1. Introduction
This study investigates the actual indoor air quality (IAQ) in Japanese houses by measuring carbon dioxide () concentrations and using questionnaire surveys.
Modern people spend most of their day indoors [
1,
2]. The IAQ in residential buildings is often poor, polluted by chemical substances emitted from heating appliances, building materials, or other household goods. Bolstered by the increase in highly airtight houses, there have been rising concerns about the increased health risks to occupants due to exposure to indoor pollutants such as
, carbon monoxide (CO), nitrogen oxides (
), and volatile organic compounds (VOCs). Despite this concern, it has proven difficult for occupants to recognize air pollution, thus preventing them from understanding the actual IAQ conditions in their homes.
Ventilation is generally recommended to prevent IAQ deterioration. In Japan, the Building Standard Law was amended to require the installation of mechanical ventilation systems in all houses built after July 2003; however, this law only mandated the installation of ventilation equipment, and it is left to occupants to ensure that they are properly ventilating their homes. In addition, houses built before July 2003 were not considered for this mandatory ventilation installation. In Japan, roughly 26% of houses were estimated to have been built since 2003 relative to the total number of houses as of 2018 [
3,
4]. Multiple studies have reported that the amount of ventilation obtained by activities like opening windows decreases in the winter season [
5,
6], and there are concerns about the deterioration of air quality in highly airtight houses in recent years.
Another factor that causes air indoor pollution is the use of combustion heaters [
7,
8,
9,
10,
11]. Combustion heating is recognized as a major source of
, sulfur dioxide (
), and
emissions [
8,
10,
12], and exposure to these substances can cause sick building syndrome [
13,
14]. Gas and kerosene heaters still account for a large percentage of heating devices used in Japan [
15,
16,
17], and the use of combustion heating has been reported to cause
concentrations to reach 4000–5000 ppm [
9,
10,
11].
The
concentration is an indicator of the retention of harmful substances, as its concentration increases with inadequate ventilation in buildings [
18,
19,
20,
21,
22]. Many countries have regulatory standards and guidelines to maintain
concentrations below a fixed level (around 1000 to 1500 ppm) within buildings such as offices, schools, and houses [
18,
22,
23]. The
concentration also increases with human breathing; in addition to being an indicator of the retention of harmful substances, high concentrations are also known to increase the risk of respiratory diseases, impair thinking, and lower concentration [
24,
25,
26,
27]. It has also been reported that high concentrations of
can affect the quality of sleep as well as overall performance the next day [
28].
From the studies mentioned above, it can be gathered that there are two major factors that lead to air pollution in houses: inadequate ventilation and the use of combustion heaters. It is also worth considering the influence of outdoor temperature, because several studies suggest that seasonal changes affect ventilation rates [
5,
6,
29,
30]. Another important aspect is the difficulty for occupants to recognize the deterioration of IAQ. If occupants are not aware of the conditions in their immediate environment, it will be difficult for them to take appropriate actions to maintain a healthy IAQ.
Existing studies on the direct measurement of ventilation rate involve fan pressurization [
5,
31,
32,
33] and tracer gas [
6,
13,
29,
30], which are extremely intrusive and require expensive instruments and experts. Non-intrusive approaches that can be measured easily and at a low cost are required for widely adopted residential IAQ monitoring.
concentration is therefore focused on because it is utilized as an indicator of IAQ and can be measured easily and at low cost.
In this study, a -based investigation was conducted on the actual conditions of IAQ in Japanese houses, focusing on the following three aspects:
- (i)
The relationship between combustion heating and indoor concentration.
- (ii)
The influence of outdoor temperature on indoor concentration.
- (iii)
The gap between occupants’ awareness of ventilation and the actual IAQ in their homes.
A sensor kit, which is inexpensive and easy to handle, was developed in this study. The concentration in houses was measured from autumn to winter, targeting houses with different housing attributes and heating methods. A questionnaire on occupants’ knowledge and awareness of indoor ventilation was also administered. Based on the results of these investigations, the influence of combustion heating, outdoor temperature, and occupants’ attitudes on the residential IAQ are discussed hereafter.
5. Discussion
The state of IAQ was investigated through the measurement of concentrations in houses and questionnaire surveys, and these findings are discussed in this section.
5.1. IAQ Pollution from Combustion Heating
The use of oil- and gas-fired heating appliances was found to cause significant increases in
concentrations, leading to indoor air pollution with concentrations ranging from 3000–5000 ppm. This is similar to the results of the existing studies described in
Section 2; such an environment is both uncomfortable and raises concerns about health effects due to long-term exposure.
In contrast, the concentration was relatively lower in houses with multiple heating appliances, such as air conditioners, than in houses with only combustion heating, suggesting the possibility of reducing air pollution by using multiple heating methods. Although it is necessary to consider other factors, such as the airtightness of the house and the amount of ventilation, this result implies that the combined use of air conditioners and other devices might lead to IAQ improvement.
All of the existing measurement case studies were conducted only for the use of combustion heating, and the results described in this paper are findings obtained by targeting houses with various heating methods.
5.2. Influence of Outdoor Temperature on Ventilation
As shown in
Figure 4 and
Figure 6, the
concentration tends to increase with decreasing outdoor temperature in many houses. This result can be attributed to a decrease in the frequency of ventilation, including window opening, in order to avoid a drop in room temperature due to air exchange. This is supported by the results of the questionnaire responses described in
Section 4.2 and by the existing study [
48].
Various pollutants such as CO,
,
, and VOCs are likely to be retained in houses with high
concentrations and insufficient ventilation, which may cause Sick Building Syndrome and respiratory diseases caused by long-term exposure [
13,
14,
36,
37,
38,
39]. In addition, even when considering the effects of
alone, high concentrations have negative effects, such as reducing the ability to think, concentrate, and sleep [
26,
27,
28].
Changes in the concentration are difficult for occupants to perceive. It is necessary to implement methods to maintain a clean IAQ without relying on human senses, such as constantly measuring the concentration and encouraging ventilation at appropriate times.
5.3. Gap between Occupants’ Awareness of Ventilation and the Actual IAQ
The results of the measurements in houses and the questionnaire survey conducted in this study showed that the actual IAQ does not match occupants’ awareness of ventilation. This could be attributed to the inability of occupants to perceive air pollution, as well as a lack of knowledge about the ventilation system of the house and the effects of combustion heating as barriers.
On the other hand, occupants are highly interested in ventilation in their homes, and there is considerable room for improvement in residential IAQ. If appropriate solutions for improvement can be presented to occupants based on the findings of this study, they will be able to understand the actual conditions in their homes, and can be expected to change their behavior to achieve a healthy and comfortable air environment.
6. Conclusions
In this study, the true condition of residential IAQ was investigated by measuring the concentration and a questionnaire survey. From the results obtained, the relationship between factors such as the heating method, outdoor temperature, and air pollution was clarified. The main findings and suggestions obtained in this study are as follows:
The concentration increased significantly with the use of combustion heating equipment, resulting in air pollution in the house. This air pollution could be reduced by using non-combustion heating methods such as air conditioners.
In many houses, mainly apartment complexes, it was found that the concentration tends to increase as the outdoor temperature decreases. This result is presumably due to the fact that the frequency of ventilation, such as window opening, decreases in order to avoid a decrease in indoor temperature in the winter season, when the outdoor temperature decreases.
The actual IAQ in each house does not match occupants’ awareness of ventilation. In addition to the occupants’ difficulty in perceiving air pollution, the lack of knowledge about the ventilation system and the effects of combustion heating might be additional barrier.
Based on these results, methods to improve residential IAQ should be studied in future work. Specifically in addition to the concentration, substances such as CO, , and VOCs should be measured to clarify the relationship between each substance, and to investigate which behaviors lead to deterioration of the IAQ. Methods to improve the IAQ can subsequently be developed through technologies such as information provision and automation systems. It is important to utilize these methods to raise awareness of IAQ and to inform occupants about the harmful effects of pollutants. Since ventilation causes a decrease in heating efficiency, energy conservation in heating use is also an important issue. Future studies should also include methods to achieve both healthy IAQ and high energy efficiency.