The current-use flame retardants (FRs) tri(2-chloroethyl) phosphate (TCEP), tri(chloroiso-propyl) phosphate (TCIPP), triphenyl phosphate (TPHP), 2-ethylhexyl-2,3,4,5-tetrabromobenzoate (EH-TBB), bis(2-ethylhexyl) tetrabromophthalate (BEH-TEBP) and hexabromobenzene (HBBz) can be transported to the Arctic over long distances, but might also be emitted locally from the use and disposal of FR-containing products. The aim of this study was to assess the importance of local sources and long-range transport for these FRs, based on predicted environmental concentrations at Nuuk, Greenland, derived from the modelling of long-range atmospheric transport and local FR emissions and fate, respectively. Following an FR mass flow of geographically stratified production, use and waste, emissions of the selected FRs to air, soil and water were estimated for the northern hemisphere and at the local level. The Danish Eulerian Hemispheric Model (DEHM) was adjusted to model the atmospheric transport to Nuuk, and a local model was established for Nuuk to calculate local concentrations in air, soil and seawater. Comparisons with measurements in Arctic air were limited by data availability. High emission scenarios showed better agreement with measured values than low emission scenarios, indicating a potential underestimation of real concentrations by the model. Generally, the locally determined concentrations exceeded the long-range transported ones for all media (air, soil, seawater), with the exception of the high emission scenario for HBBz where the two components were more similar. A preliminary screening of environmental and human health risks resulted in risk characterisation ratios ≪ 1.

Active pharmaceutical ingredients (APIs) can cause severe adverse effects if released into the environment. In response to the 2574 APIs approved in Denmark and costly environmental monitoring initiatives, a prioritisation scheme is presented to identify candidate substances for further investigation, based on their hazard, use and risk. The proposed prioritisation scheme consists of three subsequent filters; human toxicology and ecotoxicology followed by reported Danish use and lastly measured or predicted environmental concentration and risk ratios based on these. To generate a final list of 300 APIs, actions such as assigning scores for missing data were implemented. These substances may inform future monitoring campaigns focused on pharmaceutical contamination in aquatic environments through effluent discharge. All in all, 84% among the top-50 priority APIs have not previously been included in chemical analysis of any environmental samples in Denmark. These APIs belonged to therapeutic groups such as treatments for hypertension, antibiotics, antifungals, antineoplastics, and medicines affecting the nervous system. Of particular concern is metformin, clindamycin and clotrimazole as these are all amongst the highest ranked based on risk, and also appear on the EU commission's watch list of substances for EU-wide monitoring in the field of water policy. The scheme can be updated, adapted and implemented in other geographical regions.

We ranked 1528 chemicals of concern from regulatory databases using a scoring system of these 16 chemicals was ranked as representing the highest risk. Ten of these are not part any environmental monitoring program among in the Nordic countries.

The Baltic Sea is one of the world's largest brackish waters, it is drained through the Danish Straits into the Kattegat, and almost enclosed by nine countries. The Baltic Sea is one of the world's most polluted water bodies thus facing a wide range of environmental threats towards its water resources, such as fish stocks, and coastal environments and economics. Chemical warfare agents (CWAs) that were dumped following the Second World War are known to occur in intact or degraded states in sediments and are documented to affect benthic fauna and fish as well as having injured fishermen having accidentally caught lumps of CWAs in their nets. However, a thorough mapping of remaining CWAs and degradation products and an understanding of the environmental risks does not exist although more than 75 years have passed. This study compiles and analyzes published/peer-reviewed data, generated since 2005 in five selected comprehensive studies, on sediment measurements of known dumped CWAs and degradation products in the Baltic Sea. As a worst-case approach, sediment concentrations are transformed to concentrations in near-bottom water, which represents Predicted Environmental Concentrations (PECs) to marine biota. To investigate the accuracy and representativeness of toxicological values, which are fundamental in risk assessments, two cases are considered: Case 1 (specificity) uses toxicological data (EC50 or NOEC) for Daphnia magna and fish with applied assessment factors (AF) to derive Predicted No-Effect Concentrations (PNECs) for organoarsenical and non-arsenical CWAs; Case 2 (robustness) uses partly Danish Environmental Quality Standards (DK EQS) for arsenicals, and partly a Water Quality Criterion (WQC) for arsenicals, representing the marine environment. From 872 data points risk quotients (RQs=PEC/PNEC) are calculated. In Case 1 exceedances of risk for the sum of chemicals (sumRQ>1) occur 24 and 1 times for Daphnia magna and fish, respectively, without applying AFs. 263 and 120 exceedances are found for Daphnia magna and fish, respectively, when applying AFs. Case 2 shows 0 (WQC) and 25 (DK EQS) exceedances for arsenicals when using more robust toxicological values, however, at the expense of specificity of chemicals and target species. The results underline the importance of obtaining more representative and accurate toxicological data (lowering AFs) in order to increase the accuracy of the risk estimates. This quantitative state of risk towards representative marine species indicates that there are indeed potential risks, and it qualifies the understanding and debate on the challenges and future actions regarding dumped chemical munitions in the Baltic Sea.

Several chemicals with widespread consumer uses have been identified as endocrine-disrupting chemicals (EDCs), with a potential risk to humans. The occurrence in indoor dust and resulting human exposure have been reviewed for six groups of known and suspected EDCs, including phthalates and non-phthalate plasticizers, flame retardants, bisphenols, per- and polyfluoroalkyl substances (PFAS), biocides and personal care product additives (PCPs). Some banned or restricted EDCs, such as polybrominated diphenyl ethers (PBDEs), di-(2-ethylhexyl) phthalate (DEHP), bisphenol A (BPA), perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), are still widely detected in indoor dust in most countries, even as the predominating compounds of their group, but generally with decreasing trends. Meanwhile, alternatives that are also potential EDCs, such as bisphenol S (BPS), bisphenol F (BPF), decabromodiphenyl ethane (DBDPE) and organophosphate flame retardants (OPFRs), and PFAS precursors, such as fluorotelomer alcohols, have been detected in indoor dust with increasing frequencies and concentrations. Associations between some known and suspected EDCs, such as phthalate and non-phthalate plasticizers, FRs and BPs, in indoor dust and paired human samples indicate indoor dust as an important human exposure pathway. Although the estimated daily intake (EDI) of most of the investigated compounds was mostly below reference values, the co-exposure to a multitude of known or suspected EDCs requires a better understanding of mixture effects.

Abstract Armed conflicts have, in addition to severe impacts on human lives and infrastructure, also impacts on the environment, which needs to be assessed and documented. On September the 26th 2022, unknown perpetrators deliberately ruptured the two gas pipelines Nord Stream 1 and 2 with four coordinated explosions near a major chemical munition dump site near the Danish island of Bornholm in the Baltic Sea. While the massive release of natural gas into atmosphere raised serious concerns for climate, this paper assesses the overlooked direct impact of this sabotage on marine ecosystem. Seals and porpoises within a radius of four km would be at high risk of being killed by the shockwave, while temporary impact on hearing would be expected up to 50 km away. As the Baltic Proper population of harbour porpoises (Phocoena phocoena) is critically endangered, the loss or serious injury of even a single individual is considered a significant impact on the population. The rupture resulted in the resuspension of 250.000 metric tons of heavily contaminated sediment from deep-sea sedimentary basin for over a week, resulting in unacceptable risks towards fish and other biota in 11 km3 water for more than a month.

Indoor dust is a sink of hundreds of organic chemicals, and humans may potentially be exposed to these via indoor activities. This study investigated potentially harmful semi-volatile organic contaminants in indoor dust from Danish kindergartens using suspect and non-target screening on gas chromatography (GC)-Orbitrap, supported by target analyses using GC-low resolution mass spectrometry (LRMS). A suspect list of 41 chemicals with one or more toxicological endpoints, i.e. endocrine disruption, carcinogenicity, neurotoxicity and allergenicity, known or suspected to be present in indoor dust, was established including phthalate and non-phthalate plasticizers, flame retardants, bisphenols, biocides, UV filters and other plastic additives. Of these, 29 contaminants were detected in the indoor dust samples, also including several compounds that had been banned or restricted for years. In addition, 22 chemicals were tentatively identified via non-target screening. Several chemicals have not previously been detected in Danish indoor dust. Most of the detected chemicals are known to be potentially harmful for human health while hazard assessment of the remaining compounds indicated limited risks to human. However, children were not specifically considered in this hazard assessment.

Background 1.2.Objectives 2. Approaches 3. Results and discussion 3.1.Existing indicators 3.2.Indicators for leakages along the value chain and for the lifespan of plastics 3.3.Gap analysis, recommendations and outlook 4. References 5. Annexes 5.1.Information sources to be consulted, according to Terms of References for this study 5.2.Frameworks including potential indicators, sorted by the DPSIR concept 5.3.Currently used indicators, sorted by the DPSIR concept 5.4.Macrolitter indicators and recommendations for marine litter monitoring in Europe (incl.Arctic) 5.5.Microlitter indicators and recommendations for marine litter monitoring in Europe (incl.Arctic) 5.6.Plastic use, waste generation and leakage 5.7.Gap analysis for plastic loss along the plastic value chain

Armed conflicts have, in addition to severe impacts on human lives and infrastructure, also impacts on the environment, which needs to be assessed and documented. On September the 26th 2022, unknown perpetrators deliberately ruptured the two gas pipelines Nord Stream 1 and 2 with four coordinated explosions near a major chemical munition dump site near the Danish island of Bornholm in the Baltic Sea. While the massive release of natural gas into atmosphere raised serious concerns concerning the contribution to climate change—this paper assesses the overlooked direct impact of the explosions on the marine ecosystem. Seals and porpoises within a radius of four km would be at high risk of being killed by the shockwave, while temporary impact on hearing would be expected up to 50 km away. As the Baltic Proper population of harbour porpoises ( Phocoena phocoena ) is critically endangered, the loss or serious injury of even a single individual is considered a significant impact on the population. The rupture moreover resulted in the resuspension of 250000 metric tons of heavily contaminated sediment from deep-sea sedimentary basin for over a week, resulting in unacceptable toxicological risks towards fish and other biota in 11 km 3 water in the area for more than a month.

This review summarizes the current state of knowledge regarding the risk assessment of plastic-associated residual additives, i.e. residual monomers, degradation products and additives, in the marine environment, also considering effects of weathering and bioavailability. Experimental studies have found a number of organic and metal additive compounds in leachates from plastics, and the analysis of weathered plastic particles, such as polyethylene, polypropylene and polystyrene particles sampled on beaches and shorelines, has identified residual additives, such as flame retardants, plasticizers, UV stabilizers and antioxidants. While the transfer of e.g. PBDEs to organisms upon ingestion has been demonstrated, studies on uptake and bioaccumulation of plastic-associated chemicals are inconclusive. Studies on hazard and risk assessments are few, and focus on monomers and/or a limited number of high concentration additives, such as phthalates and flame retardants. The risk assessment results vary between low, moderate and high risks of specific additives, and are not necessarily consistent for the same compound. Given the large number of chemicals potentially introduced into the marine environment with plastic particles and the challenges associated with the correct quantification of exposure concentrations and toxicity thresholds, the question arises whether new risk assessment concepts may be needed.

We propose a conceptual model that describes the in situ formation of androstenedione in agricultural soil from a phytosterol, β-sitosterol, released after crop harvest and soil fertiliser amendment. Based on the recorded agricultural practice at a spring barley field, β-sitosterol and androstenedione concentrations were modelled over the year. While decomposition of crop residues created low soil levels, the application of pig slurry led to an androstenedione soil concentration of 54 μg kg-1. The elevated soil concentration of androstenedione is not due to the introduction of the endocrine disruptor in the fertiliser, but a result of the addition of large concentrations of β-sitosterol as a natural precursor. The limited available data on β-sitosterol and androstenedione concentration in soil prohibited their accurate prediction by our model. However, the potential implication of endocrine-disrupting steroid hormones being formed in situ from currently little considered phytosterols justifies a conceptual description and further research.

A risk assessment framework for direct exposure of residual additives and monomers present in ingested plastic particles, including microplastics, in the Danish marine environment, was presented. Eight cases of different polymer types and product groups were defined that represent the most significant exposures, and thus potential high-risk cases, towards marine organisms. Risk Quotients (RQ) were calculated for three trophic levels, i.e. pelagic/planktonic zooplankton: copepod, benthopelagic fish: Atlantic cod and seabird: northern fulmar. European and Danish Environmental Quality Standard (EQS) values were used as Predicted No-Effect Concentrations (PNEC). RQ larger than unity, indicating potential risks, were found for copepod and cod (pelagic community) and the flame-retardant pentabromodiphenyl ether (PeBDE) used in polyurethane (PUR), the biocide tributyltin (TBT) present as impurity in polyvinylchloride (PVC) and PUR, and the flame-retardant hexabromocyclododecane (HBCDD) used in expanded polystyrene (EPS). A potential risk was found for fulmar (secondary poisoning) and PeBDE used in PUR.

Indoor AirVolume 30, Issue 3 p. 373-376 EDITORIALFree Access The past, present, and future of indoor air chemistry Gabriel Bekö, Corresponding Author Gabriel Bekö gab@byg.dtu.dk orcid.org/0000-0001-6107-8336 International Centre for Indoor Environment and Energy, Department of Civil Engineering, Technical University of Denmark, Kgs. Lyngby, Denmark Correspondence Gabriel Bekö, Technical University of Denmark, Nils Koppels Allé 402, 2800 Kgs. Lyngby, Denmark. Email: gab@byg.dtu.dkSearch for more papers by this authorNicola Carslaw, Nicola Carslaw orcid.org/0000-0002-5290-4779 Department of Environment and Geography, University of York, York, UKSearch for more papers by this authorPatrik Fauser, Patrik Fauser Department of Environmental Science, Århus University, Roskilde, DenmarkSearch for more papers by this authorVioleta Kauneliene, Violeta Kauneliene Faculty of Chemical Technology, Kaunas University of Technology, Kaunas, LithuaniaSearch for more papers by this authorSascha Nehr, Sascha Nehr European University of Applied Sciences, Brühl, GermanySearch for more papers by this authorGavin Phillips, Gavin Phillips Faculty of Science and Engineering, University of Chester, Chester, UKSearch for more papers by this authorDikaia Saraga, Dikaia Saraga National Center for Scientific Research "Demokritos", Athens, GreeceSearch for more papers by this authorCoralie Schoemaecker, Coralie Schoemaecker orcid.org/0000-0001-7016-9432 Physicochimie des Processus de Combustion et de l’Atmosphère, Université Lille, Lille, FranceSearch for more papers by this authorAneta Wierzbicka, Aneta Wierzbicka Devision of Ergonomics and Aerosol Technology, Lund University, Lund, SwedenSearch for more papers by this authorXavier Querol, Xavier Querol Institute of Environmental Assessment and Water Research, Barcelona, SpainSearch for more papers by this author Gabriel Bekö, Corresponding Author Gabriel Bekö gab@byg.dtu.dk orcid.org/0000-0001-6107-8336 International Centre for Indoor Environment and Energy, Department of Civil Engineering, Technical University of Denmark, Kgs. Lyngby, Denmark Correspondence Gabriel Bekö, Technical University of Denmark, Nils Koppels Allé 402, 2800 Kgs. Lyngby, Denmark. Email: gab@byg.dtu.dkSearch for more papers by this authorNicola Carslaw, Nicola Carslaw orcid.org/0000-0002-5290-4779 Department of Environment and Geography, University of York, York, UKSearch for more papers by this authorPatrik Fauser, Patrik Fauser Department of Environmental Science, Århus University, Roskilde, DenmarkSearch for more papers by this authorVioleta Kauneliene, Violeta Kauneliene Faculty of Chemical Technology, Kaunas University of Technology, Kaunas, LithuaniaSearch for more papers by this authorSascha Nehr, Sascha Nehr European University of Applied Sciences, Brühl, GermanySearch for more papers by this authorGavin Phillips, Gavin Phillips Faculty of Science and Engineering, University of Chester, Chester, UKSearch for more papers by this authorDikaia Saraga, Dikaia Saraga National Center for Scientific Research "Demokritos", Athens, GreeceSearch for more papers by this authorCoralie Schoemaecker, Coralie Schoemaecker orcid.org/0000-0001-7016-9432 Physicochimie des Processus de Combustion et de l’Atmosphère, Université Lille, Lille, FranceSearch for more papers by this authorAneta Wierzbicka, Aneta Wierzbicka Devision of Ergonomics and Aerosol Technology, Lund University, Lund, SwedenSearch for more papers by this authorXavier Querol, Xavier Querol Institute of Environmental Assessment and Water Research, Barcelona, SpainSearch for more papers by this author First published: 25 April 2020 https://doi.org/10.1111/ina.12634AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat “In developed countries, we spend 80%-90% of our time indoors” is the opening sentence for most grant applications and publications in our field. But has this well-worn truism lost its impact? We know that the majority of our exposure to air pollution occurs indoors, so why has indoor air quality not received the attention it deserves and how do we as a community better communicate this message? Recently, there has been increasing interest from experts from a wide range of backgrounds, including outdoor air quality scientists. They have enhanced our community, sparking a rapid evolution in the measurement technology used indoors, and the number, diversity, and novelty of findings. The INDoor AIR POLLution NETwork (INDAIRPOLLNET) was recently supported by the European Cooperation in Science and Technology (COST), following submission of a proposal that began with the sentence highlighted above. It consists of ~200 participants from 38 countries, comprising both scientists and practitioners in chemistry, biology, aerosol characterization, toxicology, exposure, emissions and chemical risk assessments, material design, building physics, civil engineering, and standardization. Over a four-year period, INDAIRPOLLNET will address the current state of indoor air pollution, with emphasis on indoor air chemistry (IAC), including the associated research needs, challenges, and ways to address them. A liaison with the International Organization for Standardization (ISO) will facilitate the transfer of this scientific knowledge to practice. Increasing climate change awareness is driving rigorous energy efficiency measures with buildings becoming more airtight, though adverse health effects can be associated with lower ventilation rates.1 Air pollutant concentrations are often higher indoors than outdoors, particularly following activities such as cleaning, cooking, and smoking.2 More than two million healthy life years are lost across Europe because of indoor air pollution, including indoor exposure to outdoor pollutants, indoor combustion sources, moisture, and emissions from building materials and consumer products.3 There are vast differences in building types and uses, occupant behavior, geographic locations, ventilation systems, and indoor and outdoor sources. Chemical processes indoors and their relation to those occurring outdoors must be well-understood, in order to extrapolate results to a wider range of buildings and locations than in the relatively few, in which measurements have been made. In its first year, INDAIRPOLLNET used seven subgroups to mine recent literature to summarize what existing measurement and model studies reveal about IAC. The research priorities from the subgroups are now presented. 1 CHEMICAL TRANSFORMATIONS Indoor chemical processes have been identified by reviewing laboratory, field, and modeling studies both indoors and, where relevant, outdoors. The relative importance of chemical processes indoors differs to outdoors because of higher surface-to-volume ratios, and differences in pollutant sources and dispersion, light characteristics, and temperature and humidity profiles.4, 5 The following priorities were identified: Assess the potential for indoor chemistry in the gas-phase, with emphasis on the following: Role of photolysis (via attenuated solar and artificial light sources), Comparison of the relative importance of OH, Cl, NO3, and O3 as oxidants, the conditions where each dominates and routes to formation, Contribution of different reaction pathways to the formation of secondary products, and Competition between chemical reactions and ventilation rate. Advance knowledge on the abundance of organic trace constituents in indoor air and the identification of new constituents through total OH reactivity measurements compared with measurable individual trace gas measurements. Investigate the impact of solid- and liquid-phase processes indoors (eg, reactivity in the liquid-phase indoors, including acid–base chemistry). Parameterize heterogeneous processes (production yields of secondary species such as aldehydes and HONO) for representative indoor surfaces and in real environments. 2 BUILDING MATERIALS, HOUSEHOLD PRODUCTS, AND OCCUPANT BEHAVIOR Building and household products (eg, wood-based building materials, paints, varnishes, cleaning and personal care products, air fresheners, combustion appliances, electronic appliances, furniture, carpets, and toys) as well as occupant activities (eg, cooking, smoking, and cleaning) can contribute significantly to indoor air pollution. A comparison of 13 labeling schemes for construction products worldwide has identified 15 lists of target compounds, with 611 individual chemicals occurring on at least one of the lists.6 Indoor surfaces may act as both sources and sinks of gas-phase air pollutants; there is increasing interest in secondary pollutant emissions following surface interactions indoors.5, 7 The following are the identified research priorities: Improve the characterization of pollutant emissions, deposition, and chemical transformations on various indoor surfaces for the identification and development of building and furnishing materials for better indoor air quality. Design field studies with a particular focus on the role of semivolatile organic compounds (SVOCs) and their reaction products in surface chemistry and related health effects. Disentangle the role played by humidity (potential impact on aqueous chemistry) versus reactive species (eg, chlorine) in surface reactivity. Study the impact of building construction, location, and operation on IAC. 3 OCCUPANTS Humans emit a range of organic compounds from sweat and sebaceous secretions from skin, breath, and intestinal gases. The compounds associated with the presence of humans contribute 40%-57% of the volatile organic compound (VOC) concentrations indoors in daytime.8, 9 Studies on occupant-related chemical transformations have mainly focused on ozonolysis of squalene, a major skin oil constituent.10 The following research priorities were identified: Investigate the personal and environmental factors that influence human emissions (eg, diet, stress level, personal hygiene, age, sex, health condition, activity level, personal care products, clothing, and its laundering). Study the inter- and intrapersonal variability and the influence of prior exposure on human dermal and oral emissions and their reactive capacity. Perform real-time measurements of OH, NO3, Criegee intermediates, and other short-lived, highly reactive species in occupied and unoccupied indoor environments. Investigate the influence of occupancy on indoor surfaces as well as on the composition of airborne particles. 4 MICROBIAL ACTIVITY Microbial life is ubiquitous within buildings11 and is a frequent cause of indoor air quality problems and health effects.12 Microbial colonies such as molds produce a wide variety of VOCs through their metabolisms, which encompass a range of functional moieties.13 The behavior of analogous microbes in the ambient atmosphere suggests that their indoor counterparts are chemically and photochemically active and are likely to affect chemistries both in the gas-phase and on surfaces, including on aerosols. Future research priorities include the following: Determine the relative VOC load from microbial activity and the impact on IAC in an “ordinary dwelling” compared to a “problem dwelling” with similar occupancy. Determine the influence of species variety indoors and of the balance between the building biome and the occupant biome on indoor air chemistry. Investigate the levels of toxic volatile emissions resulting from microbial processing of widely used building materials. Coordinate a network of well-characterized test facilities with a minimum set of controlled variables in conjunction with a coupled indoor chemistry/dynamic model framework, in order to address the potential variability due to geographic differences in species, indoor environments, and emission factors. Individually study microbial activity by the use of standardized coupons whereby known quantities of microbes are introduced into well-characterized realistic environments and the marginal effects of the microbial activity are measured. 5 PARTICLES Airborne particles form an integral part of IAC, as dynamic changes between gas-phase and particle-phase take place continuously.14 Particles in indoor air are influenced by both physical and chemical processes, which change their physical characteristics, chemical composition, and concentrations.15 Recent studies based on real-time aerosol mass spectrometry have brought novel understanding of chemical transformations taking place indoors. Future studies are recommended in the following domains: Study the factors influencing SVOC uptake on particles (eg, chemical composition, number size distribution, surface area, and environmental parameters). Determine size-resolved particle- and gas-phase chemical emission factors and mass spectra/signatures for specific indoor sources under controlled laboratory settings with parallel use of proton-transfer-reaction mass spectrometry and aerosol mass spectrometry. Assess the oxidative reactivity of particles from specific sources and mixtures under laboratory conditions and compare with measurements in occupied real indoor environments. Study the change in physicochemical characteristics of particles upon infiltration and the transformations that occur when outdoor and indoor air pollutants in the gas- and particle-phase interact. 6 SOURCE APPORTIONMENT Receptor models apportion the measured mass of an atmospheric pollutant at a given site (receptor), to its emission sources by using multivariate analysis to solve a mass balance equation.16 Positive matrix factorization (PMF), chemical mass balance (CMB), and principal component analysis (PCA) are among the receptor models most frequently used. While the major sources of indoor air pollution have been identified, few studies have attempted to estimate the contribution of specific sources using such techniques, mainly because the presence of both indoor and outdoor sources, building-related mechanisms (eg, ventilation and infiltration), as well as outdoor meteorology and long-range transport of pollutants makes source apportionment challenging. Key research areas that warrant attention are as follows: The influence of IAC on source apportionment model applications, including the validity of assumptions and relevant constraints (eg, unstable source profiles over time). The dependence of receptor modeling applications on available decay rates, air exchange rates, and penetration factors. Improved estimates of the outdoor contribution in source apportionment of indoor pollutants. Perform source apportionment studies based on datasets obtained with real-time measurements with very short time resolution (eg, aerosol mass spectrometer and PTR-MS). Define basic guidelines for source apportionment in indoor environments and address reliability issues of online source apportionments based on low-cost sensor networks. 7 MODELING Indoor air pollutant measurement techniques are still unable to measure many pollutants at sufficient temporal frequency and with the required specificity in a wide enough range of buildings, to provide a broad and representative understanding of chemistry indoors. The development and use of indoor air models is, therefore, a substantial requirement for understanding IAC.17 Indoor air chemistry models need to include the important sources and sinks of pollutants within a building envelope, such as chemical reactions, material emissions and surface interactions, human activity, exchange of pollutants with outdoors, or transport of pollutants within/between different zones of a building. A number of challenges for modeling studies have been identified: Field experiments in real buildings, especially those with real-time measurements and real/simulated activities, are necessary to validate and improve models. Models often assume well-mixed air in buildings, when spatial variation within zones should be considered. Indoor air chemistry models typically use chemical mechanisms originally constructed for modeling outdoor chemistry and may lack appropriate degradation schemes. Current models typically include estimated photolysis rate indoors (although some measurements have become available recently18, 19) and often fail to consider the propagation of light from the windows throughout the indoor space. Modelers and experimentalists must work together; models help design experiments, and the experimental results can be used to improve models. The field of indoor air chemistry is moving forward rapidly, accelerated to a great extent by the Alfred P. Sloan Foundation's Chemistry of Indoor Environment program (whose Web site's homepage vividly flashes the “90%-sentence”). From the laboratory to the field, from test houses to climate chambers, and from extraordinary campaigns such as HOMEChem20 to modeling efforts such as the international MOCCIE consortium, invaluable data about indoor air chemistry and physics are being swiftly generated. Indoor air chemistry occupies an increasing share in the programs of the Indoor Air conference series and at meetings such as the recent joint conference of The International Societies of Exposure Science (ISES) and Indoor Air Quality and Climate (ISIAQ) in Kaunas, Lithuania. But as it often is the case in science, new answers generate new questions and we seem to have lots of them. Such new questions are to be welcomed. As Albert Einstein noted, “To raise new questions, new possibilities, to regard old problems from a new angle, requires creative imagination and marks real advance in science.” As we continue to address the unknowns of indoor air chemistry and allow our scientific curiosity to generate further insights, we should remember that we ultimately strive not only for understanding, but especially for healthier indoor environments. ACKNOWLEDGMENTS We thank those participants of INDAIRPOLLNET's Working Group 1, who actively contributed to the work, which resulted in this editorial. These participants are Elena Gomez Alvarez, Noel J. Aquilina, Steigvile Bycenkiene, Nuno Canha, Regina Duarte, Emer Duffy, Sebastien Dusanter, Renata Kovacevic, Mila Ródenas García, Pawel Misztal, Aleksandar Petrovski, Ana Maria Scutaru, Milena Jovasevic-Stojanovic, Kristina Plauškaitė-Šukienė, Teresa Vera, and Lenka Wimmerová. The authors gratefully acknowledge the support of the European Cooperation in Science and Technology (COST). REFERENCES 1Sundell J, Levin H, Nazaroff WW, et al. Ventilation rates and health: multidisciplinary review of the scientific literature. Indoor Air. 2011; 21: 191- 204. 2Nazaroff WW, Goldstein AH. Indoor chemistry: research opportunities and challenges. Indoor Air. 2015; 25: 357- 361. 3de Oliveira FE, Jantunen M, Carrer P, Seppänen O, Harrison P, Kephalopoulos S. EnVIE, Coordination action on indoor air quality and health effects. Final activity report; 2009. https://paginas.fe.up.pt/~envie/finalreports.html. Accessed October 22, 2019. 4Nehr S, Hosen E, Tanabe S-I. Emerging developments in the standardized chemical characterization of indoor air quality. Environ Int. 2017; 98: 233- 237. 5Weschler CJ, Carslaw N. Indoor chemistry. Environ Sci Technol. 2018; 52: 2419- 2428. 6Brown VM, Crump DR, Harrison PTC. 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Dechlorane Plus (DP) is a chlorinated flame retardant applied in parallel to or as a replacement product for regulated flame retardants. Detection of DP in environmental media all over the world in recent years necessitates the development of detailed global emission estimates for environmental model studies. Based on production, usage and disposal data two global atmospheric emission scenarios were made with a detailed geographical distribution. The total DP emission is estimated to be 0.02 t/year and 3.2 t/year in a low and high emission scenario, respectively, reflecting the uncertainties in production volumes and emission factors. The emission estimates are tested by implementation in the Danish Eulerian Hemispheric Model, an advanced chemistry-transport model. An evaluation against measurements in the Arctic from the early 2010s, considered to represent background concentrations, shows that the predicted concentration range for the high emission scenario is in line with the measured range, whereas the predicted concentrations for the low emission estimate are more than a factor of 100 lower than the measurements, rendering the high emission estimate most probable.