Associations of Indoor Ventilation Frequency with Depression and Anxiety in Chinese Older Adults

the original


Introduction
Depression is characterized by feelings of sadness, emptiness, and irritability and involves a wide variety of cognitive and somatic symptoms [1,2].Approximately 6% of individuals worldwide experience depression, as do at least 11.5% of older adults of ≥65 years [3].In Asian countries, the prevalence of depression ranged from 5.5% to 25.7%, although the prevalence among older adults (those aged 65 or above) is likely to be underestimated [4,5].Additionally, depression is a primary cause of disability [6].
Anxiety disorders, which are also fairly prevalent worldwide, are characterized by hyperarousal and excessive fear and worry [7].The global prevalence rates of anxiety disorders are estimated to be between 3.8% and 25% in the population at large and up to 70% in people with chronic illnesses [8].Anxiety disorders affect quality of life, daily functioning, and overall happiness [9], which are the sixth leading cause of disability globally, accounting for 260 000 disability-adjusted life years [10].In summary, depression and anxiety disorders constitute a public health burden.
In recent decades, indoor air pollution has become a growing focus.Indoor air pollution can result from the use of disinfectants, pesticides, and unclean fuels, including solid fuels and biofuels.Indoor living environments are very important for human health, especially among older people, who typically spend more time indoors than do younger adults owing to that fact that older adults are more likely to have retired and that individuals who have retired tend to spend more time indoors than outdoors [3,11,12].Environmental factors, including air pollution, are risk factors for mental health conditions.Several studies have shown that airborne pollutants, including volatile organic pollutants and particulate matter, are associated with depression and anxiety [13][14][15].Ventilation, which involves supplying air into indoor environments through mechanical or natural forces (i.e., air conditioning and windows, respectively), can reduce the concentration of indoor air pollution, such as volatile organic compounds, formaldehyde, and radon [16,17], improve indoor air quality, and is beneficial to cognitive function and respiratory health for older adults [17,18].However, to date, the relationship of indoor ventilation with depression and anxiety has not been reported.This study therefore is aimed at assessing the relationship between frequency of indoor ventilation with open windows and depression and anxiety by using a large representative population of older adults nationwide.

Materials and Methods
2.1.Settings and Participants.Data came from the 2018 Chinese Longitudinal Healthy Longevity Survey (CLHLS).Participants with the age of 65 years and older from 23 provinces of China, an area representing approximately 85% of the country's population, were selected using multistage randomcluster sampling.This sampling method was chosen to ensure adequate representativeness.The CLHLS has a systematic data collection process that involves trained staff interviewing older adults face to face.The details of CLHLS have been published on other places [19].The Biomedical Ethics Committee of Peking University approved the CLHLS (IRB00001052-13074), and each participant provided written informed consent prior to their participation in this study.
A total of 15 874 individuals were interviewed in the 2018 CLHLS.After the exclusion of cases with missing data related to depression (n = 3414), anxiety (n = 74), indoor ventilation (n = 206), and individuals aged <65 years (n = 84), 12 096 participants were included for analysis.

Indoor Ventilation
Frequency.Indoor ventilation frequency was obtained by asking the participants how frequently they opened their windows.Specifically, the participants were asked how many times they opened their windows per week in each season.Items were rated on a 3-point Likert scale including 0 (never), 1 (1 to 5 times), and 2 (>5 times).Total scores ranged from 0 to 8. Total scores of 0 to 3, 4 to 5, and 6 to 8 were classified as low, intermediate, and high ventilation frequency, respectively.Lowess smooth plots were used to evaluate the unadjusted associations of continuous indoor ventilation frequency scores with depression and anxiety to determine the cutoff score for indoor ventilation frequency [18].

Depression and Anxiety
. Depression was evaluated using the 10-item Center for Epidemiological Studies Depression scale, which was demonstrated to be valid and reliable when used in a Chinese cultural context [20,21].The scale includes ten items by a 4-point Likert scale from 0 (rarely or never) to 3 (all of the time).Scores for positive questions were reversed before summation.Total scores of ≥10, ranging from 0 to 30, indicate depression.It has adequate internal consistency with Cronbach's alpha of 0.738.
Anxiety was evaluated using the 7-item Generalized Anxiety Disorder (GAD-7) scale [22].GAD-7 comprises seven indicators by a 4-point Likert scale ranging from 0 (not at all) to 3 (almost every day).Total GAD-7 scores were from 0 to 21, with higher scores representing a greater likelihood of anxiety symptoms [23]; if the total score is ≥10, an individual is considered as having anxiety [24].GAD-7 was demonstrated to be valid in a Chinese cultural context and to have strong psychometric properties.Cronbach's alpha value for this scale was 0.920 in this study.

Covariates.
A spectrum of potentially confounding factors was controlled to obtain robust results, including sociodemographic factors, lifestyle factors, and chronic diseases.Data were collected for age group (65 to 74, 75 to 84, or ≥85), sex, urbanization level (urban, town, or rural), educational level (0, 1 to 6, or >6 years of study), marital status (married and living with spouse or other), occupation type (formal or informal), drinking status (current drinker or not), smoking status (current smoker or not), exercise habits (yes or no), and the number of chronic diseases self-reported (0, 1, or ≥2 of hypertension, diabetes, stroke, heart disease, bronchitis, cataracts, and arthritis).
2.5.Statistical Analyses.Descriptive analysis was performed to summarize the characteristics of the study sample.A chi-square test was conducted to examine differences in indoor ventilation status, depression, and anxiety according to age group, sex, urbanization level, marital status, educational level, occupational status, current drinking status, current smoking status, exercise habits, and the number of chronic diseases.Multiple logistic regression analysis was performed to calculate odds ratios (OR) with its 95% confidence intervals (CI) for depression and anxiety for multiple indoor ventilation statuses.All data were analyzed by using SPSS.Statistical significance was indicated by P < 0 05.

Results
3.1.General Characteristics.Of the 12 096 participants in this study, 259 (2.14%) had anxiety symptoms, and 3277 (27.09%) had depression symptoms.More than half of the participants (58.56%) ventilated their homes 6 to 8 times per week, and 32.62% of the participants ventilated their homes 4 to 5 times per week (Table 1).In total, nearly half of the participants were ≥85 years old, 46.59% of the participants were men, 53.41% were women, 43.85% lived in rural areas, 74.04% were engaged in informal employment, and 44.59% were married and living with their spouse.
The participants were more likely to ventilate their homes if they were young, exercised frequently, had a high educational level, had a formal occupation, lived in an urban area, and had no chronic diseases.Smoking status and alcohol consumption status were not significantly correlated with ventilation frequency.Participants were more likely to have depression symptoms if they were women, had a low educational level (no more than 6 years of education), exercised infrequently, lived in a rural or town area, had an informal occupation, were unmarried, or had at fewest one chronic disease.Differences in anxiety levels across the study population were largely consistent with those in depression levels.

Relationships of Indoor Ventilation Frequency with
Depression and Anxiety.The participants who never ventilated their homes were more probably to have depression or anxiety than those who ventilated their homes at fewest once per week, regardless of the season (P < 0 001; Table 2).
The OR for depression and anxiety according to ventilation frequency in each season are presented in Table 3.

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Indoor Air Indoor Air  Compared with the participants with low indoor ventilation frequency, those with high indoor ventilation frequency had 51% lower odds of depression (OR: 0.49, 95% CI: 0.43 to 0.57), and those with intermediate indoor ventilation frequency had 35% lower odds of depression (OR: 0.65, 95% CI: 0.56 to 0.75) after adjustment for confounders; furthermore, those with high indoor ventilation frequency had 37% lower odds of anxiety (OR: 0.63, 95% CI: 0.43 to 0.91), and those with intermediate indoor ventilation frequency had 45% lower odds of anxiety (OR: 0.55, 95% CI: 0.37 to 0.82).The results were largely similar across the seasons.Compared to participants with low indoor ventilation, there was 28% to 53% lower odds of depression in those with intermediate indoor ventilation and was 43% to 67% in those with high indoor ventilation, regardless of sex, age group, smoking status, and alcohol consumption status.In terms of exercise, compared to low ventilation frequency, moderate ventilation frequency reduced the odds of depression by 38% only among participants who did not exercise and not yet among those who exercised, while at high indoor ventilation frequency, the odds of depression were reduced regardless of whether or not they exercised.
However, although participants who cooked with fuels had 31%-54% lower odds of depression, this negative association between indoor ventilation and depression did not appear in those who never cooked at home (OR: 1.10, 95% CI: 0.31 to 3.93 for intermediate indoor ventilation and OR: 0.88, 95% CI: 0.25 to 3.16 for high indoor ventilation) (Figure 1).

Discussion
This study explored the relationship between frequency of indoor ventilation and depression and anxiety by using a large representative group of older people in China.A relatively high frequency of indoor ventilation shows a significantly negative association with depression and anxiety among older people.Several studies have reported that air pollution caused by CO 2 [25,26], cooking [12], and humidity and mold [27] may be related to depression and anxiety in older people [28,29].Indoor ventilation, in the absence of pollution of the outdoor air and in a better climate, can effectively improve the quality of indoor air and reduce pollutants [30].Natural ventilation alone can reduce 80-90% of indoor particulate matter [31]; opening a 30 cm window and a 10 cm door for half an hour reduces CO 2 to about 1/3 of the concentration and about 1/10 in about an hour, and increasing the number of openings is more effective than increasing the degree of opening fewer windows and doors for the same area [32].Reasonable indoor ventilation may improve mental health by reducing pollutants among older adults.However, our study does not refute the possible reverse effect since our data do not allow us to draw a causal relationship.That is, people with depression may themselves tend to lose control over their environment due to low environmental triggers and are less inclined to open windows and ventilate their homes; thus, not opening windows may be a consequence of their mental state rather than a cause [33].Moreover, it is worth noting that not high concentrations of carbon dioxide may instead be able to activate cognitive functions and have beneficial effects on improving depression [26,34].Therefore, perspective follow-up studies are needed to further characterize the effects of specific substances in indoor air on depression and anxiety.
Notably, our study showed that the negative association between ventilation and depression was observed in older adults who cooked with any fuel but did not appear in those who never cook at home.It might be associated with the fact that the effect of cooking fuels on indoor air quality is nonexistent in the situation of never cooking at home, and therefore, the effect of ventilation on improving indoor air quality is relatively limited and not significant.The pollutants produced by either type of cooking fuel use are associated with depression in adults [3,12,18].Moreover, large numbers of people in developing countries around the world, including China, use biofuels, coal, or kerosene, which are more likely to produce pollutants, to meet their basic energy needs than cleaner cooking facilities [35].Using solid fuels for cooking in poorly ventilated homes can lead to exposure to indoor air pollution and become a major environmental and public health challenge in developing countries [36].Therefore, older people who need to cook at home are prompted to pay more attention to ventilation to reduce the health effects of pollutants from cooking.Moreover, ventilation practices may vary by region, and in cold weather conditions, individuals tend to keep their doors and windows closed; this may lead to a lack of fresh air indoors and thus worsen indoor air quality; therefore, improving the ventilation systems in areas prone to air quality problems, such as kitchen, is very important to promote indoor air circulation [37].
Our study showed that the relationship between ventilation and anxiety differed across subgroups.Specifically, the negative relationship between them was only observed in women, nonsmokers, nondrinkers, and nonexercising older adults.The prevalence of anxiety disorders seems to be higher in women.It may be linked with the fact that women are more susceptible to negative emotions [38]; moreover, testosterone in men may have a protective effect against anxiety [39].In general, smoking and drinking are harmful to health; however, a higher prevalence of anxiety was found in nonsmokers and nondrinkers in the present study, and the negative correlation between ventilation and anxiety was present only in nonsmokers and nondrinkers.In fact, smoking and drinking may also be beneficial for 6 Indoor Air psychiatric symptoms [40,41], and moderate alcohol consumption may also be able to reduce anxiety in older adults [42].Our study also showed that the association between indoor ventilation and anxiety was relatively strong in nonexercising populations.Exercise is often considered to be an anxiety [43] protection, which improves respiratory function and counteracts the effects of low indoor ventilation frequency on depression and anxiety.In summary, individuals with healthy lifestyles are more likely to understand the benefits of good indoor air quality and so are more likely to open their windows frequently [44]  7 Indoor Air can also improve mental health and reduce the incidence of depression and anxiety [45].So more in-depth research is needed to determine the specific relationships.
This study also found that the higher the age of the elderly, the higher the prevalence of depression and anxiety, and the correlation between ventilation and anxiety was also only shown in ≥85 years old and not yet found in other age groups.This may be due to the fact that the relatively younger elderly may be healthier overall in terms of their physical and mental state, whereas as they grow older, their health deteriorates, and the elderly may become anxious because of their aging bodies [46] or depressed because of the loss of friends and peers [47].Moreover, older adults may be afraid of outdoor activities due to illness or other reasons [48], making them spent more time in living indoors.Thus, indoor air pollution may have a greater potential impact on the health for these older adults.Reducing indoor pollutants through ventilation thereby may reduce depression and anxiety, especially for the old elder.Hence, ventilation by frequently opening windows is encouraged for older people

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Indoor Air to improve indoor air quality and promote better mental health.
Our study showed that people living in rural areas had higher prevalence of depression and anxiety compared with those in urban areas; it may be associated with lower ventilation in rural areas.It is noted that urban areas generally have higher densities, taller buildings, and more pollution; ventilation is more important for people's health in urban areas than in rural areas because it is influenced by building density and height [49].However, the natural environment can also have a positive impact on mental conditions such as depression, and simply looking at the greenery outside a window may have mental health benefits [50].Therefore, further research is needed to investigate the potential mechanism about the impact of ventilation by opening windows on older people's health.
This study has several limitations.First, the crosssectional study design failed to capture changes in depression, anxiety, and indoor ventilation and did not provide a direct causal relationship.Thus, further prospective study is required to elucidate the causal relationships of indoor ventilation frequency with depression and anxiety.Second, the study only considered open window ventilation frequency; however, ventilation time, ventilation location, and other types of ventilation (e.g., air conditioning) were not included and need further research; for example, it is needed to explore the impact of different ventilation durations or time periods or specific ventilation location (living room or/and kitchen) on depression and anxiety.Third, pollutants from the outdoor environment may also affect depression and anxiety symptoms in older people [51,52], and ventilation may cause pollutants that were originally outside to enter the house instead.Further study is needed to control for outdoor environmental factor to assess the true association between ventilation and health.

Conclusions
High frequency of indoor ventilation was negatively associated with depression and anxiety regardless of seasons.Our findings suggested that frequent ventilation may be an effective measure to improve mental health in the elderly.Moreover, there are sex, age, and lifestyle-specific differences in the association between ventilation and anxiety, suggesting that more attention should be paid to women, nonsmokers, nonalcohol users, and older adults who do not exercise when making ventilation-targeted interventions or policy to improve older people's anxiety.Also, the negative association between ventilation and depression appears only in older adults who cooked with any fuel rather than those who do not cook, suggesting that older people who need to cook at home are prompted to pay more attention to ventilation to reduce the health effects of pollutants from cooking.

Table 1 :
Indoor ventilation frequency, depression, and anxiety among the participants.

Table 3 :
Likelihood of depression and anxiety according to indoor ventilation frequency.
Multiple logistic regression analysis was applied to estimate the ORs and 95% CIs for depression and anxiety according to indoor ventilation frequency after adjustment for age, sex, urbanization level, educational level, occupational status, marital status, smoking status, drinking status, exercise habits, number of chronic diseases, and fuels.CI: confidence interval; OR: odds ratio.

Table 2 :
Prevalence of depression and anxiety according to seasonal ventilation frequency.
than individuals with unhealthy lifestyles.However, healthy lifestyles themselves Figure 1: Relationships of indoor ventilation frequency with depression disorders stratified by lifestyle, sex, fuel use, and age.Resultsadjusted for age, sex, urbanization level, educational level, occupational status, marital status, smoking status, drinking status, exercise habits, number of chronic diseases, and fuels.CI: confidence interval; OR: odds ratio.* P < 0 05, * * P < 0 01, and * * * P < 0 001.