Laundering in domestic washing machines is an important pathway for microfibres, both synthetic and natural, to enter into the environment. As filtration of domestic effluent at waste water treatment plants may not be effective at retaining all small fibres and particles, dependent on the type and complexity of the plant (Palacios-Mateo et al., 2021; Ziajahromi et al., 2017), accurate quantification of the microfibre discharge from domestic laundering is essential to understanding the scale of the issue. Furthermore, quantification of microfibre release from laundry is important in the creation of effective mitigation strategies to reduce this form of pollution.
To support this, a more comprehensive understanding of the textile characteristics of the fabrics being washed, the conditions in which they are laundered, and how these factors influence microfibre release is also required. The influence of testing methodologies, laundering variables and fabric characteristics are discussed.
Influence of testing methodologies
Several studies have attempted to quantify microfibre release from domestic laundering; a summary of methodologies used, important testing variables, and estimated microfibre release quantities is provided in Table 1. Direct comparison of findings between studies is challenging due to the differences in methodologies and sampling, and as a result, estimated microfibre release ranges from a few thousands to several millions of fibres per wash depending on the method used.
Table 1
Measurement of fibre loss from domestic laundering: Literature comparison
Author | Test Method | Temp. (°C) | Time (mins) | Agitation (no. ball bearings) | Liquor Ratio | Reported microfibre release |
(Browne et al., 2011) | Washing machines | 40 | Not stated (NS) | Not applicable (N/A) | NS | > 1900 particles/wash |
(Dubaish and Liebezeit, 2013) | NS | NS | NS | NS | NS | 220–260 mg/garment/wash |
(Karlsson, 2015) | Washing machines | 40 | NS | N/A | NS | 209,000 fibres/litre |
(Napper and Thompson, 2016) | Washing machines | 30, 40 | 75 | N/A | NS | Poly-cotton: 137,951 fibres/6 kg wash Polyester: 496,030 fibres/6 kg wash Acrylic: 728,789 fibres/6 kg wash |
(Pirc et al., 2016) | Washing machines | 30 | 15 | N/A | NS | 11,300 particles/500g of fabrics or 6 mg/500g fabric |
(Hartline et al., 2016) | Washing machines | 30, 40 | 30, 48 | N/A | NS | Average 1174 mg/wash |
(Hernandez et al., 2017) | Laboratory method | 25, 40, 60, 80 | 14, 60, 120, 240, 480 | 10 | 7:1 l/m² | Average approx. 0.025 mg/g |
(Sillanpää and Sainio, 2017) | Washing machines | 40 | 75 | N/A | NS | Polyester: 223,000 particles/wash or 340 mg/wash Cotton: 973,000/wash or 809 mg/wash |
(Carney Almroth et al., 2018) | Laboratory method | 60 | 30 | 25 | 13:1 l/m² | Fleece: 110,000 fibres/garment/wash Knit: 900 fibres/garment/wash |
(De Falco et al., 2018) | Laboratory method | 40, 60 | 45, 75, 90 | 0, 10, 20 | NS | Polyester: >6,000,000 fibres/5 kg wash |
(Jönsson et al., 2018) | Laboratory method | 40 | 60 | 25 | 5:1 l/m² | N/A Focus on method development and validation |
(Belzagui et al., 2019) | Washing machines | Ambient | 15 | N/A | NS | Polyester-elastane: 175 fibres/g/wash or 30,000 fibres/m²/wash Acrylic-polyamide: 560 fibres/g/wash or 465,000 fibres/g/wash |
(De Falco et al., 2019) | Washing machines | 40 | 107 | N/A | NS | Polyester: 640,000–1,100,000 fibres/garment/wash Poly-cotton-modal: 1,500,000 fibres/garment/wash |
(Haap et al., 2019) | Laboratory method | 40 | 30 | 50 | 6:1 l/m² | N/A Focus on method development and validation |
(Kelly et al., 2019) | Washing Machines and Laboratory Method | 30, 15 | 60, 15 | NS | 120:1 l/m² and 240:1 l/m² | Standard wash (with detergent): 663,523 fibres/kg/wash Delicate wash (with detergent): 1,474,793 fibres/kg/wash |
(Yang et al., 2019) | Washing machines | 30 ,40, 60 | 15 | N/A | 17:1 l/m² | Acetate: up to 74,816 fibres/m²/wash Polyester: up to 72,130 fibres/m²/wash |
(Zambrano et al., 2019) | Laboratory method | 25, 44 | 16 | 25 | 15:1 l/m² | Cellulose-based fabrics: 0.2-4 mg/g Polyester: 0.1-1 mg/g |
(Cai et al., 2020) | Laboratory method | 40 | 45 | 0, 10, 20 | 37.5:1 l/m² and 60:1 l/m² | 210–72,000 fibres/g/wash |
(Cesa et al., 2020) | Washing machines | 24 | 20 | N/A | NS | 49.8–307.8 mg/wash 18,400–69,600 fibres/wash |
(Cotton et al., 2020) | Washing machines | 25, 40 | 30, 85 | N/A | NS | Increased microfibre release for 40°C, 85 minute wash compared to 25°C, 30 minute wash |
(Dalla Fontana et al., 2020) | Washing machines | 40, 30 | 90, 43 | N/A | NS | Delicate/silk cycle: 33.86 mg/kg Cotton cycle: 40.19 mg/kg |
(De Falco et al., 2020) | Washing machines | 40 | 107 | None | NS | Poly-cotton: 3898 fibres/g/wash Polyester: 709–1747 fibres/g/wash |
(Frost et al., 2020) | Laboratory method | 20 | 16 | 25 | NS | Reduced shedding for 70% recycled polyester compared to 40% polyester (not significantly different from virgin polyester) |
(Galvão et al., 2020) | Washing machines | 20, 30, 40, 60 | NS (automatic) | N/A | NS | Average total MF per wash: 297,400 MF/l (83% from cotton fibres) Estimate for 6 kg wash: 18,000,000 MF/6 kg synthetic fibres 1842–6259 mf/g/wash |
(Kärkkäinen and Sillanpää, 2020) | Washing machines | 40 | 75 | N/A | NS | First wash: 100,000–6,300,000 fibres/kg/wash Fifth wash: 19,000–190,000 fibres/kg/wash |
(Lant et al., 2020) | Washing machines | 40, 15 | 85, 30 | N/A | NS | Approx. 357 mg/3.13 kg wash 96% released fibres = natural (cotton, wool, viscose), 4% = synthetic (acrylic, nylon, polyester) |
(Praveena et al., 2020) | Washing machines (household study) | Room temperature | 41–60 | N/A | NS | 0.0069–0.183 g/m³ Average: 0.068 g/m³ |
(Celi̇k, 2021) | Washing machine | 40 | 53 | N/A | NS | 3.28–3.91 mg/l/wash 13.1–15.66 mg/kg/wash |
(Choi et al., 2021) | Washing machine | 40, 20 | 80 | N/A | NS | Hard twist filament: 51.6 ppm Non-twist filament: 88.7 ppm Spun yarn: 107.7 ppm |
(Dalla Fontana et al., 2021) | Washing machine | 40 | 90 | N/A | NS | 38.6–67.9 mg/kg |
(Özkan and Gündoğdu, 2021) | Laboratory method | 40 | 45 | 10 | 37.5:1 l/m² | Recycled polyester: 368,094 fibres/kg/wash Virgin polyester: 167,436 fibres/kg/wash |
(Periyasamy, 2021) | Washing machine | 30, 45, 60 | 60, 75, 90 | N/A | NS | 2,305,395–4,874,323 fibres/kg/wash |
(Raja Balasaraswathi and Rathinamoorthy, 2021) | Laboratory method | 30 | 45 | 10 | 6.67:1 l/m² | Open edge: 72.37 fibres/cm²/wash or 0.0239 mg/cm²/wash Finished edge: 18.07 fibres/cm²/wash or 0.0034 mg/cm²/wash |
(Rathinamoorthy and Raja Balasaraswathi, 2021) | Laboratory method | 30 | 45 | 5, 10, 15 | 2.22:1 l/m², 4.44:1 l/m², 6.67:1 l/m², 8.89:1 l/m² | Machine wash: 18.06 fibres/cm² Gentle hand wash: 17.33 fibres/cm² Intense hand wash: 23.7 fibres/cm² |
(Tiffin et al., 2021) | Laboratory method | 40 | 45 | 50 | 15:1 l/m² | N/A Focus on method development and validation |
(Vassilenko et al., 2021) | Washing machine | 41 | 18 | NA | 136.4:1 l/kg | 8,809 – >6,877,000 fibres/wash 9.6–1,240 mg/kg/wash |
(Zambrano et al., 2021) | Laboratory method | 25 | 16 | 25 | NS | 9,000–14,000 particles/g/wash Softener/durable press treated cotton: 1.30–1.63 mg/g/wash Untreated cotton: 0.73 mg/g |
Several important methodological factors can influence the quantity of microfibre release from a sample in these laboratory tests, including but not limited to: the type of laboratory method, for example, testing in a domestic washing machine or using a simulated laundering device (Gyrowash or similar); the means of filtering the test liquor, including the filtration method and the type and retention efficiency of the filters themselves; and the metric for quantification, by fibre count or by fibre mass (Tiffin et al., 2021).
Comparison of quantifications according to testing methodology
To explore the influence of methodological approaches from different studies on the quantification of microfibre release, the results provided in Table 1 were standardised to either fibres/kg or mg/kg of release to allow comparison. Only results which were expressed as per unit mass were included in this comparison.
An estimation for the annual microfibre release for the UK was calculated for each method using the following assumptions; the average UK wash load is approximately 5 kg, and the average UK household completes 260 wash loads each year (Webber et al., 2016). With 27.8 million households in the UK, according to the Office for National Statistics (ONS) (Sanders, 2019), this equates to 7.228 billion wash loads annually.
The estimated microfibre release counts are shown in Fig. 2. Multiple data points are provided where authors offered multiple microfibre release results for different conditions as detailed in Table 1. Estimates range from 17,167 billion microfibres (originally reported as > 1900 particles released from a 4 kg wash load (Browne et al., 2011)) to 2,602,080 trillion fibres (originally reported as 72,000 fibres/g (Cai et al., 2020)) discharged annually from domestic laundry in the UK. The mean of these estimates of annual UK release is 155,097,349 billion microfibres and the range is 2,602,063 trillion fibres. Interestingly, there is an upward trend of the microfibre count reported with more recent studies.
The estimates for annual mass of microfibre released for UK are shown in Fig. 3, with multiple data points provided where authors offered multiple microfibre release results in their original studies. The lowest estimate is 347 tonnes (originally reported as 9.6 mg/kg (Vassilenko et al., 2021)), while the largest mass estimate is 144,560 tonnes (originally reported as 4 mg/g (Zambrano et al., 2019)). The mean estimated mass of annual microfibre release from domestic laundering is 17,234 tonnes and the range is 144,213 tonnes. For comparison, the UK disposes of approximately 350,000 tonnes of clothing of all fibre types to landfill each year (Welden, 2019).
There is a huge variation in the estimates for the number and the mass of microfibres being reported. It is clear from this analysis that the discrepancies between methods do not provide confidence in the current estimates for microfibre release from laundering, and there is a clear need for a standardised reliable method for estimating microfibre release. This is particularly important for assessing the impact of mitigation strategies to reduce microfibre pollution from domestic laundry.
Influence of laundering variables
There are also a number of important laundering variables to consider which can influence microfibre release. These include wash temperature, wash duration, liquor ratio, washing agitation, and the presence of detergent or other washing products.
The impact of temperature on microfibre release has been explored through several studies with mixed results. Some studies found wash temperature had no significant effect on microfibre release (De Falco et al., 2018; Hernandez et al., 2017; Kelly et al., 2019) while others reported greater release for higher wash temperatures (Cotton et al., 2020; Napper and Thompson, 2016; Periyasamy, 2021; Yang et al., 2019; Zambrano et al., 2019)). This divergence in results may be explained by the fact studies used different textile materials, with different thermal properties, for testing.
Most studies found wash duration had little or no significant influence on microfibre release (De Falco et al., 2018; Hernandez et al., 2017; Kelly et al., 2019). Kelly et al. (2019) found that the same quantity of fibres were released during a 15 minute express wash as for a 60 minute wash, which they suggested might indicate that the majority of microfibre release occurs in the first 15 minutes of the wash cycle.
The level of agitation and friction within the washing process is considered to be a major factor affecting the quantity of microfibre released. Agitation is a function of drum size, drum configuration, rotational speeds, and wash liquor volume to load ratio. Kelly et al. investigated the impact of agitation by considering the effect of rotation speed and wash liquor volume on the quantity of microfibres released from a single laundering cycle (Kelly et al., 2019). They showed increasing rotational speed resulted in a greater release of microfibres, as did increasing the ratio of wash liquor to load. In a separate study, Lant et al. found microfibre release increased when washing smaller loads, theorising that smaller wash loads, with higher wash liquor to load ratios, caused increased flow of the wash liquor through the fabrics which promoted higher microfibre release (Lant et al., 2020). Hartline et al. also noted the influence of agitation; they found greater release of microfibres from a top-loading washing machine compared to front-loading, which they supposed to be due to the central agitator of a top-loading machine having a more abrasive effect compared to the rotating drum of a front-loading machine (Hartline et al., 2016). Rathinamoorthy and Raja Balasaraswathi also found a positive correlation between increasing agitation and microfibre release with increasing number of ball bearings (5, 10, 15) in their laboratory device method (Rathinamoorthy and Raja Balasaraswathi, 2021). On the other hand, Cai et al. found no significant difference from increasing the number of ball bearings from 0 to 20 (Cai et al., 2020). The conflicting results could be explained by differences between the two testing methods and the fact that different fabrics were tested.
In full-scale domestic washing machines when washing loads are small relative to the drum capacity, there tends to be more movement between garments and the drum, and between individual garments. Where the load is large relative to the drum capacity, there is less movement and therefore, less friction (Mac Namara et al., 2012; Yun and Park, 2015). This suggests that microfibre release could well have been overestimated in studies where only a single garment or fabric specimen was laundered individually in a full-scale domestic washing machine due to increased agitation comparative to typical washing loads.
Where studies included detergent in their laundry testing, several groups noted the detergent was a significant contaminant when analysing microfibre release. Clogging or “caking” of detergent on the filters, particularly when powder detergents were used, interfered with the analysis making it very difficult to differentiate between microfibres and residual detergent, and therefore, difficult to quantify microfibre release from these studies (De Falco et al., 2018; Hernandez et al., 2017; Jönsson et al., 2018; Yang et al., 2019; Zambrano et al., 2019).
For those studies that could exclude significant errors due to detergent contamination, the impact of detergents on microfibre release remains unclear. Some studies reported no significant impact of detergent (Kelly et al., 2019; Lant et al., 2020; Napper and Thompson, 2016; Pirc et al., 2016), while others reported increased microfibre release when detergent was present (Carney Almroth et al., 2018; De Falco et al., 2018; Hernandez et al., 2017; Periyasamy, 2021; Yang et al., 2019; Zambrano et al., 2019). It appears from such studies that powder detergent had a greater impact on microfibre release than liquid detergent (De Falco et al., 2018; Hernandez et al., 2017; Periyasamy, 2021). It has been suggested that the higher microfibre releases due to powder detergents might be explained by increased friction between small insoluble particles within the powder formulation that penetrate into the fabric, leading to fibre damage and thus microfibre fragmentation (De Falco et al., 2018). Of the studies reviewed, only Cesa et al. found that use of detergent reduced microfibre release (Cesa et al., 2020). However, this negative correlation was only observed for synthetic garments, with cotton test samples demonstrating higher microfibre release overall, possibly suggesting that the influence of fabric composition was more dominant than the detergent use in this case.
Several researchers have reported a reduction in microfibre release with increasing number of repeat wash cycles (Belzagui et al., 2019; Cai et al., 2020; Carney Almroth et al., 2018; Cesa et al., 2020; Kelly et al., 2019; Lant et al., 2020; Özkan and Gündoğdu, 2021; Pirc et al., 2016; Rathinamoorthy and Raja Balasaraswathi, 2021; Sillanpää and Sainio, 2017; Vassilenko et al., 2021; Zambrano et al., 2019). This suggests that a majority of the microfibres available for release, are lost from the fabric structure in the first few washing cycles. Only Dalla Fontana et al. found no significant reduction in microfibre release across multiple washes (up to 5 cycles), but they noted that this could be due to loose fibres having already become detached during the more intensive pre-test scouring procedure used in their method (Dalla Fontana et al., 2021). It should be noted that in the studies reviewed, fabric specimens were not subjected to any additional physical ageing between each wash. In real life situations, everyday use and wear of textile items will disturb the fabric surface, potentially mobilising and fragmenting fibres held within the fabric structure. With subsequent washing these fragments may be released into the wash liquor, resulting in increased microfibre release over the lifetime of a garment.
Influence of fabric variables
Different fabrics can be expected to differ in their microfibre release dependent on their characteristics and properties. Very few studies have been dedicated to understanding the influence of fabric characteristics on microfibre release, and there is little consensus in the data currently available.
Raja Balasaraswathi and Rathinamoorthy provide the most comprehensive investigation to date; and they found that fabric parameters such as stitch density, tightness, thickness, and filament denier were of greater influence, and provided a better indication of microfibre release than physical fabric properties such as bursting strength and pilling resistance (Raja Balasaraswathi and Rathinamoorthy, 2021). They also noted increased microfibre release with increased mass per unit area and fabric thickness, owing to an increase in fibres per unit area available for detachment and release (Raja Balasaraswathi and Rathinamoorthy, 2021). However, De Falco et al. found mass per unit area to have little influence (De Falco et al., 2018). Some researchers have noted the influence of the tightness of the fabric structure, with tighter, more compact structures deemed favourable to reduce mobility and release of fibres (De Falco et al., 2019; Raja Balasaraswathi and Rathinamoorthy, 2021; Yang et al., 2019).
The influence of some yarn types on microfibre release was explored by Choi et al. who found that, for the same woven construction, fabrics constructed from staple spun yarns released more fibre than filament yarns. Fibres in a staple spun yarn are shorter and have greater mobility than filament yarns, they are more vulnerable to release when the fabric is agitated so they are expected to release more microfibres. Filament yarns with no twist shed more than highly twisted yarns (Choi et al., 2021). The higher release from non-twist filaments is theorised to be due to reduced inter-fibre friction leading to greater fibre freedom and therefore greater potential for fibre damage (Choi et al., 2021). Zambrano et al. theorised that the generation of microfibres is largely a function of pill formation, and that the formation of fuzz would be highly influential to microfibre release (Zambrano et al., 2019). However, Dalla Fontana et al. found they could not correlate pilling performance with microfibre release, as fabrics which performed poorly in pilling testing had more pills held at the fabric surface which were not released during laundering (Dalla Fontana et al., 2021). There is a clear need for greater understanding of the influence of different fabric characteristics on microfibre release as current data is limited and findings are often contradictory.
This research explores the influence of laundering variables on microfibre release by systematically comparing different in-wash conditions. The influence of fabric characteristics is also explored by testing a range of commercially relevant fabric qualities. A reliable and reproducible test method is employed throughout to eliminate the influence of different testing methodologies. The results of this research are also used to estimate annual microfibre release from UK domestic laundering, and estimates are compared to those acquired from the existing literature.