Indoor Volatile Organic Compounds Concentrations Research Articles

Long-term emission characteristics of VOCs from building materials

Long-term emission behaviors of volatile organic compounds (VOCs) from indoor buildings materials heavily depend on the value of three key parameters (initial concentration C0, diffusion coefficient Dm, partition coefficient K) that govern emissions over time. We made the first attempt to quantitatively explore the variation of parameters through a long-lasting aging test that simulates natural indoor exposure. Over a span of 431 days, we obtained a substantial dataset consisting of ten thousand data points. The parameters of six VOCs (formaldehyde, benzene, toluene, ethylbenzene, o-xylene, p-m-xylene) from three kinds of wood-based boards with different aging intervals were determined. Our findings demonstrate that C0 decreases exponentially with aging time, while Dm and K merely fluctuate with it. With the obtained correlations, ventilation time for renovated house is proposed to meet the WHO standard. These results lay the groundwork for predicting long-term indoor VOC concentrations, which is crucial for indoor air quality pre-evaluation.

Numerical prediction model for long- and short-term concentration of indoor volatile organic compounds from building materials

ABSTRACT Emission models of volatile organic compounds (VOCs) from individual indoor building materials have been developed and validated. However, multiple indoor building materials release VOCs simultaneously, and neither single building material nor multiple building material emission models can predict the entire release cycle of VOCs accurately. This study established a long- and short-term numerical prediction model for indoor VOC concentration. The model includes an attenuation coefficient θ. To describe the decay rate of the total VOC content, which is mainly influenced by time, and by designing experiments and testing in environmental warehouses under different seasonal conditions, the value of θ was first obtained. Then, after successfully plotting the emission curve of indoor pollutant concentration over time through numerical solution and using θ, the VOC content was corrected for various seasonal conditions. On the basis of this model, an exposure dose integration algorithm was proposed to evaluate the environmental health risks, as an application of this model. In comparison with previous research results and experimental data, this model has better predictive performance.

Evaluation of the seasonal variation of VOC surface emissions and indoor air concentrations in a public building with bio-based insulation

The use of bio-based insulation materials is widely spreading in buildings. Due to their organic load, they can be an important source of Volatile Organic Compounds (VOCs). This study is among the first that evaluates the spatial and seasonal in-field VOC surface emissions from bio-based and conventional building structures as a whole, in a French public building insulated with wood wool. In addition to surface emissions, measurements of VOC concentrations in indoor air were taken. Results showed that a spatial difference (up to 5 times) in VOC emissions was observed due to the inhomogeneity of the surface. Moreover, the cardinal orientation of building structures with the same constitution induced a difference (up to a factor 30) in emission rates due to the exposure to different hygrothermal conditions. The variation in temperature and relative humidity between seasons led to higher summer VOC emissions and indoor air concentrations. In addition, indoor VOC concentrations were shown to be higher at night compared to daytime due to the decreased ventilation rate. Furthermore, an interesting approach was developed in this study to have a primary overview of the impact of surface emissions on indoor VOC levels. Results confirmed that the three bio-based walls have no significant specific VOC emissions at high rates compared to the floor and the ceiling. Bio-based insulations showed no impact on microbial indoor air concentrations during the two seasons. Moreover, no detected VOCs could be attributed to microbial development as they were also emitted from building materials.

Volatile organic compounds in children's bedrooms, Shanghai, China: Sources and influential factors

Exposure to volatile organic compounds (VOCs) in the residential environment may cause harm to children's health. During 2013–2014, we collected air samples from 358 children's bedrooms in Shanghai, China, to quantify indoor volatile organic compounds (VOCs) levels, and investigate the influential factors and the sources of indoor VOCs. This study analyzed tested and analyzed all the collected air samples by gas chromatography-mass spectrometry. Twelve kinds of VOCs were found, and the most abundant VOCs in terms of median concentration was dodecane, followed by toluene and hexadecane. This study further analyzed three influential factors on the concentration of VOCs: building materials (floor and wall covering materials), household chemical products, and ambient exposures (traffic exposure, shopping center exposure, industrial area exposure, and river/lake exposure). Most of the VOCs concentrations were mainly dominated by indoor sources or the combination of indoor and outdoor sources. Indoor VOCs concentrations were positively associated with oil painting wall covering used in children's bedrooms, frequently usage of household chemical products, and ambient traffic exposure. The rank of the significant influential factors of indoor VOCs concentrations was obtained through the stratification analysis combined with the elimination and disturbance methods. Principal component analysis was used to identify potential sources of indoor VOCs, and four main factors in children's bedrooms were generated, including household chemical products, building materials, vehicle-related emissions, and organic solvents. This study can be used to control and manage the levels of VOCs in residential environments.

A pre-assessment and pollution prevention tool for indoor volatile organic compound simulations during the interior design stage

Volatile organic compounds (VOCs) are common indoor pollutants that can deteriorate indoor air quality. The pre-assessment of VOC concentrations aimed at pollution prevention during the interior design stage is effective in terms of time and expense for VOC control, as required by engineering applications. In this study, we introduce the concept, framework, and mathematical models of a pre-assessment and pollution prevention tool for VOC simulations. This tool considers multiple emissions sources and sinks, the interior construction schedule for the building materials, internal and external airflows, and air cleaning. The emissions/adsorption patterns from building materials were numerically coupled with the combined effects of the thermal environment, ventilation, indoor VOC concentrations, and emissions/adsorption from other materials at each time step, thereby improving the accuracy of the tool in engineering applications. The proposed tool was applied in an office building with multiple rooms and building materials to illustrate the simulation procedures and validate its effectiveness at VOC preventive control. The main emissions sources were determined and the selection of building materials was modified based on the simulation results. Field measurements were performed and the measured data were compared with the simulation results after construction completion. Differences between the simulated and measured results were predominantly within 0.02 mg/m3. The normalized mean square error was 0.11 and the mean relative standard deviation was 16.9%. This method can be used for indoor VOC concentration estimations, source contribution analysis, interior design scheme optimization, and ventilation rate requirement estimation, among other applications.

A Pilot Study to Quantify Volatile Organic Compounds and Their Sources Inside and Outside Homes in Urban India in Summer and Winter during Normal Daily Activities

Indian cities have some of the poorest air quality globally but volatile organic compounds (VOCs)—many of which adversely affect health—and their indoor sources remain understudied in India. In this pilot study we quantified hundreds of VOCs inside and outside 26 homes in Ahmedabad and Gandhinagar, Gujarat, in May 2019 and in January 2020. We sampled in the morning and afternoon/evening to capture temporal variability. Total indoor VOCs were measured at higher concentrations in winter (327.0 ± 224.2 µgm−3) than summer (150.1 ± 121.0 µgm−3) and exceeded those measured outdoors. Using variable reduction techniques, we identified potential sources of compounds (cooking, plastics [with an emphasis on plasticizers], consumer products, siloxanes [as used in the production of consumer products], vehicles). Contributions differed by season and between homes. In May, when temperatures were high, plastics contributed substantially to indoor pollution (mean of 42% contribution to total VOCs) as compared to in January (mean of 4%). Indoor cooking and consumer products contributed on average 29% and 10% to all VOCs indoors in January and 16% and 4% in May. Siloxane sources contributed <4% to any home during either season. Cooking contributed substantially to outdoor VOCs (on average 18% in January and 11% in May) and vehicle-related sources accounted for up to 84% of VOCs in some samples. Overall, results indicate a strong seasonal dependence of indoor VOC concentrations and sources, underscoring the need to better understand factors driving health-harming pollutants inside homes to facilitate exposure reductions.

Probabilistic Prediction Models and Influence Factors of Indoor Formaldehyde and VOC Levels in Newly Renovated Houses

Rapid urbanization has promoted house renovations and refurbishment in urban and rural cities. Indoor pollutants emitted through renovations and refurbishment processes have raised public concerns owing to their adverse effects on human health. In the present study, the sources of formaldehyde and specific volatile organic compounds (VOCs) are used to model the health effects associated with exposure to formaldehyde and specific VOCs and the loading factors of building materials for newly renovated homes. The present study is carried out to identify the sources of formaldehyde and specific VOCs in newly renovated houses and develop probabilistic prediction models of the health effects to explore the health risks of residents and the potential contributions of multilayer wood materials responsible for indoor pollutants. In living rooms and bedrooms, the average concentrations of formaldehyde and TVOCs in closed window conditions were higher than those in opened window conditions. Multi-layer wooden structures were a significant predictor of indoor VOC concentrations in houses. The 95 percentile values of Monte Carlo simulations (MCS P95) of the hazard index and cancer risk were lower and slightly higher than the acceptable level, respectively. Prediction models for the concentrations of formaldehyde and selected VOCs in newly renovated houses were first established using probabilistic and sensitive approaches. The multi-layer wood materials, including the wooden floor, cold paint multi-layer wooden materials, and multi-layer materials for system furniture, were responsible for the contribution of these levels of formaldehyde and selected VOCs in the newly renovated houses. Our results provide a strategy for eliminating indoor pollutants emitted from construction and building/furnishing materials.

The Association between Exposure to Residential Indoor Volatile Organic Compounds and Measures of Central Arterial Stiffness in Healthy Middle-Aged Men and Women.

It is well reported that individuals spend up to 90% of their daily time indoors, with between 60% to 90% of this time being spent in the home. Using a cross-sectional study design in a population of 111 healthy adults (mean age: 52.3 ± 9.9 years; 65% women), we investigated the association between exposure to total volatile organic compounds (VOCs) in indoor residential environments and measures of central arterial stiffness, known to be related to cardiovascular risk. Indoor VOC concentrations were measured along with ambulatory measures of pulse pressure (cPP), augmentation index (cAIx) and cAIx normalized for heart rate (cAIx75), over a continuous 24-h period. Pulse wave velocity (cfPWV) was determined during clinical assessment. Multiple regression analysis was performed to examine the relationship between measures of arterial stiffness and VOCs after adjusting for covariates. Higher 24-h, daytime and night-time cAIx was associated with an interquartile range increase in VOCs. Similar effects were shown with cAIx75. No significant effects were observed between exposure to VOCs and cPP or cfPWV. After stratifying for sex and age (≤50 years; >50 years), effect estimates were observed to be greater and significant for 24-h and daytime cAIx in men, when compared to women. No significant effect differences were seen between age groups with any measure of arterial stiffness. In this study, we demonstrated that residential indoor VOCs exposure was adversely associated with some measures of central arterial stiffness, and effects were different between men and women. Although mechanistic pathways remain unclear, these findings provide a possible link between domestic VOCs exposure and unfavourable impacts on individual-level cardiovascular disease risk.

Influence of Indoor Volatile Organic Compound Concentrations on Respiratory Symptoms Among Children Living in Rural Biomass Cook Stove Using Households of Tamil Nadu and Andrapradesh, India

Influence of Indoor Volatile Organic Compound Concentrations on Respiratory Symptoms Among Children Living in Rural Biomass Cook Stove Using Households of Tamil Nadu and Andrapradesh, India

Estimates of emission strengths of 43 VOCs in wintertime residential indoor environments, Beijing

There are many sources of volatile organic compounds (VOCs) in indoor environments, leading to much higher total indoor VOC concentrations than outdoor counterparts. Given the potential health hazards associated with VOC exposure, it is necessary to estimate the indoor VOC emission strengths. In this study, the indoor and outdoor concentrations of 43 VOCs were concurrently measured in 8 urban residences, Beijing. The indoor/outdoor concentration ratio was used to screen out 36 species having significant indoor sources. A one-compartment steady-state model was developed to estimate the indoor emission strengths of these VOCs, in which ventilation and reaction with ozone were included as sink routes. The order of VOCs in terms of indoor emission strength was d-limonene (a median value of 1.05 g/h), α-pinene (82.50 mg/h), styrene (24.12 mg/h), ß-pinene (9.70 mg/h), formaldehyde (1.97 mg/h), n-dodecane (1.82 mg/h), n-pentadecane (1.66 mg/h), n-hexadecane (1.62 mg/h), n-undecane (1.20 mg/h), acetaldehyde (1.05 mg/h) and 1, 4-dichlorobenzene (0.80 mg/h). The sum of estimates of those VOCs accounted for >95% of total emission strength. Specific indoor sources of those VOCs in the tested homes were identified. Air exchange rate, indoor temperature and air humidity were found to pose significant impacts to the indoor emission strengths of VOCs.

Volatile Organic Compounds in Underground Shopping Districts in Korea.

Underground shopping districts (USDs) are susceptible to severe indoor air pollution, which can adversely impact human health. We measured 24 volatile organic compounds (VOCs) in 13 USDs throughout South Korea from July to October 2017, and the human risk of inhaling hazardous substances was evaluated. The sum of the concentrations of the 24 VOCs was much higher inside the USDs than in the open air. Based on factor analysis, six indoor air pollution sources were identified. Despite the expectation of a partial outdoor effect, the impacts of the indoor emissions were significant, resulting in an indoor/outdoor (I/O) ratio of 5.9 and indicating elevated indoor air pollution. However, the effects of indoor emissions decreased, and the contributions of the pollution sources reduced when the USD entrances were open and the stores were closed. Although benzene, formaldehyde, and acetaldehyde exhibited lower concentrations compared to previous studies, they still posed health risks in both indoor and outdoor settings. Particularly, while the indoor excess cancer risk (ECR) of formaldehyde was ~10 times higher than its outdoor ECR, benzene had a low I/O ratio (1.1) and a similar ECR value. Therefore, indoor VOC concentrations could be reduced by managing inputs of open air into USDs.

Indoor air quality of 5,000 households and its determinants. Part B: Volatile organic compounds and inorganic gaseous pollutants in the Japan Environment and Children's study

Volatile organic compounds (VOCs) are major indoor air pollutants. Quantification of indoor concentrations of VOCs and identification of factors associated with these concentrations can help manage indoor air quality. This study measured the concentrations of VOCs and inorganic gaseous pollutants in around 5000 households in Japan and utilised a random forest model to estimate these concentrations and identify important determinants. The homes of 5017 randomly selected participants in the Japan Environment and Children's Study (JECS) were visited twice, when the children were aged 1.5 and 3 years. Twelve VOCs and inorganic gaseous pollutants were measured during 7 days by passive samplers. Various factors in these households, including household appliances, building characteristics, cooking styles, use of consumer products, renovation, pets, personal behaviours and ventilation were recorded. A random forest model with recursive feature elimination was utilised to identify factors predictive of VOCs and inorganic gaseous pollutants. Toluene, formaldehyde and acetaldehyde were the dominant indoor VOCs. The 95th percentiles of indoor p-dichlorobenzene concentrations at 1.5 and 3 years were 67 μg/m3 and 71 μg/m3, respectively. Random forest models with coefficients of determination ranging from 0.34 to 0.76 outperformed the traditional linear regression models. Factors associated with indoor VOC and inorganic gaseous pollutant concentrations included their outdoor concentrations, indoor and outdoor temperature and relative humidity, month of the year, hours windows were open, kerosene heater use and times of operation and building age. The results provided basic descriptions of indoor VOCs and inorganic gaseous pollutants in Japan and identified several determinants of these concentrations. These determinants should be considered to maintain indoor air quality. These results can be used in epidemiological assessments of the effects of VOCs and inorganic gaseous pollutants on health in children.

Modelling VOC Emissions from Building Materials for Healthy Building Design

The profound qualitative changes of indoor air and the progressive increase in the absolute number of pollutants, combined with the scientific awareness of the health impacts deriving from spending more than 90% of one’s time inside confined spaces, have increased the attention onto the needs of well-being, hygiene, and the health of users. This scientific attention has produced studies and analyses useful for evidence-based insights into building performance. Among the main pollutants in the indoor environment, Volatile Organic Compounds (VOCs) play a central role, and the use of box-models using the mass balance approach and Computational Fluid Dynamics (CFD) models are now consolidated to study their concentrations in an indoor environment. This paper presents the use of both types of modelling for the prediction of the VOC concentration in the indoor environment and the proposal of a guide value for the Indoor Air Quality (IAQ)-oriented building design, specifically related to the indoor VOC concentration due to building materials. Methodologically, the topic is addressed through environmental sampling, the definition of the parameters necessary for the numerical models, the simulations with the box-model and the CFD, and the comparison between the results. They show a good correspondence between the modelling tools used, highlighting the central role of ventilation and allowing a discussion of the relationship between regulatory limits of emissivity of materials and Indoor Air Guide Values for the concentration of pollutants.

Volatile organic compounds in the Spanish National Archaeological Museum: Four years of chemometric analysis

The 19th-century building housing the National Archaeological Museum (MAN) of Spain located in Madrid's downtown was remodelled between 2008 and 2014. During the final of the reform and since its opening to the public on April 2014 and until 2017, the levels of forty different Volatile Organic Compounds (VOCs) were determined and quantified following the AirArt project. This project aimed to identify, quantify and treat the chemical and biological pollutants in the museum. This article discusses the presence, quantification and chemometric studies of the main VOCs identified in the museum. The campaigns were carried out by collecting active and passive air samples on sorbent tubes and monitoring the results over four years. The I/O ratio >1 shows that the indoor VOCs concentrations were higher than those at the outdoors of the museum from the emission of new materials used for their renovation. The Egypt and Reinos Cristianos rooms have the highest I/O ratio. The PCA analysis for four years of indoor and outdoor VOCs analysis of the museum explains the difference between the two environments and highlighted the siloxanes and aldehydes like principal VOCs inside the museum. The concentration of these compounds is below the WHO limits, but little is known about the limit and which compounds can damage the heritage in the museums. The filtration system in the airtight showcase ensures the removal of pollutants over time, avoiding damage to both the health of conservatives and the heritage on display.

A numerical model predicting indoor volatile organic compound Volatile Organic Compounds emissions from multiple building materials.

There have been many studies on the model of volatile organic compound (VOC) emissions from individual dry building material and have been validated in the chamber. Actually, VOC emitted from multiple dry building materials simultaneously indoor. The concentration of VOC indoor increases and will inhibit the VOC emission of dry building materials indoor. This paper developed a new model predicting indoor VOC concentrations caused by simultaneous emissions from multiple dry building materials, with a consideration of impact from dynamic VOC concentrations on the emission rate. The model has been used to predict the VOC emissions from a combination of medium-density fiberboard (MDF) and consolidated compound floor (CCF) simultaneously. The study demonstrated a good prediction performance of the newly proposed model, against field experimental data. The study also showed that when multiple dry building materials emit pollutants in a common space, a mutual inhibition effect could be observed. Furthermore, when multiple dry building materials emit VOC simultaneously, the change of VOC concentrations in the air followed the trends of VOC emissions from building materials with higher initial concentration (C0), diffusion coefficient (Dm), and the partition coefficient (Kma).

Potted plants do not improve indoor air quality: a review and analysis of reported VOC removal efficiencies.

Potted plants have demonstrated abilities to remove airborne volatile organic compounds (VOC) in small, sealed chambers over timescales of many hours or days. Claims have subsequently been made suggesting that potted plants may reduce indoor VOC concentrations. These potted plant chamber studies reported outcomes using various metrics, often not directly applicable to contextualizing plants' impacts on indoor VOC loads. To assess potential impacts, 12 published studies of chamber experiments were reviewed, and 196 experimental results were translated into clean air delivery rates (CADR, m3/h), which is an air cleaner metric that can be normalized by volume to parameterize first-order loss indoors. The distribution of single-plant CADR spanned orders of magnitude, with a median of 0.023 m3/h, necessitating the placement of 10-1000 plants/m2 of a building's floor space for the combined VOC-removing ability by potted plants to achieve the same removal rate that outdoor-to-indoor air exchange already provides in typical buildings (~1 h-1). Future experiments should shift the focus from potted plants' (in)abilities to passively clean indoor air, and instead investigate VOC uptake mechanisms, alternative biofiltration technologies, biophilic productivity and well-being benefits, or negative impacts of other plant-sourced emissions, which must be assessed by rigorous field work accounting for important indoor processes.

Evaluation of Indoor Air Pollution during Decorating Process and Inhalation Health Risks in Xi’an, China: A Case Study

ABSTRACT PM2.5, formaldehyde, and 8 volatile organic compounds (VOCs) were observed in 6 newly decorated apartment units to evaluate the effects of the decorating process on the indoor air quality in Xi’an, China. The comparison of indoor and outdoor formaldehyde and VOCs concentrations showed that the outdoor PM2.5 concentration exceeded the indoor one during the monitoring process, whereas the indoor formaldehyde and VOCs concentrations exceeded the outdoor ones. The levels of formaldehyde and VOCs in different rooms were investigated, and the concentrations in the bedroom were found to be the highest. Furthermore, the formaldehyde and VOCs concentrations were measured in 200 other rooms decorated within a 2-year period in Xi’an, and the results indicated that wallpapering, wooden flooring, and furniture were the major decorating processes emitting these compounds. In addition, a health risk assessment of the monitored formaldehyde and VOCs in the rooms 1 year after decorating showed that benzene posed the greatest health risk among the assessed VOCs.

Elevated Indoor Volatile Organic Compound Exposure in the Niger Delta Region of Nigeria.

The implications of environmental contamination on human health in the Niger Delta region of Nigeria remain a topic of growing international public health interest. To better understand ongoing air pollution and initiate remediation efforts, the United Nations Environmental Programme (UNEP) report recommended the monitoring of volatile organic compounds (VOCs) across different media (water, soil, and air) in Ogoniland, an at-risk population in the Niger Delta region of Nigeria. In this pilot study, we measured indoor VOC concentrations in the indoor air of 20 households in Ogale, an Ogoniland community whose groundwater system is contaminated with benzene at levels 900 times the World Health Organization guidelines and evaluated self-reported health conditions and predicted cancer risks and hazards from inhalation exposure to VOCs. We detected higher concentrations of benzene (mean = 25.7 μg/m3, SD = 23.2 μg/m3) and naphthalene (mean = 7.6 μg/m3, SD = 13.8 μg/m3) than has been reported in other regions. Although study participants reported health symptoms consistent with VOC exposure, we were underpowered to detect a significant association between select indoor VOCs and these self-reported health symptoms using univariate logistic regression models. These findings suggest that that the health symptoms reported by participants may be poor proxies for the underlying disease processes associated with adverse health outcomes due to VOC exposure in this community and that the burden of adverse health effects due to VOC exposure may stem from the contaminated groundwater system. We estimated a non-cancer hazard quotient of 3 from exposure to naphthalene and lifetime excess cancer risks from exposure to naphthalene, benzene, p-dichlorobenzene, carbon tetrachloride, and ethylbenzene of 3 × 10−4, 2 × 10−4, 6 × 10−5, 6 × 10−6, and 1 × 10−5, respectively. These results exceed common risk benchmarks in the United States, suggesting a need for further studies to characterize VOC exposures, sources, and associated health risks in the Niger Delta.

Indoor and outdoor air concentrations of volatile organic compounds in schools within different urban areas

This study included nine schools which were selected from three different areas (industrial, traffic, and residential) of Kuwait. Three schools were chosen from each area to represent elementary, middle, and secondary levels. Samples of indoor volatile organic compounds (VOCs) were collected using canisters placed in one classroom in each school in addition to outdoor VOCs captured in canisters placed at the roof of one chosen school among the three studied at each area. The VOCs levels were monitored during the weekdays and weekends (schools closed) over a period of 2 months. The results showed higher VOCs concentrations in indoor than in outdoor samples in all schools studied at the three areas during both the weekdays and the weekends, irrespective of the level of schooling. Indoor VOCs concentrations were less than 200 ppbv in all schools, with relatively higher levels recorded in schools located in the industrial area reaching 681 ppbv since the indoor air quality is influenced by that of outdoor. The total (indoor and outdoor) levels of VOCs measured during the weekdays were higher than those detected in the weekends in both the traffic and industrial areas, whereas the opposite was observed in the residential area due to its increased weekend traffic density. Percentage composition of VOCs in air samples indicated domination of halogenated and oxygenated components compared to aliphatic and aromatic constituents. The affected schools should increase ventilation rates and install devices to purify the indoor air to ensure healthy environment for students, teachers, and employees at schools.

The identification of Volatile Organic Compound’s emission sources in indoor air of living spaces, offices and laboratories

Indoor air quality is important because people spend most of their time in closed rooms. If volatile organic compounds (VOCs) are present at elevated concentrations, they may cause a deterioration in human well-being or health. The identification of indoor emission sources is carried out by comparison indoor and outdoor air composition. The aim of the study was to determinate the concentration of VOCs in indoor air, where there was a risk of elevated levels due to the kind of work type carried out or the users complained about the symptoms of a sick building followed by an appropriate interpretation of the results to determine whether the source of the emission in the tested room occurs. The air from residential, office and laboratory was tested in this study. The identification of emission sources was based on comparison of indoor and outdoor VOCs concentration and their correlation coefficients. The concentration of VOCs in all the rooms were higher or at a similar level to that of the air sampled at the same time outside the building. Human activity, in particular repair works and experiments with organic solvents, has the greatest impact on deterioration of air quality.