Emerging indoor pollutants

Emerging indoor pollutants

International Journal of Hygiene and Environmental Health 224 (2020) 113423 Contents lists available at ScienceDirect International Journal of Hygie...

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International Journal of Hygiene and Environmental Health 224 (2020) 113423

Contents lists available at ScienceDirect

International Journal of Hygiene and Environmental Health journal homepage: www.elsevier.com/locate/ijheh

Emerging indoor pollutants

T

Tunga Salthammer Fraunhofer WKI, Department of Material Analysis and Indoor Chemistry, Bienroder Weg 54 E, 38108, Braunschweig, Germany

ARTICLE INFO

ABSTRACT

Keywords: Emerging compounds Legacy compounds Déjà vu chemicals House dust Biomonitoring

There is an increasing use of so-called emerging substances or substances of emerging concern. These terms describe, inter alia, the replacement of commonly used chemicals in formulations by supposedly less harmful chemicals. A well-known example is the shift from DEHP to higher molecular weight phthalates and later the shift from phthalates to DINCH, adipates, terephthalates, etc. Similar trends can be observed in the case of solvents and flame retardants. Over the years, new compound groups such as perfluorocarbons, UV-filters, synthetic musks, parabens, siloxanes, neonicotinoids and drug residues also appeared on the scene. Today, however, the term “emerging substances” has to be defined much more broadly as regards the indoor environment. As a result of the extensive measures for energy-related renovation, contaminated waste products such as asbestos, PCBs, PAHs and PCNs are once again forming the focus of attention as re-emerging chemicals. Many relevant compounds, in particular reaction products, were unknown until recently due to the fact, that they can only now be detected using highly sensitive methods. Furthermore, already known chemicals attract scientific and public interest through reclassification or through the derivation of indoor guideline and reference values. The classical way of monitoring emerging compounds is air and dust analysis and therefore, the spectrum of analytical techniques needs to be continuously broadened. However, there is also a demand for human biomarkers, preferably in urine. A further important aspect is the post-hoc analysis of house dust and urine samples, which are stored in environmental specimen banks. The identification and temporal tracking of emerging chemicals is thereby enabled. It is strongly recommended to take advantage of the possibilities resulting from the combination of classical interior analytics and human biomonitoring to promptly detect emerging pollutants and chemicals of concern.

1. Introduction “Emerging pollutants” is a general term that describes environmentally relevant substances and substance groups, which form the focus of scientific or political interest at a certain point in time. A definition by Farré et al. (2008) from water analytics is as follows: “emerging pollutants are defined as compounds that are not currently covered by existing water-quality regulations, have not been studied before, and are thought to be potential threats to environmental ecosystems and human health and safety.” Historically, wood preservation, the origins of which can be traced back to antiquity, has the longest history as regards the use of chemical agents. The first patent for polycyclic aromatic hydrocarbons was granted in 1888 under the name “Carbolineum”. Polychlorinated naphthalenes (PCNs) were then patented in 1923 as “Xylamon”. The 1920s marked the beginning of the synthesis and large-scale production of biocides such as pentachlorophenol (PCP), lindane, dichlorodiphenyltrichloroethane (DDT), pyrethroids, etc. for outdoor and

indoor applications (Unger et al., 2001). The modern synthesis chemistry of the 20th century also created the conditions enabling the largescale production of flame retardants and plasticizers. It is often the case - examples hereby include polychlorinated biphenyls (PCBs) and polybrominated biphenyl ethers (PBDEs) - that a large number of congeners are involved. Furthermore, many substances are only of a technical quality and, as in the case of pentachlorophenol, are contaminated with other components. The industrial synthesis of formaldehyde had been possible since as early as 1867; it was not until the introduction of phenolic and urea resins, however, that the substance achieved real market significance (Walker, 1964). With regard to indoor areas, the work of Seifert (2002) offers a historical outline concerning periods of increasing and decreasing interest in specific substances up to the year 1850. Over time, it has been established that many of the substances and compounds used in indoor and outdoor applications are not only released but also exhibit ecotoxic and/or human-toxic properties (Sterner, 1999). Asbestosis had already been diagnosed as a disease at the end of

E-mail address: [email protected]. https://doi.org/10.1016/j.ijheh.2019.113423 Received 26 July 2019; Received in revised form 19 November 2019; Accepted 19 November 2019 1438-4639/ © 2019 The Author. Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

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the 19th century. The first publication on the release of formaldehyde from wood-based materials was published by Wittmann (1962). An early detection of 2-diethyl hexyl phthalate (DEHP) in human blood plasma was performed by Piechocki and Purdy (1973). As a result, the use of many substances, which sometimes came into the administrative focus decades after their market launch, was banned over the years; at the same time, the search for environmentally friendly and less-problematic alternatives began. This in turn led to constantly changing compositions of organic and inorganic substances in air and dust. The works of Weschler (2009) and Sundell (2017) provide good summaries for the period from 1948 to 2017. Human biomonitoring can also be implemented in order to track internal exposure to many indoor-relevant chemicals over specific periods of time (Salthammer et al., 2018). Since 1991, there has been an ongoing discussion concerning the fact that chemical substances react chemically in indoor areas and thereby form secondary products, some of which are themselves reactive or thermally unstable and can therefore be barely analytically detected or only with correspondingly high outlay (Weschler, 2011). For such secondary products, Weschler (2001) coined the term “stealth indoor pollutants”. For some time now, the terms emerging pollutants and chemicals of concern have also been established in indoor science. Initially, this was understood to mean newly synthesized or previously unused substitutes for plasticizers and flame retardants, which became visible in house dust and human biomonitoring following their market launch. This paper shows, however, that emerging pollutants have a much broader scope. Consequently, already known materials, nanoparticles and substances from new technologies are also included in the discussion here. The short historical overview of indoor pollutants showed that the topic is highly dynamic. The categorization of a compound as an emerging pollutant or a classical pollutant may change quickly. Consequently, the list of chemical and technologies is not exhaustive. The literature research essentially represents the situation in industrialized areas and covers published work until August 2019.

which have been present in indoor areas for quite some time, but which can only be detected using sensitive instrumental methods and which were therefore only made available for discussion and evaluation at a late stage. One example includes reaction products of skin oils with ozone, which have been identified in the ambient air through proton transfer reaction mass spectrometry (PTR-MS). Chemicals with new assessments. From time to time, substances are reevaluated toxicologically or toxicologically justified guideline values are derived, as a result of which these substances receive increased attention. The interest in formaldehyde (certainly not an emerging chemical), for example, which was first reassessed by the IARC in 2004 and then by the EU in 2014, has never really subsided (Salthammer, 2015). Over time, many indoor related chemicals were added to the annexes (elimination, restriction and unintentional production) of the Stockholm Convention. 3. New chemicals 3.1. Substitute chemicals At the beginning of the new millennium, an intensive discussion began concerning human exposure to semi-volatile organic compounds (SVOCs) in indoor areas. Rudel et al. (2003) were able to identify substances such as phthalates, alkylphenols, parabens, polychlorinated biphenyls (PCBs), polybrominated biphenyl ethers (PBDEs) and various pesticides in the indoor air and dust from US homes. Subsequently, diverse studies were published which covered further investigations into the occurrence and distribution of SVOCs in indoor areas (Lucattini et al., 2018). With the development of analytical methods for the determination of metabolites of common SVOCs in human urine, biomonitoring became established as a routine method. In the following period, e.g. within the framework of the German Environmental Survey (GerES) and the National Health and Nutrition Examination Survey (NHANES) (Koch et al., 2017), the spectrum of possible target substances for the determination of internal exposure was significantly expanded (Kolossa-Gehring et al., 2017). Many of the SVOCs originally used in building products and consumer goods, as for example phthalates (Eichler et al., 2019), are classified as endocrine disruptors (Rudel and Perovich, 2009) or feature other hazardous properties like carcinogenicity or toxicity. Consequently, the need arose for substitute products with properties which are less harmful to health. Amongst the plasticizers, the cyclohexane dicarboxylic acid ester DINCH, introduced in 2002, is the most widely known substitute for phthalates (Crespo et al., 2007) and it has swiftly developed into one of the most important target compounds for indoor dust analysis and human biomonitoring. Further phthalate alternatives are adipates (DEHA, DnBA, DIBA, DINA), sebacates (DEHS), citrates (ATBC), terephthalates (DEHTP) (Lessmann et al., 2019) and trimellitates (TOTM) (Christia et al., 2019; Fromme et al., 2016; Schossler et al., 2011). However, phthalates were also replaced with each other (see Table 1 for compounds and Table 2 for abbreviations). Schütze et al. (2015), for example, revealed an increase in the plasticizer DPHP during the analysis of 24-h urine from the Deutsche Umweltprobenbank (German Environmental Specimen Bank). As a result of the rapidly growing market share, it generally became possible to track increasing concentrations of many of these new substances in the compartments house dust (Nagorka et al., 2011) and urine (Schütze et al., 2014; Silva et al., 2013) over time. The possibility of reanalyzing samples stored in the German Environmental Specimen Bank was hereby helpful. The increase of the DINCH metabolite OH-MINCH in human urine (see Fig. 1) since the market launch in 2002 (Kasper-Sonnenberg et al., 2019) correlates with the increase of DINCH in house dust (Nagorka et al., 2011). At the same time, decreasing concentrations were observed for classical phthalates. The decrease of the DEHP metabolite MEHP in human urine (see Fig. 1) correlates with the decreasing market share of DEHP (Koch et al., 2017). When discussing data from

2. How can “emerging pollutants” be defined? In the journal “Emerging Contaminants” (ISSN: 2405–6650) these compounds are “… defined as chemicals that are not currently (or have been only recently) regulated and about which there exist concerns regarding their impact on human or ecological health …“. However, no uniform or generally accepted definition exists for the term emerging pollutants. Instead, the interpretation is flexible, depending on the respective perspective and objective. The list of individual substances and substance groups, which are considered as emerging pollutants, varies considerably over time. In the following, various criteria are named, with the help of which these chemicals can be categorized. Deviating from the definition by Farré et al. (2008) and the journal “Emerging Contaminants”, substances are also included here which had already been used previously in indoor areas or for which a re-evaluation has been performed. New chemicals. These are substances, which have only recently been detected in indoor areas. These substances are often used as substitutes in industrial products. In environmental surveys, increasing concentrations in air, dust and human biomonitoring are usually observed. A well-known example is DINCH as a substitute product for phthalates. Alternatively, new technologies, as for example 3D printing, may also cause emissions of new chemicals. Déjà vu chemicals or re-emerging chemicals. This group consists of existing substances that were not removed during renovation measures but were instead fixed in place and later re-exposed, e.g. as a result of structural measures. Polychlorinated biphenyls (PCBs), for example, can today be once again detected in elevated concentrations in indoor areas. Chemicals that can now be measured. There are many substances 2

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Table 1 Usage and sources of emerging chemicals and chemicals of concern in the indoor environment. See text and Table 2 for abbreviations. Group Chemicals New phthalates Non-phthalate plasticizers Emerging flame retardants BPA substitutes Drugs Technologies 3D-printing E-cigarettes, e-shishas Energy saving lamps Nanosprays Indoor Chemistry Ozone/squalene reaction products Metabolites of PCP Reactive species

Table 2 Abbreviations and full names of chemicals and their substitutes.

Compounds DPHP ATBC, DEHA, DEHS, DEHTP, DINCH, DnBA, DIBA, DINA, TOTM DBDPE, TBBPA-BDBPE, BEH-TEBP, BTBPE, EH-TBB, DBE-DBCH PBB, PBT, HBB, TBX, TBCT, DDC-CO V6, RDP, IDPP, TXP, BDP BPS, BPF, BPAF Methylamphetamine, tetrahydrocannabinol (THC) Caprolactam, lactide, Irganox® 1076, siloxanes (D3D6) Propylene glycol, glycerol, glycidol, acetol, diacetyl Mercury (Hg) Silver (Ag), siloxanes, MgO, ZnO, TiO2

Abbreviation

Compound name

CAS no.

ATBC BDP BEH-TEBP BPAa BPAF BPF BPS BTBPE DBDPE DBE-DBCH DDC-CO DecaBDEa DEHA DEHS DEHTP DINCH

Acetyltributyl citrate Bisphenol-A-bis(diphenyl phosphate) Bis(2-ethylhexyl)-tetrabromo phthalate Bisphenol A Bisphenol AF Bisphenol F Bisphenol S 1,2-Bis(2,4,6-tribromophenoxy) ethane Decabromodiphenyl ethane (BDPE-209) Tetrabromoethylcyclohexane Dechloran Plus Decabromodiphenyl ether (BDE-209) Di-2-ethylhexyl adipate Di-2-ethylhexyl sebacate Di-2-ethylhexyl terephthalate 1,2-Cyclohexanedicarboxylicacid-diisononyl ester Di-n-butyl adipate Di-isobutyl adipate Di-iso-nonyl adipate Di-2-propylheptyl phthalate 2-Ethylhexyl-2,3,4,5-tetrabromobenzoate 2-Ethylhexyldiphenyl phosphate Hexabromobenzene Hexabromocyclododecane Isodecyldiphenyl phosphate Isopropylphenyl phosphate Pentabromobenzene Pentabromotoulene Pentabromodiphenyl ether (technical) Resorcinol bis(diphenyl phosphate) Tetrabromobisphenol A Tetrabromobisphenol-A-bis(2,3)-dibromopropyl ether Tetrabromo-o-chlorotoluene 1,2,4,5-tetrabromo-3,6-dimethylbenzene Tri-2-ethylhexyltrimellitate Trixylenyl phosphate Tetraekis(2-chloroethyl)-dichloroisopentyl diphosphate

77-90-7 5945-33-5 26040-51-7 80-05-7 1478-61-1 620-92-8 80-09-1 37853-59-1 84852-53-9 3322-93-8 13560-89-9 1163-19-5 103-23-1 122-62-3 6422-86-2 166412-78-8

DnBA DIBA DINA DPHP EH-TBB EHDPHP HBB HBCDDa iDPP IPP PBB PBT PentaBDEa RDP TBBPA TBBPA-BDBPE

6-MHO, 4-OPA 2,3,4,6-Tetrachloroanisole O3, NO2, OH, HO2, NO3, N2O5, HONO, HOCl, Cl2, NH3

Other pollutants of concern Corrosion inhibitors Benzothiazoles, benzotriazoles UV filters Benzophenone-3, homosalate, octocrylene, 4-MBC Synthetic musk Galaxolide (HHCB), tonalide Biocides CIT, MIT, parabens, neonicotinoids Solvents NMP, NEP Monomers MDI, TDI Particles Microplastics

TBCT TBX TOTM TXP V6

biomonitoring, however, it must generally be noted that internal exposure does not necessarily result from indoor exposure. For DEHP and other phthalates, for example, it was shown that the presence in an indoor area on average accounts for less than 10% of the total exposure (Clark et al., 2003; Wormuth et al., 2006). However, the intake of SVOCs via ingestion can be relevant in case of highly contaminated house dust (Wensing et al., 2005). A similar development towards substitutes can be observed for flame retardants. For decades, polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecane (HBCDD) were among the substances with the highest market significance. When the use of pentaBDE, decaBDE and HBCDD was largely banned, the need for alternatives arose (Covaci et al., 2011), which were described by Tao et al. (2016) as “emerging flame retardants (EFRs)”. Classical substances received the designation “legacy flame retardants (LFRs)”. Tao et al. (2016) cite DBDPE, TBBPA-BDBPE, BEH-TEBP, BTBPE, EH-TBB, DBE-DBCH PBB, PBT, HBB, TBX, TBCT and DDC-CO as the significant EFRs which can be identified in indoor air or house dust (se also Ali et al., 2011; Dodson et al., 2012; Fromme et al., 2014; Kopp et al., 2012; Newton et al., 2015; Al-Omran and Harrad, 2017; Tao et al., 2019; Reche et al., 2019). Due to the changes in manufacturing practices Sjödin et al. (2019) found a decreasing trend of PBDE concentrations in the U.S. general population, since 2005–2006. There is also a trend in organophosphate esters from LFRs to EFRs with V6, RDP, IDPP, TXP and BDP (Christia et al., 2018; Kademoglou et al., 2017). Blum et al. (2019) critically discuss the emerging role of organophosphate ester flame retardants (OPFRs) as substitutes for PBDEs. The authors argue that OPFRs do not have a reduced potential for harm compared to PBDEs. For several compounds, the results of house dust analysis are presented in Table 3. In the determination of diverse LFRs and EFRs in human serum, Cequier et al. (2015) found no correlation with the concentrations in house dust. The authors conclude that the food pathway contributes significantly more to the internal exposure

a

105-99-7 141-04-8 33703-08-1 53306-54-0 183658-27-7 1241-94-7 87-82-1 25637-99-4 29761-21-5 68937-41-7 608-90-2 87-83-2 32534-81-9 57583-54-7 79-94-7 21850-44-2 39569-21-6 23488-38-2 3319-31-1 3862-12-2 38051-10-4

Classical chemical to be substituted.

Fig. 1. Decreasing trend of mono-ethylhexyl phthalate (MEHP) concentrations (median) in human urine from 1988 to 2015. The data were taken from the German Environmental Specimen Bank. Increasing trend of OH-MINCH (cyclohexane-1,2-dicarboxylic acid-mono(hydroxy-isononyl) ester) concentrations (median) in human urine from 1999 to 2017. Before 2010, all median values were below the limit of determination (LOD). The data were taken from KasperSonnenberg et al. (2019). 3

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Table 3 Concentrations of emerging SVOCs and chemicals of concern in house dust. N = number of samples, LOD = limit of detection. For more compounds and data see the reviews by Lucattini et al. (2018) and Ma et al. (2014). Compound

N

Range (ng/g)

Median (ng/g)

Reference

ΣBTRs (US) ΣBTRs (Asia) a ΣBTHs (US) ΣBTHs (Asia) a ΣBPs (US) ΣBPHs (Asia) a

40 118 40 118 40 118

2.01 – 186 1.62 – 1980 277 – 13,800 119 – 9420 121 – 37,400 9.17 – 8950

36.2 – 1290 – 612 –

Wang Wang Wang Wang Wang Wang

et et et et et et

al. al. al. al. al. al.

BP-3 (PR China) 4-MBC (PR China) Homosalate (PR China) Octocrylene (PR China)

203 203 203 203

< LOD < LOD < LOD < LOD

5261 1889 2732 55,059

241.5 118.1 83.1 325.7

Ao Ao Ao Ao

al. al. al. al.

(2018) (2018) (2018) (2018)

BPA (Sweden) b BPS (Sweden) b

100 98

< LOD – 15,000 < LOD – 22,000

1300 240

Larsson et al. (2017) Larsson et al. (2017)

TBBPA (Germany) BTBPE (Germany) EH-TBB (Germany) BEH-TEBP c (Germany) DBDPE (Germany)

20 7 8 20 17

2.9 – 233 < 10 – 34 < 3.0 – 13.6 25 – 2274 47 – 1570

28.0 < 10 < 3.0 343 146

Fromme Fromme Fromme Fromme Fromme

EH-TBB (UK) EH-TBB (Norway) BEH-TEBP (UK) BEH-TEBP (Norway) DBDPE (UK) DBDPE (Norway) EHDPHP (UK) EHDPHP (Norway) V6 (UK) V6 (Norway) iDPP (UK) iDPP (Norway) RDP (UK) RDP (Norway) TXP (UK) TXP (Norway) BDP (UK) BBP (Norway)

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

2.5 – 32.0 < 1.3 – 9.2 18 – 234 7.9 – 426 531 – 39,221 81.8 – 1802 292 – 9172 37.1 – 4011 1.3 – 756 1.2 – 8.8 114 – 1687 6.3 – 262 < 1.8 – 3.1 < 1.8 < 1.1537 2.7 – 105.0 < 3.4 – 485 < 3.4 – 697

5.0 5.4 106 27.1 1091 686 2375 195 16.6 4.1 401 51.3 1.9 < 1.8 26.5 9.1 66.8 35.4

Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou Kademoglou

a b c

– – – –

et et et et

et et et et et

(2013) (2013) (2013) (2013) (2013) (2013)

al. al. al. al. al.

(2014) (2014) (2014) (2014) (2014)

et et et et et et et et et et et et et et et et et et

al. al. al. al. al. al. al. al. al. al. al. al. al. al. al. al. al. al.

(2017) (2017) (2017) (2017) (2017) (2017) (2017) (2017) (2017) (2017) (2017) (2017) (2017) (2017) (2017) (2017) (2017) (2017)

PR China, Japan, Korea. Converted from to ng/g from μg/g. Fromme et al. (2014) used the abbreviation TBPH for this compound.

through these substances than the dust uptake pathway. A third group, which should be discussed here, are the bisphenols. Until recently, bisphenol A (BPA) had the greatest market significance as a starting material for the synthesis of polymer materials such as polycarbonates and as an additive in plastics. Due to its properties of endocrine disruptor, the use of BPA has been restricted or banned. With BPS (bisphenol S), BPF (bisphenol F) and BPAF (bisphenol AF), substitutes with similar structural properties appeared on the market, which can today be detected in house dust (see Table 3) and urine (Gyllenhammar et al., 2017; Larsson et al., 2017; Dueñas-Mas et al., 2019). Substitution by these replacements is not uncontroversial, as they may possibly possess endocrine properties, which are similar to those of BPA itself.

which aromatic substances and nicotine are dissolved in the desired composition. The steam of e-cigarettes usually contains a broad spectrum of organic substances. These can, in part, be assigned to the composition of the liquid; in part, they are created solely through the evaporation process. These include compounds which are otherwise rarely identified in indoor air, such as glycerol, glycidol, acetol, diacetyl (which is also released from microwave popcorn (Zhang et al., 2014)), vanillin and menthol (Logue et al., 2017; Schober et al., 2014; Schripp et al., 2013; Sleiman et al., 2016). Nicotine is also clearly detectable in indoor air when e-cigarettes are consumed. With regard to indoor air quality, the question now arises as to whether the consumption of e-cigarettes releases undesired substances into the ambient air, i.e. whether, analogous to passive smoking, passive vaping also exists. In the case of electronic cigarettes, the release of substances into the ambient air takes place practically solely via the consumer's respiratory gas. Propylene glycol provides the visible vapor clouds during exhalation by forming fine and ultrafine aerosols (Schripp et al., 2013). Propylene glycol is often used for fog effects during concerts and theatre performances. It has, however, been shown that employees in the entertainment industry who are regularly exposed to propylene glycol-containing aerosols are more prone to respiratory irritation than non-exposed individuals are (Varughese et al., 2005). It is due in particular to the widespread gain in popularity of the e-cigarette that an indoor guide value was derived within a short time for

3.2. New chemicals from new technologies In the original sense, Shisha refers to the classic oriental water pipe. Today, however, this term is also often used for electronic cigarettes, socalled e-cigarettes and e-shishas, which are enjoying increasing popularity as lifestyle products (Fromme and Schober, 2015). The operating principle of an e-cigarette is very simple. A fluid known as “liquid” is evaporated through a heatable wire and then inhaled. The liquid itself is composed of the carrier substances propylene glycol and glycerol, in 4

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the previously little-noticed substance propylene glycol (Ausschuss für Innenraumrichtwerte (German Committee on Indoor Guide Values), 2017). It was then shown that the use of e-cigarettes for several hours leads to room air concentrations of propylene glycol which significantly exceed the precautionary guide value I (GV I) of 0.06 mg/m³. This should be avoided for preventive reasons alone (Schober et al., 2014). One of the most important innovations of recent years is 3D printing. Of the diverse printing technologies, FFF (Fused Filament Fabrication) is the most significant within the private sector. In this procedure, filaments made from a thermoplastic material are extruded layer-by-layer onto a surface by means of a computer-controlled nozzle. Frequently used plastics include ABS (acrylonitrile, butadiene, styrene), PLA (polylactic acid), HIPS (high-impact polystyrene), polycarbonates and polyamides (Azimi et al., 2016; Gu et al., 2019). Diverse additives also exist. The plastics can be additionally reinforced using particles (Stabile et al., 2017). The extrusion procedure requires processing in excess of the softening temperature for the respective plastic. For thermoplastics used in 3D printing, this lies above 100 °C. Many of the items released during 3D printing are well-known substances such as styrene, ethylbenzene, aliphatic aldehydes (C6 – C10), alkanes and BPA. There are, however, also new compounds: caprolactam is a starting material for the production of polyamides (see Equation (1)), lactide (2,5-dimethyl-3,6-dioxo-1,4-dioxane) is the cyclic diester of lactic acid, Irganox® 1076 (octadecyl-3-(3,5-di-tert-butyl-4hydroxyphenyl)-propionate) is an antioxidant. Furthermore, diverse flame retardants, cyclic ethers and esters of acrylic acid and palmitic acid have also been detected (Azimi et al., 2016; Gu et al., 2019). The cyclic siloxanes D3 (hexamethylcyclotrisiloxane), D4 (octamethylcyclotrisiloxane), D5 (decamethylcyclotrisiloxane) and D6 (dodecamethylcyclotrisiloxane) are well-known substance groups which have been categorized as chemicals of concern by Kolossa-Gehring et al. (2017). Currently, the situation regarding the compounds emitted during 3D printing is confusing, as new filaments are constantly being developed and offered on the market.

3.3. Indoor drug exposure – an emerging problem? The drug methamphetamine is often produced under less than professional conditions in so-called meth kitchens. The “shake and bake” method is generally used, in which ephedrine is converted into methamphetamine by means of lithium and ammonia in organic solvents (Ely and McGrath, 1990). The substance is ultimately precipitated and consumed as hydrochloride. During the reaction, by-products such as cyclohexadienyl-2-methylaminopropane (CMP) are created (Person et al., 2005) (see Equation (2)) and the reaction mixture tends to explode on contact with water or air (Kunalan et al., 2012).

Furthermore, for fear of being discovered, the operators of meth kitchens change their locations regularly. With reference to the Drug Enforcement Administration (DEA), Poppendieck et al. (2015) estimate the number of accidents in illegal meth kitchens for the years 2010–2012 at around 40,000 in the USA alone. It therefore follows that a large number of homes are contaminated with methamphetamine, its by-products and other chemicals. For the European region, statistics from both the Deutsches Bundeskriminalamt (German Federal Criminal Police Office, BKA) and wastewater analyses (Ort et al., 2014) show that large quantities of the illegal methamphetamine originate from the Czech Republic. Both methamphetamine and its hydrochloride are of low volatility. The boiling point of methamphetamine lies at 215 °C, whilst the vapor pressure of the supercooled melt is 19 Pa at 298 K. Methamphetamine is therefore barely discharged with the exchange of air and instead distributes itself preferentially on surfaces. The associated potential problems for residents were highlighted in the aforementioned study by Poppendieck et al. (2015). In experiments with the substitute N-isopropylbenzylamine (NIBA), which is not subject to the drug control laws, it was determined that three weeks after dosing the substance in a test chamber with walls made from painted gypsum plasterboard under simulated indoor conditions, a maximum of 11% was discharged again via the air. At low air humidity, the discharge was even lower. Furthermore, the back diffusion into the air could not be effectively prevented through subsequent painting of the walls. It must therefore be assumed that painted plasterboard interior walls constitute a reversible sink for methamphetamine. Further works show that in particular small children are exposed to substances such as methamphetamine in indoor areas. In addition to inhalation, oral intake following hand contact or direct oral contact with the contaminated object is a fundamental path here. In model calculations assuming an air concentration of 1 ppb, the distribution of methamphetamine in upholstered furniture, clothing and toys made from fabrics was calculated; from this, the daily intake was estimated. For the oral pathway, the calculations resulted in a daily intake of 60 μg methamphetamine per kg body weight per day. This value lies significantly above the reference dose of 0.3 μg/kg/d (Morrison et al., 2015). Also of interest is the question as to which factors influence the accumulation of methamphetamine on surfaces. Parker and Morrison

The so-called nanosprays were originally developed for impregnation and disinfection purposes. They are often underestimated as emission and exposure sources, as their utilization is usually only brief. That being said, large quantities of substances are, however, released within a short time and the application takes place close to the body. During their investigations in test chambers, Norgaard et al. (2009) were able to detect, in addition to diverse VOCs, also cyclic siloxanes and fluorinated silicon compounds. Quadros and Marr (2011) determine that through the use of nanosprays and other products, people are also exposed to nanosilver (Quadros et al., 2013). Silver is a wellknown bactericide. In addition to the acute effect on organisms (Ahamed et al., 2010), the question arises as to whether the low-dose application of silver and nanosilver in everyday products contributes towards the formation of resistant microorganisms. Losert et al. (2014) state in their work that many sprays also contain other metals and metal oxides, such as ZnO, Cu, Ca, Mg, Zn, SiO2, MgO and TiO2. Further nanotechnology-modified products with relevance for indoor areas are discussed by Vance and Marr (2015).

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(PAHs) (1991 (Tar Oil Regulation), Germany; 2015 (Consumer Products), EU). Nevertheless, many substances remain in the building fabric following renovation measures and may appear as re-emerging pollutants after a certain time. It was, for example, common practice to chemically fix pentachlorophenol and prevent its release by means of diffusion-inhibiting paints. In the course of building renovations, mostly for reasons of thermal insulation and energy saving, the problem of these so-called contaminated wastes has come to light again in recent times. The situation with asbestos is particularly urgent. The focus here is on largely unknown sources of asbestos in fillers, adhesives, sealants, plasters, paints, etc., some of which were in use until around 1995. In Germany, the “Nationaler Asbestdialog” is a forum set up in 2017 by the Federal Ministry of Labor and Social Affairs, in which pragmatic solutions are sought in order to deal with this problem. Participants include all the affected social groups (home-owners, construction service providers, employers' associations, accident insurance providers, planners, official experts). Firstly, the “asbestos dialogue” searches for possibilities for determining the extent of asbestos-containing materials used in buildings, which are to be renovated. Secondly, the types of manual processing in which significant concentrations of the fibers are released are investigated (Birmili et al., 2018). In the case of PCBs, PCNs, PAHs and PCP, the renovation measures carried out must be assessed individually. From the point of view at the time, it seemed to make perfect sense to fix the weak acid PCP (pKs 4.7) as sodium phenolate (Na-PCP) in load-bearing components. In the case of PAHs as a component of adhesives in the floor area, the surface was often simply sealed. This measure, however, has the disadvantage that the sealing is only effective to a limited extent on floors, which are inherently subject to high mechanical stress. For PCBs with a total of 209 highly volatile to low-volatile congeners, however, it was common practice to remove the primary sources and, if possible, the secondary sources. PCNs were used for a limited period of time as biocides, flame retardants and plasticizers. They can often still be found in the building fabric of older buildings. Air and dust investigations as well as biomonitoring data show that residents continue to be exposed to many types of contaminated waste (Fromme et al., 2015; Kraft et al., 2018; Morrison et al., 2018). In museums, contaminated artefacts also still present a problem (Schieweck et al., 2007; Marcotte et al., 2014; Mull et al., 2015). The energy-related renovation of existing buildings which is common practice today results in difficulties due in particular to the fact that many types of contaminated waste have not yet been recognized as such or that earlier pollutant remediation measures were insufficiently documented. Table 4 summarizes the most important déjà vu or re-emerging chemicals and their standard analytes. Political decisions are also not always comprehensible from the point of view of indoor hygiene. The EU-wide ban on filament light bulbs in 2009 significantly increased the market share of so-called energy-saving lamps, in particular compact fluorescent tubes, within a short period of time. It is undisputed that this type of lamp is considerably more energy-efficient than the filament bulb. However, compact fluorescent lamps contain 1–2 mg of mercury. If a lamp is broken, the respiratory air becomes contaminated with mercury within a very short time. The concentration of mercury in the room depends not only on the mercury content of the lamp but also on the flooring. Textile floor coverings act as reversible sinks, which initially absorb mercury and then slowly release it again (Salthammer et al., 2012). For this reason, following the amalgam discussion (Schulz et al., 2007), mercury suddenly once again became the focus of scientific and public interest (Fromme et al., 2011). Once the potential mercury problem had become known, the Umweltbundesamt (German Environment Agency) carried out extensive investigations and published guidelines for action in the event of a lamp being broken in a household. In the meantime, interest in mercury has dropped sharply again, which may be due to the successful introduction of LED lamps.

Table 4 Re-emerging chemicals (déjà vu chemicals) and their standard analytes in different environmental matrices. Chemical

Standard target analyte

Asbestos Pentachlorophenol (PCP)

ab ,

Polychlorinated biphenyls (PCBs) 209 congeners

a

Polychlorinated naphthalenes (PCNs) 75 congeners Polycyclic aromatic hydrocarbons (PAHs) d a b c d e f

c

Crysothil Krokydolith PCP + Na-PCP (house dust) PCP (indoor air) PCB-28 (indoor air) PCB-52 (indoor air) PCB-101 (indoor air) PCB-138 (indoor air) PCB-153 (indoor air) PCB-180 (indoor air) PCB-118 (indoor air) e 1-Chloronaphthalene (indoor air) 2-Chloronaphthalene (indoor air) 1,4-Dichloronaphthalene (indoor air) 1,5-Dichloronaphthalene (indoor air) Benzo[a]pyrene (house dust) Naphthalene (indoor air)

f

For biomonitoring see Apel et al. (2017). See Ad hoc AG (1997). For biomonitoring see Fromme et al. (2015). For biomonitoring see Zhong et al. (2011). See Ad hoc AG (2007). See Ad hoc AG (2013b).

(2016) investigated the influence of substances such as squalene, triglycerides, fatty acids, cholesterol and cholesterol esters, which are found in various oils and in the lipid layer of human skin. At an air concentration of 1 ppb methamphetamine, the accumulation on surfaces was intensified in particular by fatty acids. Surfaces, which are frequently touched, therefore represent a particularly effective sink for methamphetamine. The same applies to house dust, which often exhibits a high proportion of skin residues. A change is currently taking place in the legislation for cannabis products with a high content of tetrahydrocannabinol (THC). As a general trend, the use of cannabis increases, whereas cigarette smoking declines. In some countries, e.g. Canada, the possession of cannabis has been legalized. This, however, leads to questions concerning exposure through secondhand cannabis smoke (Goodwin et al., 2018; Posis et al., 2019). Sheehan et al. (2019) studied the chemical composition of cannabis smoke and identified phenolic compounds, terpenoids, fatty acids, cannabinoids, sterols and long chain hydrocarbons as major constituents. Layden et al. (2019) reported about pulmonary illness associated with the use of e-cigarettes. A total of 84% of the patients reported having used THC products in e-cigarette devices. 4. Welcome back, my friends: déjà vu chemicals Since the 1950s, the manifold use in the construction sector of chemicals with possible negative health effects on residents (Salthammer et al., 2018) has often led to problems (Bake, 2003; Heudorf and Angerer, 2001; Schieweck et al., 2007; Schnelle-Kreis et al., 2000; Schwenk et al., 2002). Over the years, the use of many health-threatening chemicals and products has been banned or restricted through national and international prohibitions. With regard to indoor spaces, the following measures were of particular importance: asbestos (2005, EU), polychlorinated biphenyls (PCBs) (2001, Stockholm Convention), polychlorinated naphthalenes (PCNs) (2015, Stockholm Convention), pentachlorophenol (PCP) (1989, Germany; 2015 Stockholm Convention) and polycyclic aromatic hydrocarbons 6

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Fig. 2. Overview of chemical reactions and reaction products in the indoor environment.

5. Advanced analytical techniques and indoor chemistry: new research directions Time and time again, compounds appear in indoor areas whose origin or source is not immediately apparent. Many of these compounds are formed under differing conditions through chemical reactions in the indoor area itself (Weschler and Carslaw, 2018) (see Fig. 2). It has been known for decades that esters in the floor area decompose through hydrolysis into acids and alcohols (Uhde and Salthammer, 2007). Furthermore, thermal (Salthammer et al., 2003) and photochemical (Salthammer et al., 2002; Salthammer and Fuhrmann, 2007) reactions also occur. The formation of volatile oxidation products (OVOCs) caused through the effect of ozone on building products has also been well researched (Weschler et al., 1992). The release of chloroanisoles, in particular 2,3,4,6-tetrachloroanisole, through the microbial decomposition of technical pentachlorophenol (PCP) (Gunschera et al., 2004) has regained attention as a result of the work of Lorentzen et al. (2016). Humans also play an important role in the formation of reaction products (Weschler, 2016). By means of proton transfer reaction mass spectrometry (PTR-MS) (Wisthaler and Weschler, 2010), were able to determine substances such as 6-MHO (6-methyl-5-heptene-2-one) and 4-OPA (4-oxopentanal) as products of the reaction of ozone with squalene. Wolkoff et al. (2013) derived human reference values for 6MHO and 4-OPA and come to the conclusion that these products would not contribute substantially to sensory irritation in eyes and upper airways in office environments. However, the detection in ambient air of many of these oxidized compounds as indicators of indoor chemistry, referred to by Weschler (2001) as “stealth chemicals”, only became possible through the implementation of sensitive methods such as PTRMS. By means of semi-volatile thermal desorption aerosol gas chromatography (SV-TAG), Kristensen et al. (2019) achieved detection of galaxolides (HHCB: 4,6,6,7,8,8-hexamethyl-1,3,4,6,7,8-hexahydrocyclopenta[g]isochromene) and homosalate (3,3,5-trimethylcyclohexyl-2-hydroxybenzoate) in a family house during normal occupancy (see Fig. 3). The utilization of tandem mass spectrometers and highresolution mass detectors is also becoming increasingly important in room-air analysis. Reactive species play a central role in indoor chemistry (Waring and Wells, 2015). Whilst ozone (O3) (Salonen et al., 2018) and nitrogen dioxide (NO2) (Salonen et al., 2019) are easily accessible analytically, other components require special analytical methods. Carslaw et al. (2017) utilized laser-induced fluorescence spectroscopy (LIF) in order

Fig. 3. Identification of specific SVOCs (a–g) (gas phase and particle bound) in a family house during normal occupancy. The yellow shaded area ( ) is the total SVOC signal. The figure was adopted (and modified) from Kristensen et al. (2019). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

to detect hydroxyl radicals (OH) and hydroperoxyl radicals (HO2). Arata et al. (2018) succeeded in the direct detection of nitrate radicals (NO3) and dinitrogen pentoxide (N2O5) by implementing cavity ringdown spectroscopy (CRDS). Gligorovski (2016) attributes the formation of nitrous acid (HONO) a central role in chemical reactions in indoor environments, in particular in the formation of OH radicals (Li et al., 2018), and designates the substance an emerging indoor pollutant. Shiraiwa et al. (2019) developed a framework of different modeling tools to describe the formation and chemistry of reactive species in the indoor environment. Zhang et al. (2019) found that short-lived reactive oxygen species (ROS) and hydrogen peroxide (H2O2) are also produced during the operation of laser printers. The chemistry of inorganic compounds had until now received little attention as regards indoor areas. Sodium hypochlorite (NaOCl), however, is a common bleaching and cleaning agent which easily transforms into hypochlorous acid (HOCl), molecular chlorine (Cl2) and other chlorinated compounds (Wong et al., 2017). Hydrogen peroxide (H2O2) is available in the EU for private users as an aqueous solution in concentrations of up to 12% by weight. Ammonia (NH3) is also a reactive inorganic substance whose role in indoor chemistry is being 7

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Fig. 4. Formation of reactive inorganic and organic species in the indoor environment. The key molecule ·OH is highlighted in blue, the other important species ·Cl, ·O2−, ·HO2 and HONO are highlighted in red. The reaction scheme of HOCl is taken from Wong et al. (2017). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

increasingly discussed (Weschler and Carslaw, 2018). Fig. 4 summarizes important reactions in the indoor environment, leading to radicals and ROS. 6. New assessments Well-known substances often come back into focus as a result of changes to their assessment, their labelling obligation, due to the establishment or modification of guideline and limit values, or through their inclusion in the Stockholm Convention lists. The formaldehyde example was already mentioned, but acetaldehyde has also been classified as a 1B carcinogen in the EU since 2019, which again makes the substance a chemical of concern. In its carcinogenic effect, acetaldehyde exhibits similarities to the threshold carcinogen formaldehyde, but is less effective (Ad hoc AG, 2013a). Although the 1B classification may be justified from a toxicological point of view, it presents a large number of practical problems, as acetaldehyde cannot be easily reduced or eliminated. It is a major component of human respiratory gas (Riess et al., 2010), is released by acetyl-containing hemicelluloses and products made with them (Fengel and Wegener, 1989) and is therefore ubiquitous in indoor air (Mandin et al., 2017; Sarigiannis et al., 2011). Acetic acid is the oxidation product of acetaldehyde. The substance has been measured in indoor air since time immemorial, but has only attracted greater attention in the museum field, as it attacks metals. Acetic acid was therefore described by Tetrault (2003) as a “key airborne pollutant”. In 2017, the German Committee on Indoor Guide Values (AIR) (Fromme et al., 2019) derived provisional sum values for C1 – C8 alkanoic acids with GV I = 0.3 mg/m³ and GV II = 1 mg/m³. This substance group thereby suddenly became interesting from the point of view of indoor hygiene, as acetic acid, just like acetaldehyde, is a natural emitter of hemicelluloses. Furthermore, it is a hydrolysis product of common esters such as n-butyl acetate, ethylhexyl acetate, etc. It has, however, been shown that under normal living conditions, i.e. with an air exchange of greater than 0.5 h−1 (Salthammer, 2019), the acetic acid concentrations only reach the range of GV I in new buildings (see Fig. 5). In addition, many older data on acetic acid concentrations in ambient air are questionable as the substance is usually overvalued when measured using thermal desorption gas chromatography/mass spectrometry (TD-GC/MS) (Schieweck et al., 2018). However, Fig. 5 also shows that among the C1 – C8 alkanoic acids acetic acid is often the main component. Propylene glycol (propane-1,2-diol) has a high production volume and has been used for many years in paints, cosmetics and air fresheners. However, a serious discussion on the substance only began as a

Fig. 5. Concentrations of formic acid, acetic acid and the sum of C1–C9 alkanoic acids in newly built prefabricated houses during occupancy. House A is manually ventilated with n = 0.06 h−1 (doors and windows closed). A1: before ventilation; A2: after ventilation; B: with ventilation system (n = 0.56 h−1); C: with ventilation system (n = 0.54 h−1); D: with ventilation system (n = 0.70 h−1). The data (unpublished) were provided by A. Schieweck, Fraunhofer WKI. GV I = 0.3 mg/m³ is the preliminary guideline value I for alkanoic acids as defined by the German Committee on Indoor Guide Values (Fromme et al., 2019).

result of the growing use of electronic cigarettes, in which propylene glycol is the main component of the liquid (see Chapter 3.2). The substance currently attracting the most attention is probably nitrogen dioxide (NO2). Diverse guideline values (long and short-term) for indoor and outdoor air have been derived for NO2 (Salonen et al., 2019). The German Committee on Indoor Guide Values (AIR) has, however, refrained from deriving a long-term guideline value as, in its view, the data situation is insufficient for this purpose (Ausschuss für Innenraumrichtwerte, 2019). Generally, exposure to NO2 in indoor areas is difficult to estimate. The entry via the outside air is dependent on the ventilation conditions, indoor sources are usually temporary, and the substance is quickly degraded in the indoor environment (Salonen et al., 2019). At present, a discussion has also flared up regarding the question as to why guideline and limit values for NO2 are different for indoor and outdoor air. This aspect as well as new results concerning the contribution of NO2 to indoor chemistry (Gandolfo 8

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et al., 2017; Gómez Alvarez et al., 2014) make the substance an emerging pollutant.

substances have long been known as indoor pollution, they have attracted renewed attention through the development of human biomonitoring methods. Microplastics are persistent particles, covering a wide range of morphological properties. The problem of microplastics in the marine environment is well-known, but very few studies deal with this group of particulate pollutants in the indoor environment. Liu et al. (2019) identified different types of microplastics in house dust. On the basis of simulations Vianello et al. (2019) concluded that humans are exposed to microplastic contamination via indoor air. These studies show that microplastics is a potential problem in the indoor environment, which requires further attention.

7. Miscellaneous For some years now, the concentrations of fluorinated compounds such as fluorotelomer alcohols, derivatives of perfluorooctane sulfonic acid (PFOS) and derivatives of perfluorooctanoic acid (PFOA) (Xu et al., 2013) as well as parabens (methylbutyl, ethylbutyl, propylbutyl, butylbutyl, isobutyl paraben) (Ma et al., 2014) have been measured in house dust. In the meantime, PFOA has been added to the Stockholm Convention's list of substances to be eliminated. Reference values for human biomonitoring were derived for these substances (Apel et al., 2017). Fromme et al. (2015) found the siloxanes D4, D5 and D6 in human blood and also classified these substances as emerging pollutants. Tang et al. (2015) identified siloxanes as the main components of emissions from students in a classroom. Among the substances also newly identified in indoor areas are carbazole and halogenated carbazole, which have been detected by Fromme et al. (2018) in the air and house dust of apartments, schools and nurseries. Benzotriazoles (BTRs), benzothiazoles (BTHs) and benzophenone-3 (BP-3) (oxybenzone) are important industrial chemicals, which are used in, amongst other things, polymers and cosmetics. The substances are today ubiquitous and have been detected in indoor air (Wan et al., 2015; Xue et al., 2017b), in house dust (Wang et al., 2013) and in human biomonitoring (Asimakopoulos et al., 2013a, 2013b). Results of house dust analysis are shown in Table 3. Please note that sampling locations and dust sampling strategies are different. Especially the sampling approach might influence exposure estimation (Al-Omran and Harrad, 2017). Furthermore, BTRs, BTHs and BP-3 have been determined in textiles, in particular clothing (Liu et al., 2017; Xue et al., 2017a), which makes the dermal uptake pathway appear relevant. Through studies of clothing contaminated with BP-3, Morrison et al. (2017) show that BP-3 is effectively absorbed through the skin. Consequently, these substances must also be included in the list of emerging pollutants or chemicals of emerging concern. Other UV filters, namely 4-methylbenzylidene camphor (4-MBC), homosalate and octocrylene (Bury et al., 2018), were detected in house dust and human urine (Ao et al., 2018). Garrido et al. (2019) determined homosalate and octocrylene as major components in skin-wipe samples. Neonicotinoids represent a relatively new group of synthetic insecticides. They replaced classical organophosphates and carbamates and the market is still expanding. Indoor residual spraying with a mixture of neonicotinoids and pyrethroids is a promising technique for control of malaria (Ngufor et al., 2017). Human biomonitoring methods for the neonicotinoids imidacloprid, acetamiprid, thiacloprid and clothianidin were developed by Baker et al. (2019). Ospina et al. (2019) found that about half of the U.S. general population three years and older had been exposed to neonicotinoids. For many years, human biomonitoring (HBM) has been a standard method for measuring the internal exposure of humans to chemical substances and can therefore, also be implemented to track emerging pollutants (Yusa et al., 2012; Alves et al., 2014). In Germany, biomonitoring methods for chemicals of concern (Kolossa-Gehring et al., 2017) and emerging substances (Apel et al., 2017) are being developed within the framework of a cooperation between the Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) and the Deutscher Verband der Chemischen Industrie (German chemical industry association, VCI). Many of the identified substances are also interesting as regards the indoor environment. In addition to the compounds already discussed, these include N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP), 5-chloro-2-methyl-4-isothiazolin3-one (CIT), 2-methylisothiazol-3(2H)-one (MIT) (Nagorka et al., 2015), 4,4'-, and 2,4′-methylenediphenyl diisocyanate (MDI) (Schulz and Salthammer, 1998), 2,4- and 2,6-toluene isocyanate (TDI) (Kelly et al., 1999) and 2,6-di-tert-butyl-p-cresol (BHT). Whilst these

8. Conclusion The situation regarding indoor pollution with organic and inorganic compounds has become more confusing over the years. On the one hand, the use-restriction or banning of substances, which are potentially harmful to health must, of course, be welcomed. For many substances, which have been known for a long time, reclassifications or guideline derivations have triggered discussions on exposure reduction. On the other hand, there are large numbers of substitutes available on the market today for which no analytical methods exist. As a result of the mostly technical quality of these substances, by-products can be expected, or the substances are mixtures of isomers. Analytical standards are often not available. Despite intensive activities, it has so far only been possible to derive human biomonitoring methods for a modest number of emerging SVOCs relevant to indoor areas (KolossaGehring et al., 2017). A further problem concerns contaminated wastes (déjà vu chemicals), which are released during the course of renovation measures. It is often not even known that these substances are still present in the building fabric. The chemical composition of indoor-air and house-dust components is today subject to considerably greater dynamics than in the past. It is therefore disadvantageous that the classical room-air and house-dust analyses are increasingly being pushed into the background in environmental surveys as a result of human biomonitoring being favored. For the identification of new substances, however, the computer-aided non-target analysis of air and dust is essentially suitable. A comprehensive study by Lucattini et al. (2018) provides an excellent summary of the current situation regarding the occurrence of SVOCs in indoor air and house dust. Furthermore, indoor exposure can only be meaningfully investigated using a combination of internal and external exposure (Salthammer et al., 2018). Acknowledgements The author gratefully acknowledges the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) for financial support. He also wishes to thank Prof. Kasper Kristensen, Aarhus University, for providing data from his publication Kristensen et al. (2019), Dr. Holger Koch, Ruhr-University Bochum, for providing data prior to publication and Dr. Alexandra Schieweck, Fraunhofer WKI, for providing data prior to publication. The author is also grateful to Heike Pichlmeier (Fraunhofer WKI) and Karen McDonald (kaledonia kommunikation) for their editorial work. References Ad hoc, AG, 1997. Richtwerte für die Innenraumluft: Pentachlorphenol. Bundesgesundheitsblatt 39, 234–236. Ad hoc, AG, 2007. Gesundheitliche Bewertung dioxinähnlicher polychlorierter Biphenyle in der Innenraumluft. Bundesgesundheitsblatt 50, 1455–1466. Ad hoc, AG, 2013a. Richtwerte für Acetaldehyd in der Innenraumluft. Bundesgesundheitsblatt 56, 1434–1447. Ad hoc, AG, 2013b. Richtwerte für Naphthalin und Naphthalin-ähnliche Verbindungen in der Innenraumluft. Bundesgesundheitsblatt 56, 1448–1459.

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