A Systematic Review of Air Quality Sensors, Guidelines, and Measurement Studies for Indoor Air Quality Management

The existence of indoor air pollutants—such as ozone, carbon monoxide, carbon dioxide, sulfur dioxide, nitrogen dioxide, particulate matter, and total volatile organic compounds—is evidently a critical issue for human health. Over the past decade, various international agencies have continually refined and updated the quantitative air quality guidelines and standards in order to meet the requirements for indoor air quality management. This paper first provides a systematic review of the existing air quality guidelines and standards implemented by different agencies, which include the Ambient Air Quality Standards (NAAQS); the World Health Organization (WHO); the Occupational Safety and Health Administration (OSHA); the American Conference of Governmental Industrial Hygienists (ACGIH); the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE); the National Institute for Occupational Safety and Health (NIOSH); and the California ambient air quality standards (CAAQS). It then adds to this by providing a state-of-art review of the existing low-cost air quality sensor (LCAQS) technologies, and analyzes the corresponding specifications, such as the typical detection range, measurement tolerance or repeatability, data resolution, response time, supply current, and market price. Finally, it briefly reviews a sequence (array) of field measurement studies, which focuses on the technical measurement characteristics and their data analysis approaches.


Introduction
The WHO reported that poor air quality caused 4.2 million deaths in 2016, of which, primarily, 17% were due to strokes, 25% were due to COPD, and 26% were due to respiratory disease [1]. It is evident from many studies that the concentration levels of indoor air pollutants are two to four times higher than those of outdoor air pollutants [2][3][4][5]. In the U.S., on average, people spend 22.25 h per day inside buildings, and 1.44 h in cars or other transportation modes [6,7]. With higher concentrations of pollutants inside buildings, IAQ is one of the world's highest environmental health risks [8,9], which cannot be ignored.
The impact on human health owing to the indoor environment is, broadly speaking, either BRI or SBS. BRI relates to symptoms that are clinically defined, which are diagnosed with directly airborne building contaminants [5][6][7][8]. On the other hand, SBS is a collection of symptoms for which the cause is unclear [10][11][12]. It is to be noted that SBS is a consequence of poor indoor air quality [13]. Besides this, the symptoms caused by psychological illnesses-such as headaches, fatigue, nausea, hyperventilation, and fainting-are referred to as Mass Psychogenic Illness (MPI) [14]. Building-associated illnesses A 2015 report showed that air pollution does not affect everyone in the same way; certain vulnerable populations (e.g., children, the elderly, and cardiopulmonary patients, etc.) are more susceptible than others [33]. The US EPA defined the 'risk population' as being those who possess a significantly higher probability of developing a condition, illness or other abnormal status, and divided them into five groups, namely: (1) children aged less than or equal to 13 years; (2) older people aged greater or equal to 65 years; (3) a young person with asthma, who is less or equal than the age of 18 years; (4) legal adults with asthma; (5) people with COPD [34]. Children and older people are more sensitive than others with regards to indoor air pollution [35][36][37][38][39]. While the immune and metabolic systems of children are still developing, and their organs are immature, they are exposed to air pollutants due to which they suffer from frequent respiratory infections [40,41]. Older people are affected by IAQ due to weaker immune systems, undiagnosed respiratory conditions, and cardiovascular health conditions. A hazardous substance can aggravate heart diseases, strokes, and lung diseases such as chronic bronchitis and asthma [42,43].
Asthma is a chronic disease that often causes an exacerbation of disease activity, some of which result in hospitalizations. Air quality measures-such as PM 2.5 , NO 2 , O 3 , and dampness-related contaminants-play a significant role in asthma exacerbation, as well as disease progression. Asthmatic children spend 60% of their waking hours in school. A recent large-scale study [44] showed that co-exposure to elevated endotoxin levels and PM 2.5 was synergistically correlated with increased emergency room visits, especially for asthma among children. Exposure to higher concentrations of endotoxin and NO 2 was also synergistically associated with increased asthma attacks, despite below-normal geometric mean concentrations of PM 2.5 , O 3 and NO 2 compared to EPA NAAQ standards [44,45]. A 2015 update to the 2000 review of the Institute of Medicine [46] suggested that-in addition to endotoxin levels-dampness, and dampness-related agents are also important environmental quality indicators for asthma. variability, coagulation, and cardiac autonomic function" [64]. PM is estimated to cause of 3.3 million deaths per year worldwide [65]. Children, the elderly, and people with heart and lung disease are the high-risk populations for PM pollutants [50].
VOCs represent a diverse set of hazardous organic chemicals that participate in atmospheric photochemical reactions, which are considered to be one of the major contributors to SBS [6,66,67]. The WHO classifies both indoor and outdoor VOCs as Very-VOCs (VVOCs), VOCs, and Semi-VOCs (SVOCs) according to their boiling points [68]. Many studies have shown that the concentrations of many indoor VOCs were markedly higher than their outdoor counterparts [69][70][71]. The main indoor VOC sources include high-emission building materials, furnishings, aerosol sprays, pesticides, dry knitted products, office equipment such as copiers, and laser printers [6,67,70,72]. The US EPA issued a list of hazardous air pollutants, which include a total of 187 VOCs [73]. In addition, the ANSI/ASHRAE 62.1-2016 standard provides the Reference Exposure Levels (RELs) of 32 specific types of indoor VOCs for the general population [74]. The most common indoor VOCs-such as benzene, ethylene, formaldehyde, methylene chloride, tetrachloroethylene, toluene, xylene, and 1,3-butadiene-have been proven to be contributors of human carcinogens, irritants and toxicants [75][76][77]. TVOCs are used as a measure of the total volume of indoor VOC concentrations [78,79]. "Acute exposure to indoor TVOCs can cause eye, nose and throat irritation, headaches, loss of coordination and nausea, damage to the liver, kidney and central nervous system, respiratory disease and some cause cancer" [67]. Asthmatics, young children, and elderly people are more vulnerable to the effects of exposure to TVOCs [6,77,79,80].
In addition to common air pollutants, the indoor temperature and relative humidity significantly affect IAQ. Fang et al. (1998) found out the overt linear correlation between the acceptability and enthalpy of IAQ. The results also identified that, under a constant pollution level, IAQ would decline with the increase of temperature and relative humidity [81]. Berglund and Cain (1989) concluded that the temperature's effect on IAQ was linear and stronger than humidity; the effect of the relative humidity on the acceptability of IAQ was higher in the dew point range of 11-20 • C than in the range of 2-11 • C, and relative humidity under 50% was acceptable to the IAQ performance [82].

Air Quality Sensors, Measurement Tolerances, and Ranges
In recent years, LCAQS technology has emerged from several laboratories for practical application, as they can be used to support real-time, spatial, and temporal data resolution for the monitoring of air concentration levels [83][84][85]. Additionally, more and more companies provide their own LCAQS products. The principles of operation for the low-cost gas-phase sensors are typically based on five major components, which are OPC, MOS, EC, NDIR, and PID [86,87]. Studies have shown that modern LCAQS provide useful qualitative information for scientific research, as well as for end-users [85,88,89]. However, due to the embedded technical uncertainties and lack of cross-validation and verification, there are certain limitations when comparing them to the expensive conventional equipment [87,[90][91][92]. The US EPA has colloquially identified such devices to be low cost when their costs are less than US $2500, because this is often the limit when they are considered for capital investment by scientists and end-users [83]. The price includes the sensor module, its networks, the interactive platform, and other supply services. Therefore, hereafter, we assert that LCAQs should be less than US $500. Table 1 summarizes a series of commercially available LCAQs for primary air pollutants, such as O 3 , CO, CO 2 , SO 2 , NO 2 , PM, TVOCs. Furthermore, the specifications from the datasheet provided by the sensor companies-such as the repeatability, measuring range, circuit voltage, and response times-have been listed. The price of these LCAQS ranges between US $1 and $500, and they are capable of detecting an acceptable range of concentrations of each pollutant identified by the existing guidelines (See Table 2).    Table 2 presents a series of common air quality guidelines and standards for industrial and non-industrial environments. The majority of these guidelines are being improved constantly by implementing different criteria and procedures. The ambient air quality standards set by NAAQS and CAAQs are used for outdoor environments, and those set by OSHA, NIOSH, and ACGIH are used for industrial environments. The guidelines set by ASHRAE are designed for indoor environments, especially where building HVAC systems are used, and the WHO air quality standards are designed for the general environment. The following are the descriptions of these individual guidelines, which can provide criteria information for the decision-maker in adopting these values.

Air Quality Guidelines
The NAAQS (40 CFR part 50) are the criteria for the air pollutant standards enforced by the US EPA under the authority of the Clean Air Act (42 U.S.C.) [164,165]. The purpose of the primary standards of the NAAQS (2016) is to determine the acceptable range of seven principal pollutants (CO, NO 2 , Ozone, PM 2.5 , PM 10 , Lead, and SO 2 ) for public health protection, including the high-risk populations [164,166]. In 2019, up to 1131 counties in the US published their ambient air quality data under the NAAQs in the national platform [167]. Multiple studies indicate that NAAQS are applicable to outdoor conditions, rather than indoors, due to the technical difficulties and specific properties of indoor pollutant concentrations [166,167,170].
In 2006, the WHO published an air quality guideline, which was a global update edition based on the previous versions (WHO/Europe, 1987 and 2000) [164,165]. This guideline targeted five specific pollutants (NO 2 , Ozone, PM 2.5 , PM 10 , and SO 2 ) for application to the general environment [167, 173,175]. In 2010, the WHO's regional office in Europe released the book 'The Guidelines for Indoor Air Quality: Selected Pollutants', according to a review of the overall WHO guidelines and the related indoor air quality studies [176]. The book provided threshold concentrations of selected indoor pollutants, such as CO, NO 2 , benzene, formaldehyde, naphthalene, radon, and polycyclic aromatic hydrocarbons. However, a few of biases and limitations of the current WHO air quality guidelines were retained [177][178][179]. The meeting of the WHO Expert Consultation (2016) recommended a systematic re-evaluation of the health-related evidence, the interactions among pollutants, and the risk assessment of the biases, which are required to be performed for the new version of the WHO air pollutants guideline, which is expected to be published in 2020 (WHO, 2020) [178,180].
The ANSI/ASHRAE 62.1 and 62.2 standards of ventilation for acceptable indoor air quality are a non-judicial enforcement established by ASHRAE in 1973 [181]. The 2016 version of ANSI/ASHRAE 62.1 include contaminant concentration targets for ten types of indoor pollutants: CO, NO 2 , SO 2 , Ozone, PM 2.5 , PM 10 , Odors, Radon, Lead, and TVOCs [74,[181][182][183]. The new version of the ANSI/ASHRAE 62.1-2019 standards puts more emphasis on the consideration of the interaction of the outdoor air quality with the HVAC system. Meanwhile, it prohibits any air-cleaning equipment that generates ozone [178,180,184].
The NIOSH is the federal agency under the US CDC [173]. NIOSH and the US EPA have worked jointly on the guidance for the development, evaluation, and validation of the protocols for indoor air quality sampling since the early nineties [179]. NIOSH recommended a non-enforcement guideline for industrial environments, which includes Maximum Exposure Limits (MEL) for CO, NO 2 , SO 2 , ozone, lead, and formaldehyde [74,173,179]. These are based on industry and workplace settings, and are not applicable to the high-risk populations [174].
The OSHA is a national public health agency which is separate from the U.S. DOL [180]. The OSHA developed enforceable guidelines for maximum exposure limits, which currently contain over 600 types of hazardous substances; some of these were adopted by the NIOSH and ACGIH [181,185,186]. The OSHA Permissible Exposure Limits (PELs), which were primarily designed for commercial and institutional buildings, have not been updated since 1970 [169,180,181]. Therefore, the OHSA and its related organizations recommend that employers and participants consider referencing the alternative guidelines for the uncovered scenarios, and OSHA PELs are not suggested to protect the high-risk populations [74,169,173].
The ACGIH TLVs ® Committee has provided maximum permissible exposures for industrial workplaces since 1962 [172,187]. The current TLVs ® guidelines (ACGIH, 2019) include more than 700 chemical substances [172]. The ACGIH's TLVs ® developed time-weighted average concentration limits both for periods of 15 min (short-term) and for 8 h workdays (40-h a week) [187]. The ACGIH air quality guidelines are unenforced in the United States; they are intended to protect industrial workers, and should not be applied for sensitive or high-risk populations [181,187,188].
The CAAQS is part of the regional Air Quality Management Plans (AQMPs) developed by the CARB, and they have been updated jointly with the SCAQMD and the U.S. EPA [189]. According to the 2016 AQMP review (2016), the design value of seven principle pollutants (ozone, CO, NO 2 , SO 2 , PM 2.5 , PM 10 , and lead) and additional three VOCs (SO 4 2− , H 2 S, and C 2 H 3 Cl) are set by CAAQS, which are enacted in a manner that is often more stringent than the NAAQS [190][191][192]. Under the authority of the Clean Air Act (CAA), the CAAQS were established to prevent adverse health and welfare effects for high-risk populations, but currently, the values are not enforceable [174,[192][193][194].

Air Quality Measurements and Data Analysis
In recent years, the field measurement study of indoor and outdoor air quality has accelerated, and now includes numerous monitoring strategies. In Tables 3-5, we summarize studies that analyzed critical factors regarding the assessment of both indoor and outdoor air quality for occupant satisfaction. A total of 33 original papers, published from 2015 to 2019, are included for this narrative review; among these, 13 measurement studies were conducted in school buildings, six were focused on residential buildings, and 14 focused on other types of building (offices, hospitals, shopping malls, museums, metro stations, etc.). As the table presents, PM 2.5 , PM 10 , CO 2 , VOCs, CO, ozone, NO 2 , and SO 2 are the commonly measured pollutants across the studies. Tables 3-5 contains the list of the studies, in which most of them analyzed the correlation between indoor and outdoor concentrations, as well as the I/O ratio. They indicate that LQAS is rapidly being applied in practical applications and air quality research, but conventional and expensive quality monitors are still the mainstream equipment that is applied to IAQ research. Additionally, studies have been conducted using various equipment in different environments, and most choose their respective sampling protocols along with the approach of analyzing the output data. This shows that there is a lack of a uniform method for data quality and uncertainties control. Few of these studies considered the multicollinearity and cross-sensitivity between each of the sensors. The literature search was carried out based on the electronic databases Web of Science and Science Direct, using the keywords "Indoor air quality", "Indoor and outdoor concentrations", and "Field monitoring", and "Field measurement".         The mechanical ventilation effectively control the TVOCs and CO 2 regardless of occupant load; natural ventilation difficult to achieve standard flow rate; Ventilation scheduling significantly impact on indoor gas concentrations; The ventilation system should work before occupants arrival and shutdown after room empty and the IAQ reach the standard level;

Analysis of the Sources and Mechanisms Affecting the Concentration Measurements
The identification of the determinant factors and mechanisms affecting the indoor air quality relies on data analysis techniques and quantifiable data, such as the time series concentrations collected from the monitoring equipment, potential building defects, ventilation specifications, and sometimes local meteorological data, occupancy activities, traffic volumes and other information [228]. Descriptive statistics with trends and graphic analysis are commonly used in observational studies. They provide summaries of the initial air quality measures by describing the data's central tendency, dispersion, variability, outliers, typos, and ranges, and the time-weighted average of the concentration levels [229][230][231][232][233]. Correlation analysis is often used for the evaluation of the association of the indoor and outdoor concentrations, as well as other related time-series data [17,220,234,235]. Typically, the linear relationship between two types of air pollutants is obtained by conducting parametric tests, i.e., t-tests, ANOVA, and Pearson correlations [219,220]. For the non-parametric studies, Spearman's correlation test has often been applied in order to examine the monotonic relationship between ordinal and binary variables, such as age, sex, health performance, and the degree of building-related defects [195,210,212,217,223]. When dealing with the observational data, which are non-normally distributed, non-parametric tests-such as Mann-Whitney-Wilcoxon and Kruskal-Wallis-can be used to evaluate the difference between the average of the measured exposure variables and ordinal variables [195,202,219,236]. On the other hand, earlier field studies have found significant multicollinearity problems and temporal cross-correlations between the measured ambient air pollutants and the related influence factors [62,[237][238][239][240]. Very few studies, however, also considered the complex and nonlinear characters of indoor air pollutants [200,221,227]. Kwon et al. [227], using the principal component analysis (PCA) and self-organizing map (SOM) techniques, determined the dominant factors which increase indoor PM concentration by reducing the original set of inter-correlated variables and transforming them into principle component groups that are mutually orthogonal, or uncorrelated. Madureira et al. [200] and He et al. [221], mitigated the multicollinearity problems between the measured IAQ (CO 2 , PM 2.5 , PM 10 , and VOCs), building characteristics, and occupant activities by conducting categorical PCA (dimensionality reduction method) with a varimax rotation approach [200]. Furthermore, the mixed-effect linear regression model with random intercept provides a flexible approach to assess the association between time-series concentrations and building-related categorical variables in field measurement studies. [213,227].

Air Quality Guidelines
At present, there are several guidelines available around the world to prevent IAQ issues for different kinds of management decisions and planning processes. In most of developed or developing countries, they have and follow their respective local guidelines. The main air quality guidelines-which were reviewed in Section 4-are constantly being updated for more precise results in order to protect the target population. In spite of these efforts, the values of the guidelines are still different among each other due to many factors, such as the difference in the standard operating procedures, enforcement levels, and different design principles. Furthermore, there are various misconceptions about the interpretation of these values and guideline principles, which lead to misquotations by researchers and decision-makers. Most of the values which are represented in Table 3 are currently unenforceable because of the limited data availability, challenging deployment, and non-scalability of conventional air pollution tools such as FRM/FEM instruments. This situation is more prominent in indoor environment-related guidelines. There is also a lack of clear evidence on the exposure relevance of a different range of certain concentration values for the improvement of these guidelines, especially for the high-risk population. Log-term cluster randomized control trials and joint health impact assessment should be investigated for the development of future air quality standards.

Air Quality Sensors
In this sub-section, we discuss the critical support of LCAQS in today's world, as well as their low-cost vs. their measurement accuracies. Besides this, we also discuss the technologies used to connect and transfer data from LCAQS.

LCAQS
Air quality sensor technology is an expeditiously growing field that has the key potential to improve the applicability, reliability, and cost-effectiveness of time-resolved air pollution measurements [84,90,241,242]. Many Low-Cost Air Quality Sensors (LCAQS) products are off-the-shelf, open-source, and are becoming increasingly available on the market. Except for technical inconvenience, the information on service life maintenance and durability are insufficient in the datasheet for most of the sensors. In the US, as per the existing literature, the average cost of LCAQS for CO, CO 2 , NO 2 , SO 2 , ozone, TVOC, and PM ranges between $1 and $500, as of April 2020. There are several advantages of LCAQS besides their lower purchase and operation costs compared to regulatory-grade instruments, such as their higher spatial density; their greater number of options in the time-resolution of their data reporting; and their easier field deployment, data collection, and transmissions [90,243,244].

Cost vs. Accuracy of the LCAQS
In most cases, the measurement performance characteristics-such as the typical detection range, measurement tolerance or repeatability, data resolution, linearity, heat resistance, heater current, operating conditions, circuit condition, response time, supply voltage, supply current, and cross-sensitivity to other gases-are contained in the manufacturer's specifications of the LCAQS products. Even so, these performance indicators can vary from sensor to sensor, depending on the laboratory protocol applied, the test chamber set-up, the reference instrument used, the length of the observation period, the range of desired concentrations covered, the efficiency of the calibration algorithm, and the post-processing and data modeling [90,.

Technology of LCAQS
According to the US EPA, LCAQS technology is not considered to be mature enough to be implemented for regulatory or compliance purposes at a mass scale [83], due to their limitations of robustness and repeatability, and the lack a widely-accepted protocol for the testing and utilization of these technologies [83,[247][248][249]. Only limited numbers of the LCAQS developed are integrated with software and operational interface; most of the available program is only applied for a specific OS such as windows, android, and Linux, which increased the limits of openness. Some of LCAQS are designed to interconnect with smart equipment using Internet of Things (IoT) platforms.

Performance Evaluation of LCAQS
Numerous studies have assessed LCAQS, and can provide useful information on ambient gas species and mass particles in the range of specific conditions [79,91,92,245,250]. However, there is still no standard protocol for the evaluation of the performance and effectiveness of LCAQS against traditional monitoring equipment, such as FRM or FEM monitors, at present. In order to address these issues, three notable programs have been launched to quantitatively evaluate the performance of commercially available LCAQS compared to the high-precision equipment under both laboratory and field conditions. These are the AQ-SPEC operated by SCAQMD [251], the US EPA, Air Sensor Toolbox [252], and the EU JRC [253,254]. These platforms created opportunities to assess the data quality and stability of LCAQS by providing state-of-the-art equipment, such as a characterization chamber system, a zero-air generation system, a dynamic dilution calibrator, an air monitoring station, and the best available reference instruments [116,245,251].

Uncertainties in LCAQS
According to the reported results from the AQ-SPEC and the literature, the measurement uncertainty of all types of LCAQS is observed due to changes in the temperature, the relative humidity, cross-sensitivity, interfering compounds, and electronic component tolerances [89,90,[253][254][255][256]. There is also uncertainty due to the sensors' calibration and synchronization errors in both the fine particle sensors and gas-phase sensors [90,248,255]. The proper calibration and normalization methods for each sensor need validation through the removal of structure errors between the measured and expected sensor output. Uncertainty and ambiguity can propagate through the description of the sensor data, the sampling of the sensor data, co-location experiments, the placement of the sensor, aerosol concentrations, errors in the running code, data recovery, and inference with the results [255,[257][258][259][260]. The evaluations found that most PM 2.5 and PM 10 sensors showed strong correlations (0.85 < R 2 < 0.99) in the laboratory test, and moderate to strong correlations (0.52 < R 2 < 0.99) in the field test with the BAM and FEM equipment (at the average range between 0 to 300 µg/m 3 ¬). The laboratory results also showed extremely low intra-model variability in data recovery (98% to 100%), and RHT had minimal effects on the sensors' precision [84,90,246,[261][262][263][264][265]. In contrast, most low-cost gas sensors (CO, NO 2 , and ozone) showed more inter-sensor variability than the fine-partial sensors, especially in the field test. Variations exist from sensor to sensor (0.1 < R 2 < 0.99), with a fair to good range of data recovery (85% to 100%). The uncertainty of gas-phase sensors is generally associated with cross-sensitivity to ambient concentrations, out-of-range detection, spatiotemporal variations, and RHT conditions in the field environment [245,260,[266][267][268][269]. To date, there are limited valid SO 2 sensor evaluation reports available, for which this paper finds a curb on the provision of an overall status of SO 2 conditions. According to the DQOs defined by the European Air Quality Directive, a maximum measurement uncertainty of 15% should not be exceeded for O 3 , NO 2 , NOx, and CO sensors [191,221,222].
6.2.6. QA/QC Control Quality Assessment/Quality Control (QA/QC) protocols must systematically be conducted in order to validate the data quality by considering the elimination of obvious outliers, negative values, and invalid data points [90,259,266]. In addition, the following methods should also be taken into account when performing the field measurement. These are: (a) repeated field calibration along with the combination of different sources in a multi-sensor data fusion algorithm [270][271][272][273] (b): sensitivity analysis [274,275] (c): Monte Carlo simulation methods [276][277][278][279] (d): the mathematical modeling of the error propagation. In concert, it is not mandatory to test the existing LCAQS in these evaluating platforms as well as both the sensor and testing enterprises executed through the optional registration system. This has caused these platforms to selectively recognize a sensor type or its particular parts, resulting in the production of an incomplete evaluation of the products' features and characterizations for the end-users. Currently, these sensor testing programs are being amended on their evaluation system, along with their testing protocols being improved, in order to provide more desired results. However, several of these sensor companies prefer to choose self-evaluation or the general international organization for product standardization. Finally, this study is an extensive review of the integrated sensor system which analysed the characteristics based on various factors, in order to examine indoor and outdoor air quality for the built environment. Therefore, such examinations elaborate on the importance of sensing systems to the monitoring of holistic air quality and the mitigation of pollution levels by impacting the occupants' health levels.

Conclusions
Human health is adversely impacted by indoor air pollutants. Various international agencies have incessantly developed quantitative air quality guidelines and standards to meet the requirements for proper indoor air quality management. This paper set out to gain a better understanding of the existing major standards and guidelines related to indoor air pollutants and their health impacts. The different limiting range for the identified pollutants, enforcement levels, applicable people, and operating procedures of each was reviewed. For the large-scale implementation of air quality management, this study indicates that the importance of monitoring air quality, in real-time, at spatial and temporal data resolutions cannot be understated. Furthermore, this paper also reviewed the existing LCAQS technologies, and discussed the corresponding specifications, such as the typical detection range, measurement tolerance or repeatability, data resolution, response time, supply current, and market price. LCAQS have changed the paradigm of indoor air pollution monitoring, and can provide beneficial information. This technology is not considered advanced enough to be implemented for regulatory purposes ata large scale, due to the limitations of their robustness, repeatability, and lack of a widely-accepted protocol for testing and utilization. Compared to the fine particulate matter sensors, gaseous sensors generally perform with added uncertainties and data variation. There is a need for unified industry-standard QA/QC protocols to analyze and validate overall LCAQS performance. Conclusively, this systematic review addressed the requirements of future research and design practices in order to protect occupants' health and achieve optimal indoor environmental quality.

Conflicts of Interest:
The authors declare no conflict of interest.