of the Kanto region, where the building envelopes have no insulation. In each region, some dwellings of 'Grade 3' and 'Grade 4' insulation can be seen, and the relatively high rate of grade 3 or 4 dwellings in the Tohoku and Hokkaido regions indicates that the number of insulated buildings has been increasing.
The rate of respondents who answered 'Neither' to indoor 'Dryness' was the highest in the bedroom at 47.5%. However, the perception of dryness (Very dry, Dry, Slightly dry) was also close to 50%, indicating that more than 50% of respondents felt dryness indoors during the winter. In addition, responds who answered that they felt dampness was low were more likely to feel dryness in the winter. In each region, nearly 50% of respondents in all regions reported feeling dry, indicating that residents are likely to feel dry in their bedrooms during winter. A similar trend can be observed for the living room and residents seem to feel dryness in the living room more than in the bedroom strongly.
As for 'Using a humidifier', more than 50% of respondents in all regions stated that they had never used one. The rate of respondents in the Tohoku region (23.9%) who answered that they are 'Currently using' a humidifier was lower than in the other regions.
Furthermore, 50% of respondents answered that they experienced static electricity shocks 'Rarely' indoors, while a small number of households answered that these shocks occur 'Almost every day'. A similar trend could be seen for 'Air pollution'. Figure 2 shows the results of the relationship between perception of dryness and acceptance of dryness for in the living room and the bedroom. It indicates that the stronger the perception of dryness, the higher the rate that the dryness was perceived to be unacceptable, and almost half of the respondent reported responses of 'Unacceptable' for a 'Very dry' in the room. Some respondents also answered that indoor dampness was 'Unacceptable', and that the rate that 'Acceptable' was reported was as same as the rate for 'Neither'. A similar trend can be observed for in the bedroom, with 81.8% of residents declaring 'Unacceptable' when it is 'Very damp'. Figure 3 shows the relationship between perception of dryness and receiving static electricity shocks for in the living room and the bedroom. It indicates that the frequency of static electricity occurrence tends to be higher among residents who feel it is dry in the living room or the bedroom. In particular, 33.3% of the residents who answered that it is 'very dry' in the living room responded that they received static electricity  shocks 'Almost every day'. Including the respondents who answered 'Sometimes', 61.9% of households answered that static electricity shocks occurred in spaces that were 'very dry'.

Static electricity occurrence
The low humidity of the indoor environment is thought to be a factor in causing indoor static electricity shocks. Although the relationship between the perception of dryness and low humidity is unclear, the results suggest that there is an indirect relationship between the low-humidity environment and perception of dryness. Figure 4 shows the survey results pertaining to perception of dryness and awareness of problems with dryness. More than half of the households (63.5%) feel dryness in winter, and furthermore, 40.1% of all households are aware of problems related to dryness. 'Discomfort' was identified as the most common problem at 20.8%, followed by health hazards at 11.9% and static electricity shocks at 9.1%. The most commonly reported symptoms or health hazard was 'Pain in the nose or throat' at 75.8%, followed by 'Skin irritation' at 49.5% and 'Catch colds easily' at 42.9%. Compared to previous studies [3], the rate of respondents that answered 'Health damage' was lower, but the results are similar to those for 'Discomfort' at nearly 20% of the total. 'Building damage' was reported relatively infrequently. More than half of the households felt dryness, and this high rate confirms that dryness is a major problem in the winter seasons. Furthermore, some households reported that the perception of dryness may have indirect health risks. Figure 5 shows the responses relating to subjective symptoms in parts of the body during the winter. 91.4% of all residents were aware of symptoms, with the highest rate (84.4%) seen for awareness of skin symptoms. In addition, the rate of residents reporting symptoms of dryness in any part of the body was nearly 10%. These results could suggest that dryness is a major health risk in dwellings.

Statistical method
Using the results of the questionnaire survey, a statistical analysis was conducted to determine the relationship between the adverse health symptoms caused by indoor dryness and those caused by nondryness using multivariate logistic regression analysis. Subjective symptoms were classified into two categories of 'Due to dryness' and 'Due to causes unrelated to dryness', and adjusted odds ratios were calculated including the 95% confidence interval.
First, univariate analysis was carried out to calculate the odds ratio of the 'Symptomatic group' to the 'No symptoms group' and to ascertain the magnitude of the effect of each site on subjective symptoms individually. Next, factors meeting a significance level of p<0.2 in the univariate analysis were entered as independent variables, and multivariate logistic regression analysis using the variable reduction method based on the likelihood ratio was conducted. Among the factors that were found to be significant in this analysis, factors for which significant results have been obtained in previous studies were selected. Forced-entry variables were used so that the independent variables were the same for each part, and a final analysis was conducted. In addition, 'Gender', 'Age' and 'Smoking' were entered as confounding factors in the independent variables, and adjusted odds ratios were estimated taking confounding into account. IBM SPSS Statistics Ver. 24 was used for the analysis. Table 3 summarizes the results of the analysis with 'Presence of subjective symptoms due to dryness' and 'Presence of subjective symptoms due to causes unrelated to dryness'.

(a) Subjective symptoms due to dryness
Although 'Thermal insulation' or 'Frequency of heating' were not associated with subjective symptoms, the adjusted ORs for 'Humidifier use' (AOR: 1.67, p<0.01) were statistically significant for subjective symptoms of the upper airway.
Residents who answered 'Easily stressed' for the question regarding stress were significantly more aware of symptoms in all parts of the body than those who answered 'Not easily stressed'. It is possible that residents who are more susceptible to stress are more sensitive to indoor dryness and are also more sensitive to subjective symptoms.
For 'Dryness (Living room)', respondents who felt more aware of symptoms caused by dryness more frequently reported 'Dry' (AOR: 3.73, p<0.001 for eyes) or 'Slightly dry' (AOR: 1.98, p<0.001 for eyes) compared with 'Neither' in all body parts. Additionally, Table 3. Multivariate analysis results individuals reporting 'Dry' including 'Slightly dry' were more likely to be aware of symptoms caused by dryness in all body parts. These results indicate that it is necessary to prevent indoor low humidity for reduction of health risks.
For 'Static electricity shocks', it can be confirmed that, overall, the higher the frequency of occurrence reported, the more aware of symptoms the respondent tended to be. In the skin, AOR of receiving shocks 'Almost every day' (AOR: 4.82, p<0.05) was the largest.
Significant associations were also found for 'Air pollution' and 'Mildew smell'. This suggests that air quality is also associated with subjective symptoms caused by dryness.
For 'Awareness of the problems of indoor dryness', significant associations were found between symptoms in all body parts and 'Health damage', 'Discomfort', or 'Building damage' (excluding skin). Therefore, it can be said that residents who perceive dryness as a problem in their lives are more likely to be sensitive to dryness and to recognize dryness as a cause of subjective symptoms.

b) Subjective symptoms due to causes unrelated to dryness
No association with subjective symptoms was found with 'Thermal insulation' or 'Frequency of heating', nor with 'Humidifier use'.
Residents who reported being 'Easily stressed' for were significantly more aware of symptoms in all body parts; however, compared to 'Due to dryness' the AOR tended to be lower, except for the skin.
No significant association was found between symptoms and 'Dryness (Living room)'. Therefore, residents who reported dryness as a cause among their subjective symptoms were more likely to feel dryness.
As for 'Static electricity shocks', an association was found for symptoms relating to the nose, but no association was observed for other body parts.
For 'Air pollution' and 'Mildew smell', a similar tendency to 'Due to dryness' is seen. These results suggest that health risks can be reduced by preventing the deterioration of the indoor environment.
For 'Awareness of the problems of indoor dryness', no significant associations were found. Therefore, it can be inferred that clarifying the relationship between the subjective effects of dryness and 'Awareness of the problems of indoor dryness', and that clarifying what problems residents perceive with dryness could lead to health risk reduction.

Conclusions
A questionnaire survey was conducted in the winter of 2022 in order to understand the effects related to dryness in housing on human health and to clarify the relationship between residential environmental factors and health effects caused by indoor dryness. The following findings were obtained.
A) It was confirmed that the more dryness was felt indoors during the winter, the less the indoor environment was deemed acceptable. On the other hand, the tendency to deem the indoor environment unacceptable was also observed in the case of dampness. Therefore, it can be inferred that it is important to create residential spaces such that occupants do not feel indoor dryness or dampness.
B) 63.5% of respondents felt dryness during the winter season, and 40.1% were aware of the problem of dryness. In particular, a high rate of households reported 'Discomfort' (20.8%). Since around 10% of the households reported that there was 'Health damage', it can be inferred that dryness may be a critical important health risk factor. C) 91.4% of respondents were aware of symptoms in some body part. Nearly 10% of respondents reported dryness in all parts of body, suggesting that dryness may be a health risk. In addition, a high rate of respondents were aware of skin symptoms, with a particularly high rate of respondents reporting specific symptoms of dryness such as 'Drying of skin on the body' (65.7%) and 'Drying of hands' (64.7%). The high rate of these symptoms of dryness in body parts indicated that such symptoms are more likely to appear in housing during the winter.

D)
Regarding 'subjective symptoms due to dryness', there was an association with 'Dryness (Living room)'. Therefore, to decrease health risk, it is necessary to create indoor environments that do not cause a strong perception of dryness. However, the mechanism of perception of dryness is unclear and remains a subject for future study. In addition, associations with factors answered as psychological evaluations, such as 'Stress' and 'Mildew smell' were also confirmed, and these results may be influenced by individual differences, such as in level of sensitivity to the environment.