Quantification and exposure assessment of microplastics in Australian indoor house dust

Highlights

• Mean annual inhaled MPs were 12,891 ±4472 fibres/year.

• Petrochemical-based fibres comprised 39% of deposited particles of indoor dust.

• Carpeted floors were associated with significantly more dust than hard floors.

• Frequency of vacuum cleaner significantly correlated to deposition rate of MPs.

• Ingestion and inhalation rates of MP greatest in younger children.

Abstract

Limited attention has been given to the presence of MPs in the atmospheric environment, particularly in indoor environments where people spend about 90% of their time. This study quantitatively assesses the prevalence, source and type of MPs in Australian homes with the goal of evaluating human health exposure potential. Thirty-two airborne indoor deposited dust samples were collected in glass Petri dishes from Sydney (Australia) homes, over a one-month period in 2019. Participants completed a questionnaire on their household characteristics. Samples were analysed using a stereomicroscope, a fluorescent microscope and micro-Fourier transform infrared (FTIR) spectroscopy for their colour, size, shape and composition. Inhalation and ingestion rates were modelled using US EPA exposure factors. Microplastic fibre deposition rates ranged from 22 to 6169 fibres/m²/day. Deposited dust comprised 99% fibres. The highest proportion of fibres (19%) were 200–400 μm in length. The majority were natural (42%); 18% were transformed natural-based fibres; and 39% were petrochemical based. A significant difference was observed between the deposition rate and the main floor covering (p-value <0.05). Polyethylene, polyester, polyamide, polyacrylic, and polystyrene fibres were found in higher abundance in homes with carpet as the main floor covering. Where carpet was absent, polyvinyl fibres were the most dominant petrochemical fibre type, indicating the role of flooring materials (e.g. wood varnishes) in determining MP composition. Vacuum cleaner use was significantly related to MP deposition rates (p-value <0.05). MP ingestion rates peaked at 6.1 mg/kg-B_w/year for ages 1–6, falling to a minimum of 0.5 mg/kg-B_w/year in >20 years age group. Mean inhaled MP weight and count was determined to be 0.2±0.07 mg/kg-B_w/year and 12891±4472 fibres/year. Greatest inhalation intake rates were for the <0.5-yr age group, at 0.31 mg/kg-B_w/year. The study data reveal that MPs are prevalent in Australian homes and that the greatest risk of exposure resides with young children. Notwithstanding the limited number of global studies and the different methods used to measure MPs, this study indicates Australian deposition and inhalation rates are at the lower end of the exposure spectrum.

Introduction

Since the invention of plastics at the beginning of the 20th century, global production has increased annually (Cole et al., 2011; Plastics Europe, 2019; Thompson, 2006) reaching 359 million tonnes in 2018 (Plastics Europe, 2019). It is expected that this trend will continue to about four times the current rate by 2050 (Plastics Europe, 2019; Roland Geyer, 2017; Suaria et al., 2016). Due to their low cost and durability, plastics are now the most used material due to their versatile applications from packaging to household products. The exponential growth in plastic production has continued unabated despite environmental concerns raised by governments and community organisations (Seltenrich, 2015).

Plastics undergo different degradation processes that leads to decay products composed of increasingly smaller sized plastics. A ubiquitous subgroup of plastic decay products with a dimension ratio (length/width) of >3:1) are known as microplastics (MPs) (Cole et al., 2011). The term ‘microplastics’ was first used by Thompson (2006) for fragments, pellets and fibres with a dimension (length) between 5 mm–1 μm. Subsequently, other researchers have used different size definitions to characterise MPs (Barnes et al., 2009; Betts, 2008; Browne et al., 2007, 2011; Claessens et al., 2011; Graham and Thompson, 2009; Ng and Obbard, 2006; Ryan et al., 2009). In addition to the size characterisation of MPs, other researchers have used the chemical composition of MPs for definitional purposes with researchers applying different conditions for categorisation. For example, Song et al. (2015) determined that only particles composed of petrochemical based materials were MPs. In contrast, Frias and Nash (2019) contended that transformed natural-based fibres, including viscose, rayon and cellophane should also classified as MPs due to harmful dyes and additives such as flame retardants. In this study, MPs are defined as particles derived from petrochemicals.

Microplastics research has had a predominant focus on aquatic ecosystems and human exposure via water and seafood ingestion pathways (Dehaut et al., 2016; Li et al., 2015; Sharma and Chatterjee, 2017; Van Cauwenberghe and Janssen, 2014). In addition, ingestion of MPs from the use of table salt and salt grinders, take away containers, beer, milk, water, honey and tea bags has also been speculated (Diaz-Basantes et al., 2020; Hernandez et al., 2019; Karami et al., 2017; Peixoto et al., 2019). Nevertheless, MPs are ubiquitous in the environment and exposure potential is an ever present risk because they enter the atmosphere at all steps of their life cycle (Bank and Hansson, 2019). Recent research indicates presence of MPs in indoor environments (Chen et al., 2020; Dris et al., 2017; Liu et al., 2019a; Prata et al., 2020a; Vianello et al., 2019; Zhang et al., 2020b, 2020c). Given that people spend up to 70% and 90% of their time in homes and indoors, respectively (Klepeis et al., 2001), and the fact that plastics are used ubiquitously in homes, investigation of the health risk from exposure to MPs in the primary living environment is warranted. Moreover, there are surprisingly a limited number of airborne MPs exposure studies (Catarino et al., 2018; Chen et al., 2020; Prata, 2018; Wright and Kelly, 2017).

The form of MPs is a product of its original shape (Zhang et al., 2020c), which is a key component to understanding their source and potential for becoming airborne (Rochman et al., 2019). Given that textiles and garments, furniture, and laundry activities are composed of both transformed natural-based fibres as well as petrochemical materials it is not surprising that fibrous microplastics are the main source of indoor MPs (Browne et al., 2011; Dris et al., 2015, 2016).

In this study, the prevalence and characteristics of indoor MPs in Australian homes is assessed via a standardised passive dust time-weighted sampling collection over a 30-day period (Australian/New Zealand Standard, 2016). Sampling of ambient suspended particles using high-volume air sampling carried out over a standard 24-hr period are less likely to fully characterise indoor atmospheric fluctuations over time, compared to passive household dust sampling over a 30-day period. This research study seeks to quantitatively assess the prevalence, source and type of MPs in Australian homes with the express purpose of quantifying human health exposure potential. This evaluation is achieved by collecting and analysing the following from 32 Australian homes:

• depositional rate of MPs in household dust;

• colour, size, shape and composition of fibres in deposited dust;

• common types and sources of MPs in homes; and

• the relevance of household meta-data on MPs deposition rates and types including: frequency of using a vacuum cleaner, main floor covering (carpet vs. other types), number of residents in each home, density of occupants in each home (number of people/home area), presence/absence of children between 1 and 15 years), and geographic location and associated road network density as an outdoor source of MPs.

The above data was then used to calculate annual MPs ingestion and inhalation rates across different age groups to estimate human exposure potential.