1. Introduction
The COVID-19 pandemic, caused by the SARS-CoV-2 virus, emerged in December 2019 in Wuhan, Hubei Province, China, and by 11 March 2020, the outbreak had spread to over 114 countries around the world, with more than 118,000 cases, rapidly affecting global health and causing an acute respiratory distress syndrome [1]. This urged the World Health Organization (WHO) to declare the infection as a pandemic, indicating a significant public health emergency of international interest [2]. The use of a facemask was one of the main preventive measures, recognized by WHO [3], implemented to control the transmission of the virus and mitigate the epidemic impact of COVID-19, particularly when involving person-to-person contact, along with practicing social distancing to reduce cross-transmission, hand hygiene and other environmental hygiene measures [4]. Since then, the facemask has rapidly become a mandatory personal protective equipment (PPE), recommended or even enforced by health authorities [2,5].
Depending on the end-user, filtration capacity, and usage context, various types of facemasks are available in the market, ranging from disposable ones, such as the surgical, or any medical-grade facemask, to filtering facepiece (FFP) respirators (e.g., FFP1, P1, FFP2, P2, N95, KN95, N99, FFP3, P3, N100), and reusable cloth or paper facemasks, made from a variety of fabrics [6]. The most widely used is the surgical facemask [7], usually intended for healthcare workers, and comprising three different polymer layers.
The large majority of disposable facemasks available on the market are made from synthetic polymers, such as polypropylene, polystyrene, polyurethane, polyacrylonitrile, and polyethylene terephthalate (known as polyester), among others. For their production, these polymers include different additives in their matrix, aiming to improve the final product quality and prevent deterioration [8]. Polymer additives can be divided into four main categories: (i) functional additives (e.g., thermal and UV stabilizers, plasticizers (e.g., phthalates), lubricants, flame retardants, biocides, and antistatic, curing, foaming and slipping agents); (ii) colorants (e.g., pigments and soluble azocolorants); (iii) fillers (e.g., clay, mica, kaolin, talc, calcium carbonate and barium sulphate); and (iv) reinforcements (e.g., carbon fibers and glass fibers) [9]. Most additives are not chemically bonded to the polymer. Only reactive organic additives, such as some flame retardants, are polymerized and are part of the polymer matrix. Other additives or chemical compounds present in plastics, like volatile (VOCs) and semi-volatile (SVOCs) organic compounds, can migrate to the environment, during the plastic use, disposal or recycling phases, and consequently, humans are exposed to them via ingestion, inhalation or dermal contact [10] (Figure 1). Some of these chemical compounds can potentially cause adverse health effects [11]. Consequently, as new facemask producers entered the market to meet the growing demand, and large-scale changes began to be implemented in production processes and materials, the need arose to develop product quality and safety control protocols, such as testing the facemask’s materials to confirm that they do not release VOCs and other compounds that have harmful effects on human health during their normal use [12].
Examples of chemical compounds of concern detected in facemasks include, for example, phthalates that are widely used in industry (e.g., in clothing, packaging, etc.), for their ability to improve the softness and flexibility of plastic materials. These are not covalently bonded to the polymer but are combined by hydrogen bonds or Van der Waals forces with the plastic matrix [13]. Thus, these are easy to leak or volatilize from plastic materials. Phthalates are a family of SVOC, known as endocrine disruptors, which can have adverse effects on human hormonal balance and development [14]. In addition, some phthalates (e.g., di(2-ethylhexyl) phthalate) and/or their metabolites are suspected to be human carcinogenic [15]. Thus, this is a safety issue that cannot be ignored, in particular for susceptible populations such as children, pregnant women, and the elderly, among others [16]. Moreover, facemasks are in direct contact with our respiratory system, i.e., humans can inhale volatile compounds from their use, which will enter the body directly through the respiratory system, potentially threatening human health [9]. Facemasks can therefore be a source of human exposure, by inhalation and dermal contact, to VOCs released from them. This aroused the concern of the general public, as many of these compounds have toxic effects on human health and the environment [17,18].
The few studies [19,20,21,22,23] that analyzed VOCs and other chemicals in different types of disposable facemasks showed a great variability of these compounds, revealing their uneven quality.
Table 1 summarizes the studies available in the literature that analyzed VOCs and other chemical compounds present in the facemasks’ materials.
For example, Jin et al. [19] identified and quantified total VOCs, reactive carbonyls, polycyclic aromatic hydrocarbons and phthalate esters in commercially available surgical facemasks, having detected a wide spectrum of these chemical compounds in the analyzed facemasks. In addition, these authors found that heating the facemasks to 50 °C for just 60 min can reduce the total VOC content by up to 80%, without losing the facemask’s filtering capacity. The US Centers for Disease Control and Prevention observed that heating a facemask to 70 °C for about 3 h did not significantly affect bacterial filtration efficiency [19]. Thus, this simple measure can serve to limit our exposure to the facemask’s VOCs. Min et al. [20] analyzed three types of facemasks (surgical, N95 and activated charcoal) to detect phthalates (e.g., di-hexyl phthalate), showing for some of which concentrations above 10 μg/g or 200 μg/m2. Xie et al. [21] detected 12 phthalates in 56 disposable facemask samples (KN95 and other disposable facemasks for adults and children) collected from different countries, with total levels ranging from 115 ng/g to 37,700 ng/g, with estimated daily intakes from the facemasks ranging from 3.71 to 639 ng/kg-bw/day. The authors of [21] state that the estimated daily intakes of phthalates for children were approximately 4–5 times higher than those for adults. In addition, Vimalkumar et al. [23] estimated that the inhalation exposure to phthalate and non-phthalate plasticizers from the use of disposable facemasks is in the range of, respectively, 0.1 to 3.1 and 3.5 to 151 ng/kg-bw/day. Xie et al. [22] explored the occurrence and health risks of the SVOC exposure from the facemasks, showing that PAHs, UV-filters and OPFRs were detected in more than 90% of the facemasks analyzed. A total of 26 compounds were detected with total concentrations ranging from 8.83 to 9200 ng/g.
Regarding the potential health risks for humans, due to the prolonged daily use of facemasks, few studies have evaluated or addressed this problem [24,25,26,27,28,29]. Although unavoidable during the COVID-19 pandemic, the use of a facemask can cause discomfort and difficulty breathing, among other adverse effects. For instance, more exposure to the CO2 build-up from respiration, because much of the exhaled air (typically composed of 15.3% oxygen, 4.2% carbon dioxide, 6.2% water vapor and 74.3% nitrogen) accumulates in the space under the facemask, due to poor ventilation [25]. Individually, 3 of the 30 models of FFP respirators tested by Sinkule et al. [29] produced an average inhaled CO2 concentration above 4%, higher than the CO2 concentration in the freely exhaled air. Özdemir et al. [30] also observed that the FFP2 respirator with a surgical facemask cover used by healthcare professionals due to the COVID-19 outbreak significantly increases the end-tidal carbon dioxide (EtCO2) and the fractional inspired carbon dioxide (FiCO2) pressure. The measurements were carried out in healthcare professionals without any disease and in a resting position. Through a survey, Rosner [26] identified several adverse effects of prolonged use of N95 and surgical facemasks experienced by health professionals working on the front lines during COVID-19, such as headaches (71.4% of respondents), acne (53.1%), skin breakdown (51.0%), urticarial, contact dermatitis, lesions and rashes, impairs cognitive capacity (23.6%) and also, it interferes with vision and communication. Although most headaches are short-lived and without long-term sequelae, they can affect the occupational health and work performance of healthcare workers [27]. Regarding the temperature of the exhaled air, it depends on the ambient temperature and relative humidity (RH). For instance, at an ambient temperature of 23 °C, the temperature of the freely exhaled air is approximately 34 °C, which is almost independent of the RH, while at an ambient temperature of −5 °C and 50% RH, the temperature of the nasal exhaled air is about 23.5 °C and orally, it is 29 °C [25]. This explains the feeling of local heat, especially in hot environments, and the interference with the body’s thermal balance. Irritation of the facial tissue was also reported [31], where the surgical facemask was potentially less irritating to the facial skin than the KN95 respirator as it applies lesser forces and facilitates a faster return of facial temperatures to baseline levels.
Other adverse health effects may result from the use of a facemask, due to the user’s direct contact with the toxic chemical compounds present in the facemask’s material. For example, Fernández-Arribas et al. [24] detected organophosphate esters (OPE) in different types of facemasks used for preventing COVID-19 transmission, including surgical, self-filtering (KN95, FFP2, and FFP3) and reusable. OPE are organic esters of phosphoric acid containing either alkyl chains or aryl groups, halogenated or non-halogenated, which are used as plasticizers in many consumer products and construction materials. OPE can also result as oxidation products of phosphites that are commonly used as antioxidants in plastics. The authors of [24] detected OPEs in all facemask samples, which ranges from 0.02 to 27.7 μg/mask, with the KN95 facemasks being the ones with the highest mean concentration (11.6 μg/mask), and surgical facemasks being the ones with the smallest mean concentration (0.24 μg/mask). OPEs were the most common analytes found in the facemasks analyzed and the ones present in the highest concentration. However, other analytes were detected, including Triethyl phosphate (TEP), Triphenyl phosphate (TPHP), Triphenylphosphine oxide (TPPO), Tri-n-butyl phosphate (TNBP), Tris(2-ethylhexyl) phosphate (TEHP) and Tris(2-chloroisopropyl)phosphate (TClPP). The non-carcinogenic and carcinogenic risks of OPE inhalation estimated by the authors of [24] are low, since the amounts detected in the facemasks are below the threshold levels, making their use safe. However, it is important to note that exposure to OPE can also occur through other routes [32,33], such as inhalation, dermal absorption, dust ingestion and/or food ingestion, and in different environments. Taking this into account, the sum of all the pathways can bring the exposure values closer to the established safety limits. For example, exposure to brominated flame retardants and organophosphorus flame retardants is common to occur in subway stations [34] and other indoor environments [35], by inhaling PM2.5 in the air. Scientific studies [36] associated organophosphate compounds with altered hormone levels and decreased semen quality in men. On the other hand, the synthetic dyes and plastic additives used in facemasks manufacture are toxic and persistent in the environment [37,38], causing short and long-term negative impacts on human health, fauna and flora [28]. They are composed of a range of chemical compounds, including VOCs, with various known health effects [37,39,40]. However, their impact on human health is never or rarely considered when choosing a facemask. The most common hazard due to inhaling dye particles is respiratory problems, which symptoms include itching, watery eyes, sneezing, coughing, wheezing, and asthma symptoms [39]. However, there are still no standards or regulations that impose restrictions on these chemical compounds in facemasks. In addition, there are still few studies on the occurrence and risks of chemical exposure from facemasks [22].
In brief, since the facemask is an accessory that can be used several hours a day, through which the air we breathe passes, and which is in contact with the face skin, it must not cause discomfort to the wearer and must not contain toxic, irritating or sensitizing substances. Thus, the present study aims to discuss a proposal for a low-VOC emission label, to be assigned to facemasks, after the characterization of these compounds present in their manufacturing materials, as a precautionary measure in terms of respiratory and skin health. This label, as far as we know, is the first to assess the respiratory and dermal safety of facemasks, and it will allow the general population and in particular, vulnerable people, to take informed choices. In addition, it is presented the results of the chemical characterization of common types of facemasks materials available in the market, regarding the chemical compounds with properties of concern related to human health, using methods adapted from existing ones for another type of products. These results should be seen as examples only, as the methodology can still be refined.
2. Materials and Methods
2.1. Facemasks’ Materials Analyzed
Nine different facemasks, commercialized in Europe, were analyzed in this work, of which, 3 disposable surgical types, 3 disposable self-filtering respirator types (KN95 and FFP2) and 3 reusable cloth types.
The following two types of tests were performed for all the facemasks:
(1) Analysis of VOC emissions to assess the user exposure by inhalation;
(2) Analysis of the VOC content for evaluating the user exposure by dermal contact.
All the facemasks were certified in terms of filtering efficiency and breathability and presented the CE marking. Their characteristics are presented in Table 2.
2.2. Quality Control
The VOC analysis was subjected to tight control. Every day, a calibration solution with 15 compounds (the accredited ones) was injected in Tenax tubes, jointly with an internal standard, cyclodecane. Data from this calibration solution was plotted on a control chart to confirm the good analytical performance of the system. The tubes of the samples were also injected, together with the internal standard, before the analysis.
To note that the method for VOC analysis is accredited by EN ISO/IEC 17025 [41] for the compounds benzene, toluene, octane, ethylbenzene, 2-ethoxyethylacetate, 1,2,4-trimethylbenzene, 2-ethyl-1-hexanol, limonene, dodecane, 2-phenoyethanol, tridecane, styrene, and naphthalene.
The solvent used for VOCs standard solutions was methanol (Fisher Chemical, Loughborough, UK, 99.99%) and pure compounds of the highest possible quality were used: benzene (Sigma-Aldrich, St. Louis, MO, USA, >99.9%), toluene (Sigma-Aldrich, St. Louis, MO, USA, 99.5%), octane (Sigma-Aldrich, St. Louis, MO, USA, 99%), ethylbenzene (Fluka, Basel, Switzerland, ≥99%), 2-ethoxyethylacetate (Sigma-Aldrich, St. Louis, MO, USA, 98%), 1,2,4-trimethylbenzene (Sigma-Aldrich, St. Louis, MO, USA, 98%), 2-ethyl-1-hexanol (Fluka, Basel, Switzerland, ≥99%), limonene (Fluka, Basel, Switzerland, 98%), dodecane (Sigma-Aldrich, St. Louis, MO, USA, ≥99%), 2-phenoxyethanol (Fluka, Basel, Switzerland, ≥99%), tridecane (Sigma-Aldrich, St. Louis, MO, USA, 99%), styrene (Sigma-Aldrich, St. Louis, MO, USA >99%), tetrachloroethylene (Sigma-Aldrich, St. Louis, MO, USA, ≥99.9%), 2-butoxyethanol (Sigma-Aldrich, St. Louis, MO, USA, 99.8%), naphthalene (Sigma-Aldrich, St. Louis, MO, USA, 99.9%) and cyclodecane (Fluka, Basel, Switzerland, 95%).
2.3. Analysis of Exposure to Inhaled VOCs
To assess the user exposure to inhaled VOC emissions, it was performed an analysis based on ISO 18562-3 [42]. This standard specifies tests for the VOC emissions from the gas pathways of a medical device, its parts or accessories, which are intended to provide respiratory care or supply substances via the respiratory tract to a patient in all environments. The tests of this standard are intended to quantify the VOC emissions that are added to the breathing gas stream by the materials themselves. Those materials include, but are not limited to, ventilators, anesthesia workstations (including gas mixers), breathing systems, oxygen-conserving equipment, oxygen concentrators, nebulizers, low-pressure hose assemblies, humidifiers, heat and moisture exchangers, respiratory gas monitors, respiration monitors, facemasks, mouthpieces, resuscitators, breathing tubes, breathing system filters and Y-pieces as well as any breathing accessories intended to be used with such medical devices [42].
In the present study, to measure the VOCs emitted by the facemasks’ materials, a Personal Environmental Monitor (PEM) from SKC was used, commonly used to collect airborne particulate matter PM2.5 in 37 mm quartz microfiber filters, where the sample of the facemask filter fabrics is positioned instead of the filter. Using a personal sampling pump from Casella, the air passed through the facemask filter fabrics, and then through a Tenax® tube, where the VOCs were trapped. The airflow rate varied between 0.050 and 0.120 L/min, and the collection time was between 50 and 180 min. The PEM was positioned inside a test chamber, with temperature and relative humidity control (T = 23 ± 1 °C, RH = 50 ± 5%). A blank test, using the PEM without the facemask materials, was always performed before each facemask test. The expanded uncertainty associated with the sampling step varied between 4.2 and 4.7%. Figure 2 shows the scheme of the experimental setup and the equipment and materials used for the experimental setup, required to measure VOCs emitted by the facemask materials.
For the identification and quantification of VOCs, thermal desorption of Tenax® tubes was performed in consonance with gas chromatography (GC), coupled to a mass spectrometer detector (GC/MSD), based on the ISO 16000-6 standard [43].
The GC equipment used is from Agilent Technologies, model 7890A, and the mass spectrometer detector is also from Agilent, model 5975C. The thermal desorption system is from DANI, model TD Master. In the thermal process, the samples were desorbed at 260 °C, for 10 min.
Desorbed VOCs were first captured in a Tenax-TA-filled cold trap at −25 °C, and then, quickly heated to 300 °C to introduce analytes to the GC. Compounds were separated in an HP-5MS capillary column (length: 50 m, diameter: 0.25 mm, ID: 0.32 μm) with helium as carrier gas (purity > 99.9995%).
The quantification of the selected compounds was performed using their specific response factors. The standard solutions were prepared by weighing the pure compounds (in an analytical balance from Scaltec) and diluting them with methanol. The correlation factor of the analytical calibration curve exceeded 0.99 and the limit of detection reached 0.47 µg/m3 for toluene and 0.30 µg/m3 for benzene. With an expanded uncertainty of 4% for toluene, the analytical method was linear in the range of 10 to 5000 ng.
The total volatile organic compound (TVOC) concentration was calculated for all compounds with a concentration above 2 µg/m3, eluted between hexane and hexadecane, using the toluene response factor. To estimate the daily dose of exposure to VOCs by inhalation, the procedure presented in ISO 18562-3 standard [42] was followed: the concentration calculated in µg/m3 was multiplied by the volume (in m3) inhaled during 7.5 h by an adult, assuming 70% of filtration efficiency. Considering that the breathing volume of an adult during 24 h is 20 m3 [42], a daily volume of 6.25 m3 of inhaled air can be derived, filtered by the facemask.
2.4. Analysis of Dermal Contact Exposure
To assess the skin (or dermal) exposure, the analysis of VOCs was based on a direct thermal extraction method described by VDA 278 [44]. Following the method, a stainless-steel tube compatible with the equipment of thermal desorption (STD) was filled with the material to be tested, having approximately 0.150 g of mass. The tube was subjected to thermal desorption at 40 °C, for 60 min, using a Dani STD—Sequential Tube Desorber, model Master, coupled to a GC equipment, of Agilent Technologies brand, model 7890A and an MSD—mass selective detector, also of Agilent Technologies brand, model 5975C. This way, the VOCs were analyzed by GC with a mass selective detector (GC/MSD) for the quantification and identification of their composition. The analytical method is similar to the one described in the previous section, for the analysis of inhaled VOCs.
As the objective was to assess skin exposure by dermal contact, the VOCs identified were the compounds known to be skin sensitizers [45], with a report on skin sensitization (category 1 and subcategory 1A and 1B) [45,46], i.e., substances that will lead to hypersensitivity or an allergic response following skin contact. These substances can be classified as skin sensitizers if there is evidence, from experimental tests, or in humans, that they can lead to sensitization by skin contact.
Based on information from the “ECHA Classification and Labeling Inventory” [45], there are on the EU market more than 14,000 substances with an indication of skin sensitization [47]. These substances have many purposes and can be found in products for a variety of reasons, such as dyes in textiles.
3. Results
3.1. Exposure to VOCs by Inhalation
The compounds analyzed in all types of facemasks were mainly alkanes and alkenes, but compounds like siloxanes and terpenes were also identified. Benzene, a carcinogen, was not detected in any product. The chromatograms obtained from the air sampled through all the facemasks analyzed, where the VOCs identified are presented, are included in the Supplementary materials.
The results of TVOC concentration, obtained for all the facemask types analyzed, are presented in Table 3. The individual compounds with the highest concentration that could be identified in each mask are also presented, considering only compounds with a concentration greater than 2 μg/m3. Note that some compounds, such as alkanes, were aggregated in a family, as their identification is not unequivocal.
Comparing all the analyzed facemask types, the calculated TVOC dose resulting from the exposure by inhalation is very diversified, with low and high values varying between 0 (in M7) and 2374 µg/day (in M6), calculated assuming a daily volume of 6.3 m3 of air breathed while using the facemask. However, they are consistently higher for the three analyzed self-filtering FFP2 respirators (M4–M6).
Considering the individual compounds, it can be observed that some of them, such as 2-isopropyl-5-methyl-1-heptanol and the alkanes C9–C16, are presented in seven of the nine facemasks studied. The compound caprolactam is only present in the three facemasks FFP2. These aspects could indicate that they are caused by a common material used, such as non-woven fabric, a material that is part of the major facemasks produced.
3.2. Exposure by Dermal Contact
Regarding the exposure to skin sensitizers by dermal contact [46], results of the VOCs analyzed for the nine types of facemasks are presented in Table 4, in terms of the mass of each compound (in nanogram, ng) per area of facemask material (in cm2).
The skin-sensitizing compounds detected in the types of facemasks analyzed are diverse. Some compounds are more common than others, such as 2-ethyl-1-hexanol and limonene, which appear in six facemasks, followed by α-pinene and decanal, which appear in five facemasks. The concentration values are also diversified, for example, M8 presents a 2-ethyl hexanol concentration value 370 times higher than M1. In addition, M1 presents a limonene concentration value 17 times higher than M3. Because it is difficult to compare the facemasks in terms of individual compounds, it was calculated the sum of the skin-sensitizing VOCs (Σ SS VOC) identified, as shown at the bottom of Table 4.
It is not possible to take generalist conclusions, but the reusable facemasks analyzed in this work (M7–M9) consistently present higher values of skin-sensitizing compounds than the disposable facemasks (M1–M6). This is explained due to the additives and dyes used in the manufacture of fabrics from which the facemasks are made.
Nevertheless, as reusable facemasks are supposed to be washed several times between uses, it is expected that the levels of these compounds will decrease. This supports the importance of warning consumers to wash the reusable facemask before the first use. Still, it was verified that facemasks M7 and M9 did not have this warning in the instructions contained in the packaging.
3.3. Exposure by Inhalation and Dermal Contact
Figure 3 presents the results in a percentage of TVOC inhalation (Figure 3a) and the sum of skin-sensitizing VOCs (Σ SS VOC) (Figure 3b), in relation to the highest value obtained for respectively the daily exposure to VOCs by inhalation (highest in mask M6) and dermal contact (highest in mask M8) to the facemask materials.
It can be seen that the results obtained for the exposure to VOCs by inhalation (TVOC) are different from those obtained for dermal exposure to the skin-sensitizing compounds (Σ SS VOC). For example, facemask M7 has no VOC above 2 µg/m3 and presents a TVOC with a null value, while by thermal extraction at 40 °C, it presents a skin-sensitizing compound (2-Propenoic acid, (1-methyl-1,2-ethanediyl) bis[oxy(methyl-2,1-ethanediyl)] ester) with a high concentration level. In addition, the results obtained for the same compound might be different depending on the method. However, it should be highlighted that the sampling methods are different and the results obtained by each method are not comparable to each other, as they are intended to assess different types of human exposure to the VOCs present in the facemasks: one by inhalation and the other by dermal contact. In addition, the skin-sensitizing VOCs are only a small part of the compounds detected.
3.4. Low Emission Label for Facemasks with a Low Risk of VOCs Inhalation
Considering that for vulnerable people, in terms of the respiratory tract, it is important to reduce the risks of chemical exposure, in this work it is proposed the creation of a low emission label or low inhalation risk label for facemasks. The proposed label will be useful to help vulnerable consumers find a facemask material more appropriate for their condition. The establishment of criteria for the attribution of this label is always debatable, but the authors of [48] followed the criteria established for the indoor air emissions by building materials, in particular the EMICODE® criteria that was introduced in 1997. The EMICODE® classification system allows consumers to compare and evaluate the emission characteristics of flooring installation and construction materials. At the same time, it can be seen as an incentive to further improve these products. EMICODE® has three different classifications for building materials: EC1 Plus, EC1 and EC2.
In the case of facemasks, it was proposed a unique classification of low emission for vulnerable people, because more than one label can confuse the consumer. Thus, the criteria for EC1 Plus, for 28 days, was the basis for the establishment of the dose:
-
TVOC: 378 µg/day (calculated from the limit of emission of 60 µg/m3),
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Benzene (Carcinogenic C1): 6 µg/day (calculated from the limit of 1 µg/m3 emission).
By applying the proposed label criteria to the facemasks studied in this work, only M1, a surgical facemask for infantile use, and M7, a reusable facemask for adult use, would receive the proposed label, as shown in Table 5. However, it should be noted that reusable facemasks will be washed and cleaned several times, so a decrease in VOC content is expected.
3.5. Information about Skin-Sensitizing VOCs in Products’ Datasheet
Skin sensitization evaluation is also a key part of the safety assessment of chemical compounds in consumer products. The facemasks, whose use has become global during the COVID-19 pandemic, deserve this caution.
Several cases of skin irritation have been reportedly associated with the use of facemasks during an extensive period [31]. However, the presence of different chemicals should be an important factor. In the work performed by Nishijo et al. [49], an assessment of consumer products based on the dermal sensitization threshold (DST) concept is proposed. The concept of the DST approach has been proposed based on the principles of the TTC (threshold of toxicological concern) concept, as a level below which there is no appreciable risk of skin sensitization, even when the sensitization potency of a chemical has not been identified [50]. It has been reported that the reactive DST value of 64 μg/cm2 would be protective against 95% of reactive chemicals [51]. Nishijo et al. [52] proposed in their work a threshold level for HPC (High potency category) chemicals, a protective value of 1.5 μg/cm2. The values obtained for the Σ SS VOC, in the order of nanograms, are far below the DST values in the order of microgram values. Nevertheless, the authors warn that the HPC DST would be protective for the induction phase of skin sensitization but not for the elicitation phase.
As it is well known, elicitation reactions in individuals already sensitized to chemicals occur at exposure levels lower than those required to induce skin sensitization. Therefore, a more extensive study on the risk assessment, concerning the chemical compounds identified, should be done to allow the proposal of a low-emission label for dermal exposure and risk of skin sensitization. However, from the present study results, it is recommended at least to include information on the content of Skin Sensitizing VOCs in the product data sheet, so that vulnerable people can opt for a facemask with a low content of these compounds.
3.6. Comparison with Other Studies
It is difficult to compare with other studies in the literature, as these are scarce and both the researched compounds and the methods used are very different from those of the present study. Only one study focused on VOCs, which was the study of Jin et al. [19], that quantified total VOCs in surgical facemasks by headspace followed by GC−MS analysis and the quantification of the inhaled VOCs, where sampling of VOCs was performed in the breathing zone of volunteers with facemasks.
Comparing the headspace test with our thermal extraction test, it should be highlighted that the temperatures of extraction were very different. In the test of Jin et al. [19], the headspace vials were equilibrated at 120 °C for 45 min before injection, while our test was performed at 40 °C for 60 min. A high temperature will increase VOC emissions, so higher levels of VOCs can be expected. In fact, the TVOC levels reported were around 0.12–36.8 μg/mask. In the present study, the objective was different, it was to quantify sensitizing VOCs identified, and to compare with DST values, so the results are presented in ng/cm2. Therefore, the results of these tests are not comparable.
Comparing the tests for quantifying inhaled VOCs, the big difference was the sampling method. Jin et al. [19] performed the sampling in the breathing zone of volunteers with facemasks and compared it with ambient concentration levels. They found for facemasks with higher VOC levels, an increase in the total VOC concentration in the breathing zone. These results are consistent with our results, which show VOC transfer from the facemask when the air passes through it. Quantitively, however, the results are incomparable again, as the results of Jin et al. [19] are presented in μg/m3, while our results are transformed in a dose, in μg/day. However, the sampling method in the present study seems to be more objective, as it does not involve people and it is realized in controlled conditions. A study realized by Cabanas-Garrido et al. [53] revealed that facemasks can retain VOCs from exhaled air, but also from the environment, by exposition of users. On the other hand, the ambient conditions, such as temperature and relative humidity, are more realistic in the case of Jin et al. [19] study. However, for regulatory purposes, the authors consider that a more controlled method better fits the objective.
4. Conclusions
In this work, it was proposed that a low-VOC emission label was assigned to facemasks that meet the VOC emission requirements. It is the first proposal of a label for facemasks to assess health safety, in terms of respiratory and dermal exposure. To receive this label, the facemasks’ materials must be characterized for the presence of VOCs and in which concentrations to guarantee that they do not cause harmful effects to the user in terms of respiratory and skin health. Despite the possible impact on human health, few studies were carried out to characterize the existence of VOCs and whether they were within their limited values in the facemasks’ materials. So, it is of great importance that, in addition to their ability to protect against viruses, facemasks are also tested for VOC emissions, as they will be used for long periods.
In this study, two different experimental methods for VOC sampling were used, one to simulate inhalation exposure, and the other to simulate dermal exposure. They were adapted from existing methods, and they can be refined, to obtain results more representative of real exposure. At this stage, they should be seen as preliminary methods. The establishment of criteria for the attribution of a label is always arguable, but the authors of this study followed the EMICODE® criteria EC1 Plus that was the basis for the establishment of the dose for TVOC of 378 µg/day and for benzene of 6 µg/day concerning respiratory safety. Nine facemasks were tested and classified according to this label. Seven facemasks failed the criteria on TVOC dose. This can pose the question if the criteria are realistic regarding the materials available in the market to produce the facemasks massively. The authors of this study think that it will be necessary to perform more tests in different facemasks to verify if the criteria are not too demanding.
Concerning dermal exposure, the authors proposed to quantify the sum of the skin-sensitizing VOCs (Σ SS VOC) identified in the different facemasks. The values obtained for the nine facemasks were in the order of nanograms, very below the DST (dermal sensitization threshold) values in the order of microgram values. However, knowing that reactions in individuals already sensitized to chemicals can occur at exposure levels lower than those required to induce skin sensitization, the authors suggest at least including in the product data sheet, information on the content of Skin Sensitizing VOCs, so that vulnerable people can opt for a facemask with a low content of these compounds. Nonetheless, a more extensive study should be done to allow for the proposal of a low-emission label for both dermal exposure and the risk of skin sensitization.
The main limitation of this study was the small universe reached, but the authors expect that the results of this study may inspire new studies and future implementation of standards and regulations regarding the chemical compounds of concern present in the facemask materials.
Conceptualization, methodology, validation, writing, review, and editing, G.V.S., A.O.M., S.D.S.M. and T.M.M. All authors have read and agreed to the published version of the manuscript.
Not applicable.
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
Footnotes
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Enlarge this image.Figure 1. Exposure to volatile organic compounds present in the facemask materials, by inhalation and dermal contact (authors own creation).
Enlarge this image.Figure 2. (a) Experimental setup and (b) equipment and materials used in the experimental setup required to measure VOCs emitted by the facemask materials (figure and photo credits to the authors).
Enlarge this image.Figure 3. Exposure to VOCs from the facemasks’ materials by (a) inhalation (TVOC) and (b) dermal contact (sum of skin-sensitizing VOCs (Σ SS VOC)).
Summary of chemical compounds analyzed in facemask’s materials.
| Type of Facemasks | Compounds | Values Obtained/Main Findings | References |
|---|---|---|---|
| Surgical facemasks. | Total VOCs, reactive carbonyls, polycyclic aromatic hydrocarbons and phthalate esters. | Detected a wide spectrum of these chemical compounds in the analyzed facemasks. TVOC between 0.12–36.8 μg/mask. | Jin et al. [ |
| Surgical, N95 and activated charcoal facemasks. | Phthalates (e.g., di-hexyl phthalate). | Some facemasks presented concentrations above |
Min et al. [ |
| KN95 and other disposable facemasks for adults and children. | Phthalates. | Total levels ranged from 115 ng/g to 37,700 ng/g, with estimated daily intakes from the facemasks ranging from 3.71 to 639 ng/kg-bw/day. | Xie et al. [ |
| KN95 and disposable facemasks | SVOCs, polycyclic aromatic hydro- carbons (PAHs), organophosphorus flame retardants (OPFRs) and UV-filter. | 26 SVOCs detected, including 10 PAHs, 12 UV-filters and 4 OPFRs. Total concentrations of SVOCs ranged from 8.83 to 9200 ng/g, with a median value of 263 ng/g. PAHs, UV-filters and OPFRs were detected in 90.6%, 96.2% and 92.5% of the mask samples, respectively. The detection frequencies of individual compound for the OPFRs were found to be generally higher than those for the PAHs and |
Xie et al. [ |
| Disposable facemasks. | Phthalate and non-phthalate plasticizers. | Inhalation exposure in the range of 0.1 to 3.1 and 3.5 to 151 ng/kg-bw/day, respectively, for phthalate and non-phthalate plasticizers. | Vimalkumar et al. [ |
| Surgical, self-filtering (KN95, FFP2, and FFP3) and reusable facemasks. | OPEs and other analytes, including TEP, TPHP, TPPO, TNBP, TEHP and TClPP. | OPEs were detected in all facemasks with amounts ranging from 0.02 to 27.7 μg/mask, being the KN95 the one with the highest mean concentration (11.6 μg/mask). and surgical facemasks, the ones with the smallest mean concentration (0.24 μg/mask). | Fernández-Arribas et al. [ |
Characteristics of the nine facemasks analyzed in this work of three different types: surgical (M1–M3), FFP2 (M4–M6) and reusable (M7–M9).
| Facemask Code | Type/User | Facemask |
Recommended |
Number |
Country of Origin |
|---|---|---|---|---|---|
| M1 | Disposable surgical type/infantile use | [Image omitted. Please see PDF.] | 4 | 1 | China/ |
| M2 | Disposable surgical type/adult use | [Image omitted. Please see PDF.] | 4 | 1 | China/ |
| M3 | Disposable surgical type/adult use | [Image omitted. Please see PDF.] | 4 | 1 | China/ |
| M4 | Disposable FFP2 type/adult use | [Image omitted. Please see PDF.] | n.a. | 1 | China/ |
| M5 | Disposable FFP2 type/adult use | [Image omitted. Please see PDF.] | 8 | 1 | Portugal/ |
| M6 | Disposable FFP2 type/adult use | [Image omitted. Please see PDF.] | n.a. | 1 | China/ |
| M7 | Reusable type/ |
[Image omitted. Please see PDF.] | 4 | 15 | Portugal/ |
| M8 | Reusable cloth type/adult use | [Image omitted. Please see PDF.] | 4 | 50 | Portugal/ |
| M9 | Reusable cloth type/adult use | [Image omitted. Please see PDF.] | 4 | 100 | Portugal/ |
n.a. = not available, information not provided by the producer.
Major VOCs identified (with concentration > 2 μg/m3) and TVOC (µg/day) concentration for nine facemasks analyzed of 3 types: surgical (M1–M3), FFP2 (M4–M6) and reusable (M7–M9).
| Compound | CAS | M1 | M2 | M3 | M4 | M5 | M6 | M7 | M8 | M9 |
|---|---|---|---|---|---|---|---|---|---|---|
| S | S | S | FFP2 | FFP2 | FFP2 | R | R | R | ||
| hexane | 110-54-3 | + | + | + | ||||||
| acetic acid | 64-19-7 | + | ||||||||
| propanoic acid | 79-09-4 | + | ||||||||
| toluene | 108-88-3 | + | ||||||||
| hexanal | 66-25-1 | |||||||||
| m/p-xylene | 108-38-3/106-42-3 | + | + | + | ||||||
| 1,2,4-trimethylbenzene | 95-63-6 | + | ||||||||
| 2-ethyl-1-hexanol | 104-76-7 | + | + | + | + | + | ||||
| 2,2′-azobis(2-methylpropionitrile) | 78-67-1 | + | ||||||||
| nonanal | 124-19-6 | + | + | + | + | |||||
| caprolactam | 105-60-2 | + | + | + | ||||||
| tetradecamethylhexasiloxane | 107-52-8 | + | + | |||||||
| 1,3-diacetylbenzene | 6781-42-6 | + | + | |||||||
| hexadecamethylheptasiloxane | 541-01-5 | + | ||||||||
| 2-ethylhexyl acetate | 103-09-3 | + | ||||||||
| 2,4-di-tert-butylphenol | 96-76-4 | + | ||||||||
| 2-isopropyl-5-methyl-1-heptanol | 91337-07-4 | + | + | + | + | + | + | + | + | |
| alkanes C9-C16 | --- | + | + | + | + | + | + | + | ||
| TVOC (µg/day) | 37.1 | 548 | 858 | 1103 | 2264 | 2374 | n.d. | 630 | 712 |
S = surgical; FFP = filtering face piece; R = reusable; n.d.= not detected; TVOC = total volatile organic compound.
Skin sensitization VOCs (ng/cm2) concentration for the nine facemasks analyzed of 3 different types: surgical (M1–M3), FFP2 (M4–M6) and reusable (M7–M9).
| Compound (ng/cm2) | CAS | M1 | M2 | M3 | M4 | M5 | M6 | M7 | M8 | M9 |
|---|---|---|---|---|---|---|---|---|---|---|
| S | S | S | FFP2 | FFP2 | FFP2 | R | R | R | ||
| hexanal | 66-25-1 | 6.73 | 1.42 | |||||||
| α-pinene | 80-56-8 | 0.06 | 2.83 | 0.57 | 0.95 | 6.12 | ||||
| benzaldehyde | 100-52-7 | 2.82 | ||||||||
| β-pinene | 127-91-3 | 0.30 | 0.08 | 0.41 | ||||||
| 5-Hepten-2-one, 6-methyl | 110-93-0 | 0.28 | 0.39 | |||||||
| β-myrcene | 123-35-3 | 0.03 | ||||||||
| octanal | 124-13-0 | 0.15 | 2.22 | 0.90 | ||||||
| 3-carene | 13466-78-9 | 2.07 | 0.23 | 3.72 | ||||||
| 2-ethyl-1-hexanol | 104-76-7 | 0.24 | 0.92 | 0.91 | 1.45 | 89.81 | 5.68 | |||
| limonene | 138-86-3 | 2.92 | 0.17 | 1.99 | 0.36 | 0.44 | 4.26 | |||
| γ-terpinene | 99-85-4 | 0.16 | ||||||||
| linalool | 78-70-6 | 0.10 | ||||||||
| nonanal | 124-19-6 | 1.55 | 1.01 | 1.09 | 11.85 | |||||
| octanoic acid, methyl ester | 111-11-5 | 1.84 | ||||||||
| 4-piperidinone, 2,2,6,6-tetramethyl- | 826-36-8 | 0.42 | ||||||||
| decanal | 112-31-2 | 1.24 | 0.86 | 0.48 | 0.78 | 0.87 | ||||
| 2-ethylhexyl acrylate | 103-11-7 | 4.47 | ||||||||
| 2-Propenoic acid, (1-methyl-1,2-ethanediyl) bis[oxy(methyl-2,1-ethanediyl)] ester | 42978-66-5 | 17.76 | ||||||||
| Σ SS VOC | 6.73 | 1.87 | 0.65 | 7.81 | 3.34 | 3.15 | 19.72 | 115.66 | 26.98 |
Σ SS VOC = sum of the skin sensitizing VOCs.
TVOC and Benzene (µg/day) concentration and application of the proposed label.
| Facemask Code | M1 | M2 | M3 | M4 | M5 | M6 | M7 | M8 | M9 |
|---|---|---|---|---|---|---|---|---|---|
| S | S | S | FFP2 | FFP2 | FFP2 | R | R | R | |
| TVOC (µg/day) | 37.1 | 494 | 858 | 1103 | 2323 | 2374 | n.d. | 630 | 712 |
| Benzene (µg/day) | n.d. | n.d. | n.d. | n.d. | n.d. | n.d. | n.d. | n.d. | n.d. |
| Low-VOC emission label | ✓ | × | × | × | × | × | ✓ | × | × |
Supplementary Materials
The following supporting information can be downloaded at:
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; Martins, Anabela O 1
; Martins, Susana D S 2 ; Mata, Teresa M 1
1 Institute of Science and Innovation in Mechanical and Industrial Engineering (INEGI), Associated Laboratory for Energy and Aeronautics (LAETA), R. Dr. Roberto Frias, 400, 4200-465 Porto, Portugal
2 Institute of Science and Innovation in Mechanical and Industrial Engineering (INEGI), R. Dr. Roberto Frias, 400, 4200-465 Porto, Portugal