The results highlight the ubiquitous presence of microplastics in inhaled indoor air (Fig. 1). The use of the BTM as a sampling device yields an accurate local airflow at the chosen conditions. Although the simple design of the BTM cannot replicate the complexity of the branching airways of the human lung, the use of the BTM as a sampling device yields an accurate potential dose at the chosen conditions24, ensuring a natural mixing of local airstreams, and thus a truer mix for that given situation, than using a standard air sampling device25. On average, the measured MP concentration was 9.3 ± 5.8 N MP m−3, corresponding well to what Dris et al.11 reported on indoor air (a median value of 5.4 fibres m−3). However, a direct comparison between the studies is problematic, as the analytical approach they applied (manual sorting followed by ATR-FTIR vs µFTIR-Imaging) was different, as was the investigated size range and particle morphology (Dris et al. looked solely at fibres down to a major dimension of 50 µm). The highest exposure concentration (16.2 N MP m−3) was measured at Location 3 (L3S3), which corresponds to an inhalation rate of 11.3 MP per hour. At such a rate, an average male person doing light activity would potentially inhale up to 272 MP over 24 hours. Cellulose materials were similarly abundant in the inhaled air, and presumably came mainly from cotton and paper products. The most abundant material group, the protein-based materials, most likely almost entirely came from shed skin26. Looking solely at the particles of manufactured origin (MP and cellulose), MP accounted, on average, for 50%. Dris et al.11 reported a slightly different proportion between nonsynthetic and synthetic fibres (67% nonsynthetic and 33% petrochemical). The composition of the inhaled MP (Fig. 3) also differed from the indoor MP composition reported in that study, where polypropylene was the most abundant polymer, while no polyester was found.

In the present study, polyester was by far the most abundant synthetic polymer in all the samples (81%; Fig. 3). The ubiquitous presence of polyester in the inhalable indoor air can be explained by the fact that there are multiple potential sources of polyester fibres and fragments in an indoor environment. Nowadays, most cloths include this type of fibre, as do the majority of the textiles involved in furniture and carpet production. Nylon accounted for 5% of the total identified MP. Although this polymer finds fewer applications in indoor environments than polyester, nylon is still likely to be found in indoor fabrics. Polyethylene and polypropylene accounted for 6% and 2% of the total identified MP, respectively. Even though polyolefin fibres, such as polyethylene and polypropylene, are used for several applications in the textile industry27, the reported values were probably influenced by the presence of particles which originated from other sources. While it is possible to account for several sources of fragments and fibres in an indoor environment (carpets, sofas, chairs, etc.) for polypropylene, polyethylene does not find a broad range of applications in the common fibres market, being mostly used for technical textile production28 (e.g. high-performance textiles like Dyneema® and Spectra®). Therefore, polyethylene micro-particles probably originated from other sources, like micro-debris fragmenting from packaging materials or other plastic items inside the apartments. Among the polymers identified by FPA-µFTIR-Imaging at a lower percentage (<1%), it is worth mentioning the presence of polyurethane and paint (acrylic and alkyd) micro-particles. Polyurethanes (PU) constitutes a wide group of polymers with a broad range of applications. Some of the chemicals involved in PU production are considered to be harmful substances, and, as for many other additives in plastics, there is the possibility that they could be released in the environment. Moreover, several polyurethanes used in furniture are also treated with flame retardants, of which almost all are considered harmful29,30. The potential risk associated with the micro-paint particles could be derived from the organic compounds and heavy metals used as biocides in many paints31, as well as the fillers and the pigments32, all of which could potentially be released in the environment and could also lead to a direct impact regarding human exposure. The presence of airborne polyurethane and micro-paint particles, and moreover their availability to be inhaled, hence constitutes a potential tread to human health even though they occur at low concentrations.

Besides the ubiquitous exposure to MP pollution, the results also highlight a large variability in the concentration among the samples, both when considering the inter-location variations (58% for MP; 22% for nonsynthetic particles; 23% for total particles) and the intra-location variations (16–77% for MP; 25–49% for nonsynthetic particles; 24–50% for the total particles). The difference in MP concentration between the three apartments was significant (p = 0.041;) when comparing L1 and L3 (p = 0.037), but not when comparing L2 to L1 (p = 0.143) or L3 (p = 0.562) (Fig. 2). L1 also tended to have relatively fewer polyester particles and relatively more polyethylene particles than the other apartments, while L3 tended to have a larger fraction of nylon and “other polymers” compared to L1 and L2. However, the abundance of nonsynthetic particles was comparable between all apartments (p = 0.487). Differences in building materials, furniture, cleaning procedures, and activities among the apartments could explain the inter-location variations of the measured MP exposure concentration. Intra-location variations could be related to activities happening during the sampling, which could have temporarily modified the particle concentration in the indoor air. Sample preparation could be another parameter which influenced the obtained results. While probably only of minor importance, the transfer of the sample from the filtration membrane to the analytical substrate could have caused some loss of particles.

As highlighted by the dots shown below the black dashed line of Fig. 4a, most MP classified as fibres were composed of polyester (87%), followed by polyethylene (6%), polyacrylonitrile (PAN - acrylic fibre, 4%) and polypropylene (1%). The other 2% identified as MP fibres were composed of acrylic polymers, acrylic paints, ethylene vinyl acetate, and polycarbonate. Due to their polymeric composition, these latter particles were probably elongated fragments and not true fibres, as these polymers are not commonly used in textile manufacturing. Surprisingly, no nylon particles were classified as fibres. Although polyester was the most abundant polymer among the synthetic fibrous material, fibres only constituted 13% of the total amont of this polymer. A probable cause is that single fibres might have been entwined or interwoven in fragments of fabric, and so identified as particles. Moreover, polyester sources other than textiles could occur in an indoor environment, as this polymer is also widely used in packaging and plastic items manufacturing. Among the identified polymers, 50% of the PAN particles were classified as fibres, highlighting that this polymer is mainly used in the textile industry. Half of the identified MP were smaller than 50 µm, as shown by the D50 values (Table 1), confirming the presence of small MP (<50 µm) in the air compartment. MP size distributions at the three locations were statistically different for both major and minor dimensions (all p values were below 0.05).

The overall shape of the size distributions when binning particle sizes in intervals of 0.1 on a logarithmic scale (Fig. 5), showed that few particles were present in the larger size bins. The curve then peaked at some size, upon which it trailed off towards zero as particles approached the (size) detection limit. In this study, the size distribution showed that MP were most abundant at the 36 µm major dimension, while nonsynthetic materials peaked at a somewhat larger particle size (47 µm). Such size distribution is not uncommon in microplastic studies33, and it is unclear what causes the trailing off of particle counts when approaching small particle sizes. First of all, it cannot be excluded that the measured size distributions reflect the true particle size distributions. On the other hand, it is possible that sampling, sample preparation, or sample analysis introduces a systematic error when sizes become small. In the present case, the sample preparation was probably not the cause, as it was limited to transferring particles from a silver filter to an IR-transmissive window without introducing any digestion steps as otherwise is common in MP studies. Neither was the sampling itself a likely cause, as it was conducted on a 0.8 µm pore size filter, and the probability of fibres slipping through was, hence, low. The analysis itself, though, might contribute to the phenomenon. The smaller a particle, the less IR light it absorbs, resulting in a poorer spectrum, which again will result in an increase in false-negative detections. Finally, surface forces might cause a higher tendency of entanglement for smaller particles compared to larger particles, leading to several agglomerated particles being identified as one. These phenomena might also have affected the counts and sizes for the natural particles.

It is interesting to note that the distribution at Location 3 had the smallest D50 values, and at the same time the highest MP exposure concentration, while Location 1 had the lowest MP concentration, but the highest D50 for the major dimension and the second highest for the minor dimension (Table 1). A linear regression using MP exposure concentrations and the relative D50 values highlights a negative correlation between the MP concentration and the median value of the correspondent size distributions. For increasing MP concentration, a decrease of D50 was observed (major dimension: R2 = 0.702; minor dimension: R2 = 0.735). This relation was limited to the MP particles, as no correlation between the nonsynthetic particles and the D50 value was found (major dimension: R2 < 0.001; minor dimension: R2 = 0.044). A further comparison between the size parameters of the MP and the nonsynthetic particles shows that the MP inhaled by the manikin tended to be smaller than the nonsynthetic particles. The major dimension D50 of the MP was 23% smaller than that of protein-based particles, and 53% smaller than that of the cellulose-based particles (Table 1). For the minor dimensions, the ratios were 32% and 40%, respectively.

The overall median value of the MP detected using this method (D50 = 36 µm) suggests that most of the particles inhaled are likely to undergo deposition by impaction, and therefore then eliminated by the mucociciliary escalator, so that a limited number is likely to reach the deeper airways13,34. Smaller fragments and fibres (<11 µm, sub-micrometric and nanometric particles) that can enter the lower airways may also have been present in the samples, but not detectable with the instrumental parameters used in this study. Although µFTIR-Imaging spectroscopy is a suitable technique for identifying particles potentially down to a few micrometres, further investigation is required to test if an enhanced analytical sensitivity (higher magnification, better resolution, better particle separation) could provide results for even smaller particles.