Reactive indoor air chemistry and health-A workshop summary

Associations between fine particulate matter, gene expression, and promoter methylation in human bronchial epithelial cells exposed within a classroom under air-liquid interface

Associations between indoor air pollution from fine particulate matter (PM with aerodynamic diameter dp < 2.5 μm) and human health are poorly understood. Here, we analyse the concentration-response curves for fine and ultrafine PM, the gene expression, and the methylation patterns in human bronchial epithelial cells (BEAS-2B) exposed at the air-liquid interface (ALI) within a classroom in downtown Rome. Our results document the upregulation of aryl hydrocarbon receptor (AhR) and genes associated with xenobiotic metabolism (CYP1A1 and CYP1B1) in response to single exposure of cells to fresh urban aerosols at low fine PM mass concentrations within the classroom. This is evidenced by concentrations of ultrafine particles (UFPs, dp < 0.1 μm), polycyclic aromatic hydrocarbons (PAH), and ratios of black carbon (BC) to organic aerosol (OA). Additionally, an interleukin 18 (IL-18) down-regulation was found during periods of high human occupancy. Despite the observed gene expression dysregulation, no changes were detected in the methylation levels of the promoter regions of these genes, indicating that the altered gene expression is not linked to changes in DNA methylation and suggesting the involvement of another epigenetic mechanism in the gene regulation. Gene expression changes at low exposure doses have been previously reported. Here, we add the possibility that lung epithelial cells, when singly exposed to real environmental concentrations of fine PM that translate into ultra-low doses of treatment, may undergo epigenetic alteration in the expression of genes related to xenobiotic metabolism. Our findings provide a perspective for future indoor air quality regulations. We underscore the potential role of indoor UFPs as carriers of toxic molecules with low-pressure weather conditions, when rainfall and strong winds may favour low levels of fine PM.

CO2 in indoor environments: From environmental and health risk to potential renewable carbon source

In the developed world, individuals spend most of their time indoors. Poor Indoor Air Quality (IAQ) has a wide range of effects on human health. The burden of disease associated with indoor air accounts for millions of premature deaths related to exposure to Indoor Air Pollutants (IAPs). Among them, CO₂ is the most common one, and is commonly used as a metric of IAQ. Indoor CO₂ concentrations can be significantly higher than outdoors due to human metabolism and activities. Even in presence of ventilation, controlling the CO₂ concentration below the Indoor Air Guideline Values (IAGVs) is a challenge, and many indoor environments including schools, offices and transportation exceed the recommended value of 1000 ppm_v. This is often accompanied by high concentration of other pollutants, including bio-effluents such as viruses, and the importance of mitigating the transmission of airborne diseases has been highlighted by the COVID-19 pandemic. On the other hand, the relatively high CO₂ concentration of indoor environments presents a thermodynamic advantage for direct air capture (DAC) in comparison to atmospheric CO₂ concentration. This review aims to describe the issues associated with poor IAQ, and to demonstrate the potential of indoor CO₂ DAC to purify indoor air while generating a renewable carbon stream that can replace conventional carbon sources as a building block for chemical production, contributing to the circular economy.

Impact of glycol-based solvents on indoor air quality-Artificial fog and exposure pathways of formaldehyde and various carbonyls

Artificial fog is commonly employed in the entertainment industry and indoor household celebrations. The fog is generated from glycol-based solvents, which can also be found in e-cigarettes and personal care products. Although potential health impacts of glycol inhalation are frequently cited by studies of e-cigarette smoking, the dynamics and the chemical composition of glycol-based aerosols have never been studied systematically. The objective of this work is to investigate the impact of glycol-based aerosol on indoor air quality. Specifically, we targeted artificial fogs generated with common glycols, including propylene glycol (PG) and triethylene glycol (TEG). With the aid of a novel aerosol collecting and monitoring instrument setup, we obtained time-resolved aerosol profiles and their chemical compositions in an experimental room. Artificial fog has given rise to a significant amount of ultra-fine particulate matter, demonstrating its negative impact on indoor air quality. Additionally, we found a high concentration (9.75 mM) of formaldehyde and other carbonyls in fog machine fluids stored for months. These compounds are introduced to the indoor air upon artificial fog application. We propose that carbonyls have accumulated from the oxidative decomposition of glycols, initiated by OH radicals and singlet oxygens (¹ O₂ ) and likely sustained by autooxidation. Oxidation of glycols by indoor oxidants has never been reported previously. Such chemical processes can represent an unrecognized source of toxic carbonyl compounds which is also applicable to other glycol-based solvents.

Reactions and Products of Squalene and Ozone: A Review

This critical review describes the squalene-ozone (SqOz) reaction, or squalene ozonolysis. Ambient ozone penetrates indoors and drives indoor air chemistry. Squalene, a component of human skin oil, contains six carbon-carbon double bonds and is very reactive with ozone. Bioeffluents from people contribute to indoor air chemistry and affect the indoor air quality, resulting in exposures because people spend the majority of their time indoors. The SqOz reaction proceeds through various formation pathways and produces compounds that include aldehydes, ketones, carboxylic acids, and dicarbonyl species, which have a range of volatilities. In this critical review of SqOz chemistry, information on the mechanism of reaction, reaction probability, rate constants, and reaction kinetics are compiled. Characterizations of SqOz reaction products have been done in laboratory experiments and real-world settings. The effect of multiple environmental parameters (ozone concentration, air exchange rate (AER), temperature, and relative humidity (RH)) in indoor settings are summarized. This critical review concludes by identifying the paucity of available exposure, health, and toxicological data for known reaction products. Key knowledge gaps about SqOz reactions leading to indoor exposures and adverse health outcomes are provided as well as an outlook on where the field is headed.

COVID-19 and Lockdown: The Potential Impact of Residential Indoor Air Quality on the Health of Teleworkers

In addition to outdoor atmospheric contamination, indoor exposure to pollutants is a prime contributor to the overall human exposure, and may condition the expressiveness and severity of respiratory, cardiovascular, and allergic diseases. This situation has worsened due to COVID-19, as people have spent more time indoors to comply with social isolation and mandatory telework. The primary purpose of this study was to assess and compare indoor air quality (IAQ) in a significant sample of dwellings of workers from a Higher Education Institution (HEI) in Portugal who were teleworking and their usual workplace. The levels of carbon dioxide, carbon monoxide, and formaldehyde, particles with equivalent diameters of less than 10 μm, 5 μm, 2.5 μm, 1 μm, 0.5 μm, and 0.3 μm, and ultrafine particles, as well as the level of thermal comfort, were measured at both of the sites assessed. It was found that most of the houses studied, as well as the HEI, had good IAQ, although there were places where the concentrations of some pollutants were above the legal standards. On the other hand, a link was identified between the IAQ and the symptoms and diseases observed in the workers who participated in the study. These results offer the opportunity to make corrective interventions, thereby controlling the sources of pollutants and promoting better ventilation in order to reduce the risk for workers.

Genotoxic effect of microplastics and COVID-19: The hidden threat

Microplastics (MPs) are ubiquitous anthropogenic contaminants, and their abundance in the entire ecosystem raises the question of how far is the impact of these MPs on the biota, humans, and the environment. Recent research has overemphasized the occurrence, characterization, and direct toxicity of MPs; however, determining and understanding their genotoxic effect is still limited. Thus, the present review addresses the genotoxic potential of these emerging contaminants in aquatic organisms and in human peripheral lymphocytes and identified the research gaps in this area. Several genotoxic endpoints were implicated, including the frequency of micronuclei (MN), nucleoplasmic bridge (NPB), nuclear buds (NBUD), DNA strand breaks, and the percentage of DNA in the tail (%Tail DNA). In addition, the mechanism of MPs-induced genotoxicity seems to be closely associated with reactive oxygen species (ROS) production, inflammatory responses, and DNA repair interference. However, the gathered information urges the need for more studies that present environmentally relevant conditions. Taken into consideration, the lifestyle changes within the COVID-19 pandemic, we discussed the impact of the pandemic on enhancing the genotoxic potential of MPs whether through increasing human exposure to MPs via inappropriate disposal and overconsumption of plastic-based products or by disrupting the defense system owing to unhealthy food and sleep deprivation as well as stress. Overall, this review provided a reference for the genotoxic effect of MPs, their mechanism of action, as well as the contribution of COVID-19 to increase the genotoxic risk of MPs.

Early Chemistry of Nicotine Degradation in Heat-Not-Burn Smoking Devices and Conventional Cigarettes: Implications for Users and Second- and Third-Hand Smokers

Nicotine exposure results in health risks not only for smokers but also for second- and third-hand smokers. Unraveling nicotine's degradation mechanism and the harmful chemicals that are produced under different conditions is vital to assess exposure risks. We performed a theoretical study to describe the early chemistry of nicotine degradation by investigating two important reactions that nicotine can undergo: hydrogen abstraction by hydroxyl radicals and unimolecular dissociation. The former contributes to the control of the degradation mechanism below 800 K due to a non-Arrhenius kinetics, which implies an enhancement of reactivity as temperature decreases. The latter becomes important at higher temperatures due to its larger activation energy. This change in the degradation mechanism is expected to affect the composition of vapors inhaled by smokers and room occupants. Conventional cigarettes, which operate at temperatures higher than 1000 K, are more prone to yield harmful pyridinyl radicals via nicotine dissociation, while nicotine in electronic cigarettes and vaporizers, with operating temperatures below 600 K, will be more likely degraded by hydroxyl radicals, resulting in a vapor with a different composition. Although low-temperature nicotine delivery devices have been claimed to be less harmful due to their nonburning operating conditions, the non-Arrhenius kinetics that we observed for the degradation mechanism below 873 K suggests that nicotine degradation may be more rapidly initiated as temperature is reduced, indicating that these devices may be more harmful than it is commonly assumed.

Hydrogen Peroxide Emission and Fate Indoors during Non-bleach Cleaning: A Chamber and Modeling Study

Activities such as household cleaning can greatly alter the composition of air in indoor environments. We continuously monitored hydrogen peroxide (H₂O₂) from household non-bleach surface cleaning in a chamber designed to simulate a residential room. Mixing ratios of up to 610 ppbv gaseous H₂O₂ were observed following cleaning, orders of magnitude higher than background levels (sub-ppbv). Gaseous H₂O₂ levels decreased rapidly and irreversibly, with removal rate constants (k_H2O2) 17-73 times larger than air change rate (ACR). Increasing the surface-area-to-volume ratio within the room caused peak H₂O₂ mixing ratios to decrease and k_H2O2 to increase, suggesting that surface uptake dominated H₂O₂ loss. Volatile organic compound (VOC) levels increased rapidly after cleaning and then decreased with removal rate constants 1.2-7.2 times larger than ACR, indicating loss due to surface partitioning and/or chemical reactions. We predicted photochemical radical production rates and steady-state concentrations in the simulated room using a detailed chemical model for indoor air (the INDCM). Model results suggest that, following cleaning, H₂O₂ photolysis increased OH concentrations by 10-40% to 9.7 × 10⁵ molec cm^-3 and hydroperoxy radical (HO₂) concentrations by 50-70% to 2.3 × 10⁷ molec cm^-3 depending on the cleaning method and lighting conditions.

Indoor Surface Chemistry: Developing a Molecular Picture of Reactions on Indoor Interfaces

Chemical reactions on indoor surfaces play an important role in air quality in indoor environments, where humans spend 90% of their time. We focus on the challenges of understanding the complex chemistry that takes place on indoor surfaces and identify crucial steps necessary to gain a molecular-level understanding of environmental indoor surface chemistry: (1) elucidate key surface reaction mechanisms and kinetics important to indoor air chemistry, (2) define a range of relevant and representative surfaces to probe, and (3) define the drivers of surface reactivity, particularly with respect to the surface composition, light, and temperature. Within the drivers of surface composition are the roles of adsorbed/absorbed water associated with indoor surfaces and the prevalence, inhomogeneity, and properties of secondary organic films that can impact surface reactivity. By combining laboratory studies, field measurements, and modeling we can gain insights into the molecular processes necessary to further our understanding of the indoor environment.

Evaluation and impact factors of indoor and outdoor gas-phase nitrous acid under different environmental conditions

As an important indoor pollutant, nitrous acid (HONO) can contribute to the concentration of indoor OH radicals by photolysis via sunlight penetrating into indoor environments, thus affecting the indoor oxidizing capability. In order to investigate the concentration of indoor HONO and its impact factors, three different indoor environments and two different locations in urban and suburban areas were selected to monitor indoor and outdoor pollutants simultaneously, including HONO, NO, NO₂, nitrogen oxides (NO_x), O₃, and particle mass concentration. In general, the concentration of indoor HONO was higher than that outdoors. In the urban area, indoor HONO with high average concentration (7.10 ppbV) was well-correlated with the temperature. In the suburban area, the concentration of indoor HONO was only about 1-2 ppbV, and had a good correlation with indoor relative humidity. It was mainly attributed to the heterogeneous reaction of NO₂ on indoor surfaces. The sunlight penetrating into indoor environments from outside had a great influence on the concentration of indoor HONO, leading to a concentration of indoor HONO close to that outdoors.

Indoor air chemistry: Terpene reaction products and airway effects

Reactive chemistry is ubiquitous indoors with a wealth of complex oxidation reactions; some of these are initiated by both homogeneous and heterogeneous reaction of ozone with unsaturated organic compounds and subsequent the hydroxyl radical, either in the gas-phase or on reactive surfaces. One major focus has been the reaction of common and abundant terpene-based fragrances in indoor air emitted from many wood-based materials, a variety of consumer products, and citrus fruits and flowers. Inhalation of the terpenes themselves are generally not considered a health concern (both acute and long-term) due to their low indoor air concentrations; however, their gas- and surface reactions with ozone and the hydroxyl radical produce a host of products, both gaseous, i. a. formaldehyde, and ultrafine particles formed by condensation/nucleation processes. These reaction products may be of health concern. Human cell bioassays with key reaction products from ozone-initiated terpene reactions have shown some inflammatory reactions, but results are difficult to interpret for human exposure and risk assessment. Acute effects like sensory irritation in eyes and airways are unlikely or present at very low intensity in real life conditions based on rodent and human exposure studies and known thresholds for sensory irritation in eyes and airways and derived human reference values for airflow limitation and pulmonary irritation. Some fragrances and their ozone-initiated reaction products may possess anti-inflammatory properties. However, long-term effects of the reaction products as ultrafine particles are poorly explored. Material and product surfaces with high ozone deposition velocities may significantly impact the perceived air quality by altered emissions from both homogeneous and heterogeneous surface reactions.

Investigation of the PM2.5, NO2 and O3 I/O ratios for office and school microenvironments

Differentiation of the exposure to PM_2.5 (particulate matter less than 2.5 μm in aerodynamic diameter), NO₂ and O₃ i.e. pollutants of outdoor origin, due to the occupation of office and school microenvironments, was investigated through the quantification of the respective Indoor to Outdoor (I/O) ratios, in simple statistical terms. For that cause, indoor and outdoor observation data were retrieved from the HEALS EDMS database, and more specifically the data from the OFFICAIR and the SINPHONIE EU projects. The I/O ratios were produced and were statistically analyzed in order to be able to study the influence of the indoor environment against the pollutants coming from outdoors. The present statistical approach highlighted also the differences of I/O ratios between the two studied microenvironments for each pollutant. For exposure estimation to the above-mentioned pollutants, the probability and cumulative distribution function (pdf/cdf) empirical approximations led to the conclusion that for offices the I/O ratios of PM_2.5 follow a normal distribution, while NO₂ and O₃ a gamma distribution. Respectively, for schools the I/O ratios of all pollutants follow a lognormal distribution.

Interface Bonding Properties and Mechanism of Poplar Board-Veneered Wood Fiber/Polypropylene Composites with Chlorinated Polypropylene Films as an Intermediate Layer

It is difficult to decorate wood plastic composites with wood veneer because their surface energy is low and there are no pores on the surface. In the present study, we developed an environmentally friendly and convenient method to decorate the wood fiber/polypropylene (WF/PP) composite board. We used chlorinated polypropylene (CPP) as an intermediate film to laminate wood veneer to WF/PP composite boards by hot-pressing at 110 °C and then cooling down. The interface bonding mechanism between the wood veneer, CPP layer, and WF/PP composite was analyzed using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDS), surface roughness test, and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). The results indicated that CPP penetrated the wood pores and formed a firm anchor structure. SEM images showed small cracks at the interface between CPP and PP when WF/PP contained less WF. The results of SEM-EDS and ATR-FTIR showed that WF/PP composites with a higher WF content would have more fibers exposed at the surface, thus making the surface rough and providing more specific surface area. The veneered WF/PP composite with 80% WF content had the highest surface bond strength and water resistance.

Ozone in urban China: Impact on mortalities and approaches for establishing indoor guideline concentrations

Reducing indoor ozone levels may be an effective strategy to reduce total exposure and associated mortality. Here we estimate (a) premature mortalities attributable to ozone for China's urban population ≥25 years of age; (b) the fraction of total exposure occurring indoors; and (c) mortalities that can be potentially avoided through meeting current and more stringent indoor ozone standards/guidelines based on 1-hour daily maxima. To estimate ozone-attributable premature mortalities, we used hourly outdoor ozone concentrations measured at 1497 monitoring stations located in 339 Chinese cities and a published concentration-response model. We proceeded to estimate province-specific infiltration factors and co-occurring hourly indoor ozone concentrations. For the year 2015, we estimated that indoor exposures accounted for 59% (95% confidence interval (CI): 26%-79%) of the total ozone exposure that resulted in 70800 (95% CI: 35 900-137 700) premature all-cause mortalities in urban China. If the current Chinese indoor ozone standards (80 ppbv (160 µg/m³ ); 56 ppbv (112 µg/m³ )) were met, the mean estimates of reduction in mortalities would be indistinguishable from zero. With stricter 1-hour indoor ozone guidelines, the expected mortality reductions increase exponentially per unit decrease in indoor ozone. The analysis in this paper should help facilitate formulating present and future indoor ozone guidelines.

Heat Stress in Indoor Environments of Scandinavian Urban Areas: A Literature Review

Climate change increases the risks of heat stress, especially in urban areas where urban heat islands can develop. This literature review aims to describe how severe heat can occur and be identified in urban indoor environments, and what actions can be taken on the local scale. There is a connection between the outdoor and the indoor climate in buildings without air conditioning, but the pathways leading to the development of severe heat levels indoors are complex. These depend, for example, on the type of building, window placement, the residential area's thermal outdoor conditions, and the residents' influence and behavior. This review shows that only few studies have focused on the thermal environment indoors during heat waves, despite the fact that people commonly spend most of their time indoors and are likely to experience increased heat stress indoors in the future. Among reviewed studies, it was found that the indoor temperature can reach levels 50% higher in °C than the outdoor temperature, which highlights the importance of assessment and remediation of heat indoors. Further, most Heat-Health Warning Systems (HHWS) are based on the outdoor climate only, which can lead to a misleading interpretation of the health effects and associated solutions. In order to identify severe heat, six factors need to be taken into account, including air temperature, heat radiation, humidity, and air movement as well as the physical activity and the clothes worn by the individual. Heat stress can be identified using a heat index that includes these six factors. This paper presents some examples of practical and easy to use heat indices that are relevant for indoor environments as well as models that can be applied in indoor environments at the city level. However, existing indexes are developed for healthy workers and do not account for vulnerable groups, different uses, and daily variations. As a result, this paper highlights the need for the development of a heat index or the adjustment of current thresholds to apply specifically to indoor environments, its different uses, and vulnerable groups. There are several actions that can be taken to reduce heat indoors and thus improve the health and well-being of the population in urban areas. Examples of effective measures to reduce heat stress indoors include the use of shading devices such as blinds and vegetation as well as personal cooling techniques such as the use of fans and cooling vests. Additionally, the integration of innovative Phase Change Materials (PCM) into facades, roofs, floors, and windows can be a promising alternative once no negative health and environmental effects of PCM can be ensured.

Miniaturized Biotrickling Filters and Capillary Microbioreactors for Process Intensification of VOC Treatment with Intended Application to Indoor Air

Typical biofilters and biotrickling filters used for volatile organic compounds (VOCs) control have treatment rates limited to 30-200 g m^-3 h^-1, mostly because they are exposed to dilute VOC streams, have moderate biomass density and activity, and moderate mass transfer coefficients. For these reasons and the concern over releasing bioaerosols and humidity, traditional biofilters and biotrickling filters are not ideal for the treatment of indoor air. Here we report on the development and evaluation of microbioreactors for the intensive treatment of VOCs that could be used for indoor air quality control, when coupled with a VOC microconcentrator (developed separately). The microconcentrator will function to adsorb VOCs from indoor air and release them to the microbioreactor at a higher concentration. The miniaturized bioreactors, with maximized surface area-to-volume ratios, allow for increased mixing and mass transfer of pollutants to the biofilm, resulting in a greater degradation rate of the VOCs. Three different microbioreactors were designed, constructed and their performance for removing vapors of toluene and methanol was assessed. Results showed that they were able to achieve maximum elimination capacities (ECs) for methanol around 1000 g m^-3 h^-1, 780 g m^-3 h^-1, and 12 600 g m^-3 h^-1 for the glass beads packed bed, polyurethane (PU) foam biotrickling filters and capillary microbioreactor, respectively, and around 120 g m^-3 h^-1, 250 g m^-3 h^-1 and 3050 g m^-3 h^-1, respectively, when treating toluene vapors. These values, especially for the capillary microbioreactor, are 40-80 times greater than the rates generally obtained in conventional biofilters and biotrickling filters. The interphase mass transfer coefficient (K_La) was determined. The capillary microbioreactor had values 13-17 times greater than the other two bioreactors, suggesting that improved mass transfer could have contributed to the very high performance observed in the capillary microbioreactor. The results demonstrate that microbioreactors are promising novel technologies for controlling small amounts of organic pollutants.

Healthy Indoor Environments: The Need for a Holistic Approach

Indoor environments have a large impact on health and well-being, so it is important to understand what makes them healthy and sustainable. There is substantial knowledge on individual factors and their effects, though understanding how factors interact and what role occupants play in these interactions (both causative and receptive) is lacking. We aimed to: (i) explore interactions between factors and potential risks if these are not considered from holistic perspective; and (ii) identify components needed to advance research on indoor environments. The paper is based on collaboration between researchers from disciplines covering technical, behavioural, and medical perspectives. Outcomes were identified through literature reviews, discussions and workshops with invited experts and representatives from various stakeholder groups. Four themes emerged and were discussed with an emphasis on occupant health: (a) the bio-psycho-social aspects of health; (b) interaction between occupants, buildings and indoor environment; (c) climate change and its impact on indoor environment quality, thermal comfort and health; and (d) energy efficiency measures and indoor environment. To advance the relevant research, the indoor environment must be considered a dynamic and complex system with multiple interactions. This calls for a transdisciplinary and holistic approach and effective collaboration with various stakeholders.

Assessment of Indoor Air Quality Problems in Office-Like Environments: Role of Occupational Health Services

There is an increasing concern about indoor air quality (IAQ) and its impact on health, comfort, and work-performance in office-like environments and their workers, which account for most of the labor force. The Scientific Committee on Indoor Air Quality and Health of the ICOH (Int. Comm. Occup. Health) has discussed the assessment and management of IAQ problems and proposed a stepwise approach to be conducted by a multidisciplinary team. It is recommended to integrate the building assessment, inspection by walk-through of the office workplace, questionnaire survey, and environmental measurements, in that order. The survey should cover perceived IAQ, symptoms, and psychosocial working aspects. The outcome can be used for mapping the IAQ and to prioritize the order in which problems should be dealt with. Individual health surveillance in relation to IAQ is proposed only when periodical health surveillance is already performed for other risks (e.g., video display units) or when specific clinical examination of workers is required due to the occurrence of diseases that may be linked to IAQ (e.g., Legionnaire’s disease), recurrent inflammation, infections of eyes, respiratory airway effects, and sensorial disturbances. Environmental and personal risk factors should also be compiled and assessed. Workplace health promotion should include programs for smoking cessation and stress and IAQ management.

Indoor Chemistry

This review aims to encapsulate the importance, ubiquity, and complexity of indoor chemistry. We discuss the many sources of indoor air pollutants and summarize their chemical reactions in the air and on surfaces. We also summarize some of the known impacts of human occupants, who act as sources and sinks of indoor chemicals, and whose activities (e.g., cooking, cleaning, smoking) can lead to extremely high pollutant concentrations. As we begin to use increasingly sensitive and selective instrumentation indoors, we are learning more about chemistry in this relatively understudied environment.