Potential Pollutants and Indoor Air Quality Variables Association towards Implementation of the Breathing Architecture Concept: A Review

Indoor air quality is essential in improving indoor well-being since most people spend their time there. The breathing Architecture concept promotes a strategy for designing indoor space to maintain the indoor air quality. This concept has various challenges, such as indoor pollutants determining the indoor environment to be healthy and decent. This study aims to identify the relationship between IAQ variables related to Breathing Architecture and air pollutants based on secondary sources of past and existing research ten years ago. The review identified 16 articles collected by selecting several relevant keywords in ScienceDirect and screening the title and abstract. Content analysis was conducted to analyze the relationship between IAQ variables and pollutants and their potential association with the breathing architecture concept. The results show that air exchange rate, volume of spaces, relative humidity, and indoor-outdoor temperature are the most potential variables and have a possible relationship while implementing the BA concept. Using these variables, the optimal condition might be perceived since the possibility of lower pollutant contamination might be lower, and the purpose of breathing architecture might be obtained. However, the relationship between other variables and pollutants requires future studies since it is undescribed or may impact other variables.


Introduction
Nowadays, most people spend their time mostly indoors [1], making it crucial to maintain high Indoor Air Quality (IAQ) to minimize exposure to air pollutants either from outdoor or indoor activity.Poor indoor air quality (IAQ) can cause health problems and reduce work productivity due to air pollutants [2].Several factors that cause the decline of IAQ include tobacco smoke, dampness, mold, or household pollution that produces several pollutant gases [3] [4], and it is exacerbated by insufficient air circulation inside.Otherwise, air pollution reduction will increase life expectancy and well-being [5].Therefore, improving IAQ through sufficient ventilation is essential [6] as it could change indoor air pollutants with fresh air.
Natural ventilation is one of the strategies to maintain IAQ by having lower energy consumption [6].This device induces fresh air into the building using wind and buoyancy force, which arise caused by temperatures and pressure [7].This passive design strategy will be resulting on an adequate IAQ.Various architectural design strategies, such as wind towers, courtyards, etc., are used to control natural ventilation and are effective for improving IAQ and thermal comfort inside buildings [8] [9].This device could decrease CO2 levels [10], decrease energy consumption [11], dilute pollutants [12], and other advantages.Studies about natural ventilation related with social and health aspect [13], various type of devices [14], indoor environment quality [15], and school environment [16], already conducted.However, studies about relation between natural ventilation and IAQ should be elaborated especially in relation of how pollutant relate with IAQ variables.
"Breathing Architecture" (BA) is a concept permitting buildings to breathe, bringing outdoor fresh air to achieve good IAQ and thermal comfort through natural ventilation by inducing oxygen-rich air, removing indoor pollutants, and cooling the building [12].Several studies showed controllable natural ventilation decreases energy consumption [11], decreases CO2 levels [10], and other advantages that of it simultaneously have a similarity with the aims of breathing architecture (BA).Research on the theme of breathing architecture was first published in 2014, marking the first academic report using 'breathing architecture' terminology.Stavridou's [12] research papers in 2011, 2013, and 2015 stated the main parameter of breathing architecture, including (i) inspiration and analogical transfer, (ii) initial conception of the main idea using computational fluid dynamics (digital design), (iii) development of the main idea through an investigatory process toward building form optimization, and (iv) form configuration, shape investigation, and other morphogenetic prospects.This concept mainly relies on natural ventilation to induce outdoor fresh air, enabling buildings to "breathe" and maintain an indoor environment.
The first parameter of BA concept is inspiration and analogical transfer, defined as a source of inspiration, which on the BA concept was air is analyzed through computational or laboratory simulation intended to improve the architectural design.The result of the analysis, which is analogical, was transferred creatively into the form that offers the potential of choice, selection, estimation, evaluation, and creation of elements of each synthesis.In summary, this parameter takes inspiration from the analysis and implies the analogical in creative form.A second parameter, the initial conception of the main idea using computational fluid dynamics (digital design), refers to making an early concept through computational or laboratory analysis, which produces data, digital information, configurations, and shapes useful for the design process.This second process will have several pieces of information, which deliver options for design to the architect and offers a potential idea.Subsequently, the third parameter, the main idea's development through an investigatory process toward building optimization parameters, is a further step of the second parameter that refers to an elimination process of analyzing data.Potential ideas will be selected based on the most efficient natural ventilation performance in this stage.The last parameter, which forms configuration, shape investigation, and other morphogenetic prospects, was described as creating a dimension that harmonized with the laws of nature and improved the efficiency of physical phenomena, such as airflow.
The discussion on the BA concept is still limited since the topic was firstly proposed in 2017 by [12], and it requires further study to elaborate on the information.Additionally, the terminology of BA is still widely interpreted and creates uncertainty.A study by [17] describes "breathe" as a biomimicry strategy to collect water through a façade.Another study by [18] interprets "breathe" as an interaction between the building with the world, which had similarity to [19] 's investigation that "breathing" is not the only connection between the indoors and outdoors air, but also includes other elements surround the building such as climate, landscape, another organism, etc., which as well as the meaning of breathable as permeable boundaries that allows outdoor elements to go through inside the building.Additionally, "breathe" in [20]'s study means allowing the air to ventilate indoors aims to energyefficiency and remove indoor contaminants.Subsequently, [21]'s investigation describes "breathe" as a building device that aims to improve ventilation and insulation performances.However, [22] had a similarity in defining "breathe" with this study, which is an ability to absorb outdoor air through the facade by wind characteristics.The definition of "breathing" in this study, retrieved from [12]'s investigation and similar to the meaning from [22], is a strategy following air characteristics via computational and laboratory simulation to achieve the most effective way to induce outdoor air via natural ventilation.Consequently, computational or laboratory analysis is necessary to apply this concept in the design process.
Several studies about using computational analysis as a tool in increasing IAQ, such as for system validation [23][24] [25] and strategy decision [26][27] [28], have been conducted.Another research that had similarities with the aims of the BA concept, which used CFD as an analysis tool, has been established.The study conducted using CFD analysis through based back propagation neural network (BPNN) collaborated with a particle swarm optimizer (PSO) known had an effective and intelligent control strategy for ventilating indoor environments [29].This strategy could reduce air pollutants by up to 6.44% and computational costs by as much as 23.53%.Another investigation used CFD as an analysis tool to predict evacuation models of toxic gas leakage accidents [30].This study uses gas leakage and dispersion characteristics through CFD to examine several evacuation strategies, similar to the BA concept that uses air characteristics to obtain data for decision-making of the design process.However, investigations specifically about the review of pollutants that have opportunity while implementing the BA concept are still limited.
This study aims to discuss several studies that showing the association between pollutants and natural ventilation variables.Subsequently, the potential variables of IAQ that related with breathing architecture will be determined as aids creating optimal condition by breathing architecture concept in the indoor environment.

Method
This study aims to identify relationship between variables of IAQ that related to Breathing Architecture and air pollutants through research review.This literature review uses a content analysis method to collect and categorize research papers to get the most relevant articles.The first stage of this review is by collecting research articles from the decade 2012 -2022 collected from ScienceDirect with following keywords: a. natural ventilation parameters b. breathing architecture c. indoor air quality parameters These keywords show the result about 128.152 studies.Afterwards, these studies were sorted into the most recent and the most related with BA and IAQ variables using "relevance" tools by Sciencedirect.Afterward, the title and the abstract were screened to get the most relatable studies, and 16 articles were selected.

Figure 1. The stages of the review
Subsequently, the data will be classified based on the pollutants and their relation with several variables related to indoor air quality.Additionally, the relation will be determined whether these have a negative or positive relationship.Lastly, the association between the result of the classification of variables and the breathing architecture concept will be determined as preliminary findings to develop this concept.
The opening of building had an impact of removing indoor CO2 concentrations [37].Opening time and opening ratio had a negative relationship with the improvement of O2 level and dilution of CO2 concentration [38][39][40] [41].The [42] and [43] studies show that ventilation rate and wind speed had a significant negative correlation with CO2.The space volume had a negative correlation with the CO2 concentration, which higher volume of space increase pollutant dilution [44].
The [39] study shows that occupant density increase CO2 levels which found when the windows are closed and there is low air permeability of schools.The number of occupants is known as a source of CO2 which creating positive association with the CO2 [44].Higher air exchange rate due to opened windows resulting in lower of CO2 levels.Higher air exchange rate had a connection in the improvement of subjective indoor air quality by occupant [45].Subsequently, air exchange rate had a significant influence of indoor CO2 pollutant from the outdoor [44].Additionally, Positive association were found between temperature and indoor CO2 concentration, and positive correlation were found between relative humidity and CO [43], which indicated by high air exchange rate [44].The temperature might had a relationship with solar irradiation since low solar irradiation decrease CO2 levels [46].However, another study shows contrary result that lower airtight of indoors resulting in higher CO2 concentration, which also had indicate had a connection with the installment of mechanical ventilation [47].Additionally, the [47]'s study emerge that several research discussing about the airtightness of the building show negative association with the CO2 levels, while the rest show there is no clear relationship.Nevertheless, there are possibility in higher exposure of particles generated indoors [48].
The low opening time and opening ratio in some cases related to the low level of the outdoor temperature in some seasons, with aims to increase indoor thermal comfort [49].The low outdoor temperature (less than 10 o C) will make the occupant less frequently open the windows, and the frequency of opening the windows will be increased when the outdoor temperature exceeds 10 o C [40].The occupant activity also had a relation in producing CO2, such as from the combustion of heating or any regular activity [43][41] [44].However, outdoor sources such as vehicles should be considered since there is a connection between traffic and indoor CO [43] [44].The length of the nearby roads also had an inverse relationship with CO levels [43].

Variables
Correlation with COx Sources Wind speed Negative [45], [38], [39], [49], [43], [40], [37], [47], [41], [44], [46], and [42] Spaces The outdoor PM level influences indoor PM concentration, which will be advantageous if the outdoor level is lower than the indoor and will be detrimental if the outdoor level is higher than indoor [42].This detrimental effect happen in other studies showing that longer opening times worsened indoor air by increasing PM levels, this due to high concentration of outdoor PM levels [39] [48].Another source of the PM concentration is activity of occupant [39][43] [50], such as cooking, smoking, or any combustion activity, which exacerbated by the increasement of occupant density [44].However, increasement of cleaning activity might resulting in the lower PM concentration due to less pollutant deposition [44].These conflicting results should be considered while investigating the IAQ using PM as a parameter since the result depend on the source location, either it is indoor or outdoor.
These contradictory result of PM sources create contrary effect in reducing PM concentrations.The [47]'s study shows increasement of airtightness resulting reduction in the PM I/O ratios, which this relationship depends on the outdoor climate.Other study shows that keeping the windows close could maintain indoor PM2.5 at low level with help of air purifier [49].Negative relationship were found between PM2.5 levels and wind speed due to pollutant dilution from the outdoor air [43].Hence, air exchange rate become important variable affecting indoor PM levels [44], especially when the PM source is from indoors.Increasing the space volume could reduce the PM2.5 concentration [44].
Temperature showing had a positive association with the PM2.5 levels [43].Other study found a significant association between relative humidity and the PM levels [44].The [42]'s study shows that PM10 and PM2.5 negatively associated with temperature and humidity.These both results about relationship of temperature and PM concentration should have future investigation since it has contrary result.Additionally, humidity on PM formation were insignificant since the humidity levels not reaching the range of influence on PM concentration.

Variables
Correlation with PM Sources Volume of spaces Negative [39], [41], [43], [44], [47], [48], [49], and [ The [49]'s study shows that airtightness, indoor temperature, furniture surface area per unit volume, and humidity had a positive correlation with amount of TVOC.Another study shows that an increasement of relative humidity followed by rising of VOCs [41].The window-opening duration shows positive correlation that had no significant impact, since the effect covered by the effect of increasing of indoor temperature [49].Longer opening time decrease amount of VOCs [39].VOC concentration will be high in low airtight condition [47].TVOC had a relationship with the building materials and air exchange rate.However, it requires future investigation to define the relationship with VOCs levels [50].

Variables
Correlation with VOCs Sources Activity Positive [41], [45], [49], and [ The [39]'s investigation shows that longer opening time decrease amount of NOx.Another contrary study shows that lower concentration of NO2 had a connection with the lower penetration from the building envelope [43].This might due to outdoor pollutant levels that influence the indoor NOx concentrations.This indication might be related with the positive association between traffic volume and NO2 exposure [43].The airtight condition will be useful in areas with high concentration of NOx [47].The [49] study shows that HCHO or formaldehyde levels increase in airtight conditions, which worsens by the increasing outdoor temperature.Contrary, other study shows that airtight conditions resulting in low HCHO levels [47].However, this study requires future investigation since there is insufficient information about the relationship with the airtightness.The window opening duration also has a negative correlation with HCHO levels [49].Window opening helps reduce HCHO concentration, which leads to increasing outdoor temperature and occupant tent opening the window to perceive thermal comfort.Subsequently, relative humidity and indoor temperature negatively correlated with the HCHO levels.Due to an increase in the indoor temperature, this will be followed by a window-opening activity, which results in a decrease in HCHO concentration.The indoor materials also affect indoor HCHO levels, or due to increasement of indoor temperature and humidity create HCHO levels rise [50].The PM with IAQ variables relation that show a connection with the BA concept are air exchange rate, temperature, volume of space and relative humidity.The window opening and wind speed are not included since it is part of the air exchange rate.Other variables with no direct correlation were omitted, including occupant density, length of the nearby road, occupant density, occupant activity, airtightness, and outdoor air pollutant level.The relationship between VOCs and IAQ variables, which correlated with the BA concept, shows that indoor temperature, outdoor temperature, indoor humidity, and air exchange rate.Occupant activity and furniture surface area per unit volume were not included since both variables have no direct connection with the BA concept.

Figure 5. The relationship diagram of VOCs and IAQ variables
The air exchange rate is a single variable related to the BA concept and NOx.This variable was found because of the relation between opening time and infiltration variables.However, the other variable, the indoor-outdoor ratio, is not accounted for because it has no direct relationship with the BA concept.The IAQ variables still related to the HCHO and BA concept are air exchange rate, relative humidity, and temperature.Furniture surface area per unit volume was not accounted for because there is no direct relationship with the BA concept.

Figure 7. The relationship diagram of HCHO and IAQ variables
Four variables, including air exchange rate, volume of spaces, relative humidity, and indooroutdoor temperature, are categorized as variables related to breathing architecture.These variables could maintain indoor air quality through controlling based on each relationship.The high air exchange rate and large space volume will effectively remove indoor air pollutants.On the contrary, increasing relative humidity and temperature majorly increase indoor air pollutant levels.However, this condition is still affected by external variables such as material types, occupant activity, outdoor pollutants level, etc.This condition leads to future investigation towards other variables.

Conclusion
This systematic review identified 16 studies related to air pollutants and IAQ variables due to find the relationship of pollutants and indoor air quality variables that might affect in the implementation of Breathing Architecture concept after sorting from the most recent and relevant through relevance tools from the sciencedirect website.Variables that should be considered while designing spaces using BA concept including air exchange rate, volume of spaces, relative humidity, and indoor-outdoor temperature.Through using these variables, optimal condition might be perceived since the possibility of lower pollutant contamination might be lower and the purpose of breathing architecture might be acquired.Other variables and pollutants relationship required future studies since it is undescribed of its relation.Another gap that should be investigated are environment factors such as outdoor air pollutant level, which might give significant result of each condition.

Figure 2 .
Figure 2. Flow of literature review

3. 2
Relationship between IAQ Variables, Indoor Pollutants, and Breathing Architecture ConceptThe quality of BA concept implementation will be determined through IAQ variables and pollutants since the BA concept relies on natural ventilation as a breathing device.The related IAQ variables with COx still relevant to the BA concept are relative humidity, temperature, volume of space, and air exchange rate.Wind speed, ventilation rate, opening time, and opening ratio are part of the air exchange rate since these variables are components of air exchange rate calculation.The other variables, including solar irradiation, length of the nearby roads, occupant density, outdoor air pollutant level, and airtightness, were not included since these variables have no direct relationship with natural ventilation in terms of the BA concept.This is because the relationship between the BA concept and the outdoor environment is ignored.

Figure 3 .
Figure 3.The relationship diagram of COx and IAQ variables

Figure 4 .
Figure 4.The relationship diagram of PM and IAQ variables

Figure 6 .
Figure 6.The relationship diagram of NOx and IAQ variables

Figure 8 .
Figure 8.The relationship diagram of BA concept, pollutants, and IAQ variables

Table 4 .
Correlation between variables and NOx

Table 5 .
Correlation between variables and HCHO