# Microplastics and human health

The effects of microplastics on human health are a growing concern and an actively increasing area of research. Tiny particles known as microplastics (MPs), have been found in various environmental and biological matrices, including air, water, food, and human tissues. MPs, defined as plastic fragments smaller than 5 millimeter (mm), and even smaller particles such as nanoplastics (NPs), particles smaller than 1000 nanometer (nm) in diameter (0.001 mm or 1 micrometer [μm]), have raised concerns impacting human health. The pervasive presence of plastics in our environment has raised concerns about their long-term impacts on human health. While visible pollution caused by larger plastic items is well-documented, the hidden threat posed by NPs remains under-explored. These particles originate from the degradation of larger plastics and are now found in various environmental matrices, including water, soil, and air. Given their minute size, NPs can penetrate biological barriers and accumulate in human tissues, potentially leading to adverse health effects.
Plastics continue to accumulate in landfills and oceans, leading to pollution that negatively affects both human and animal health. Notably, MPs and NPs are now ubiquitous, infiltrating our food chain and water supplies. Studies indicate that humans ingest significant amounts of MPs daily through food, especially seafood and inhalation, with estimates ranging from 39,000 to 52,000 particles per person annually. Additionally, the presence of MPs in human feces suggests widespread exposure and absorption. In scientific literature, combined MPs and NPs are referred to as micro- and nanoplastics (MNPs), nano- and microplastics (NMPs), or nano-and microplastic particles (NMPPs).
Understanding the sources and health effects of NPs is crucial for developing effective public health policies. As plastics are an integral part of modern life, balancing their benefits with the associated health risks is essential. This research aims to provide evidence-based recommendations to mitigate the adverse health effects of NPs, thereby informing future regulatory and policy decisions. The increasing presence of NPs in the environment has raised concerns about their potential impacts on human health. Research has shown that NPs can penetrate biological barriers, induce toxicity, and accumulate in organs, leading to various health issues. NPs have been found in drinking water, food, and air, making human exposure ubiquitous.

## Routes of exposure and bioaccumulation

The major pathways of human exposure to MNPs are ingestion, inhalation, and dermal contact, with bioaccumulation varying based on particle size, composition, and physicochemical characteristics. Research suggests that MNPs above 150 μm typically remain confined to tissues and do not enter systemic circulation, whereas particles below 200 nm can breach cellular and tissue barriers, potentially reaching the bloodstream and other organs. This diversity in bioaccumulation pathways underscores the widespread yet nuanced risks of MNP exposure to human health.
Plastics are extensively used in the construction and renovation industry. Airborne microplastic dust is produced during renovation, building, bridge and road reconstruction projects and the use of power tools.

### Inhalation

Inhalation is a critical but understudied route of MNP exposure, with airborne MNPs originating from urban dust, synthetic fibers from textiles, rubber tires, and household plastic items. These airborne particles may become suspended in the air due to wave action in aquatic environments or the spread of wastewater treatment sludge on agricultural fields. Once inhaled, these particles may lodge in the lungs or, through mucociliary clearance, be ingested and enter the digestive system. Airborne MPs have been detected in urban atmospheres, with reports showing a fallout of 29–280 particles per square meter per day on an urban rooftop, underscoring the potential for routine exposure. Annual inhalation exposure rates are estimated at around 39,000–52,000 MP particles, with studies highlighting the significant contributions from synthetic textiles and urban dust sources.
These findings collectively suggest that MNPs may accumulate in multiple organ systems depending on the exposure route, potentially leading to long-term health consequences as their presence in human tissues becomes more pervasive over time.

### Ingestion

Ingestion is one of the primary pathways of MNP exposure due to the omnipresence of these particles in food, beverages, and drinking water. Studies show that MNPs are detected in a variety of consumables, including drinking water, beer, honey, sugar, table salt, and even airborne particles that settle on food. Indirect ingestion includes toothpaste, face wash, scrubs, and soap.
Marine products are particularly concerning sources of ingestion-related exposure due to the accumulation of MNPs in aquatic environments. Fish, bivalves, and other seafood are frequently contaminated with MNPs ingested through water and food, and humans consuming these animals are thus directly exposed to MPs embedded in tissue. The entire soft tissue of bivalves, for instance, is eaten by humans, which increases the direct transfer of MNPs. In a study along the Mediterranean coast of Turkey, 1,822 MPs were extracted from the stomachs and intestines of 1,337 fish specimens, with fibers accounting for 70% of these particles.
Contamination is further compounded by plastic packaging and storage materials, which can leach MNPs over time, leading to additional ingestion from common foods and drinks. Fecal sample analyses estimate a daily intake of approximately 203–332 MNPs, translating to an annual ingestion rate of around 39,000–52,000 particles. This suggests that daily MNP exposure from food and drink may be substantial, with significant implications for gastrointestinal and systemic health.

### Maternal exposure

Maternal transfer of MNPs represents an emerging exposure route that affects infants directly. Recent studies have shown the presence of MPs in breast milk, often leading to exposures in very young children. While it has already been established that chemicals such as flame retardants and pesticides have been detected in breast milk, knowledge about MPs is limited in comparison. A 2022 study detected MPs smaller than 5 mm in 75% of analyzed breast milk samples, raising concerns about infant exposure during critical developmental windows.
Exposure during developmental stages can lead to long lasting developmental defects or other issues later in life. While these detected levels were not above the currently established thresholds for unsafe levels, they show another possible route for MP ingestion. For some native population in North Canada and people who live near industrial factories, it is sometimes suggested by pediatricians that mothers not nurse their children, over fear of ingestion of MPs and other potentially harmful chemicals. It has been suggested that mothers should directly breastfeed their children instead of from a bottle. Studies have shown that pumping milk, freezing it in plastic bags, then subsequently heating it up will increase the contamination of MPs in the milk. Similar results have been seen from heating plastic reusable food containers in a microwave, showing the release of both MPs and NPs. It has been suggested that mothers try to avoid ingesting MPs themselves, to try and avoid passing them onto their children through breastfeeding. Studies have shown that drinking water from plastic bottles has significantly greater detectable plastic content than tap water.
These findings suggest that breastfeeding may inadvertently expose infants to endocrine-disrupting plastics, which could have lasting effects on growth and development. To mitigate these risks, pediatricians recommend reducing the use of plastic bottles and avoiding the heating or freezing of breast milk in plastic containers, as temperature fluctuations can increase MNP leaching.

### Dermal contact

Though less frequently examined, dermal exposure to MNPs occurs through contact with contaminated media like soil, water, and personal care products, including facial and body scrubs containing MNPs as exfoliants. Although the skin generally acts as a barrier, conditions such as skin lesions or high exposure environments may allow for enhanced absorption of MNPs, particularly NPs, which can penetrate the stratum corneum. Studies on dermal exposure highlight the potential for these particles to enter systemic circulation, especially if the skin barrier is disrupted by wounds or conditions that increase permeability, like pores such as sweat glands and hair follicles.

## Occupational exposure

Incidental generation of MNPs is mechanical or environmental degradation or industrial processes such as plastic manufacturing (heating and chemical condensation) and intentional generation of MNPs occur during 3D printing.
The main route of workplace exposure is acute inhalation. Workplace exposure can be high concentration and lasting the duration of a shift and thus short-term whereas exposure outside of work is at low concentration and long-term. The concentration of worker exposure is orders of magnitude higher than the general population (e.g., 4×10$^{10}$ particles per cubic meter [m$^3$] from extrusion 3D printers versus 50 particles per m$^3$ in the general environment).
High chronic exposure to aerosolized MNPs occur in: the synthetic textile industry, the flocking industry, and the plastics industry consisting of the Vinyl Chloride supplier and the Polyvinyl Chloride (PVC) manufacturer.

### Manufacturing and processing of plastic

* 3D printing. Additive Manufacturing such as commercial extrusion printing and multi-jet fusion printing with thermoplastics and resin emit MNPs and organic vapors (Volatile Organic Compounds) into the ambient workplace air. There is emerging evidence of allergic, respiratory, and cardiovascular adverse effects from 3D printing. For extrusion printing, Acrylonitrile butadiene styrene (ABS) filaments emit more MNPs than Polylactic acid (PLA) filaments.
* Nylon flocking is the process of applying, cutting, sanding, and machining of nylon polymers on surfaces where dust emission peaks during air blowing flocked surfaces.
* Coating utensils and cookware: polytetrafluoroethylene, and high energy or heat processing of plastic products (Bello et al. 2010; Walter et al., 2015).
* Dust generation occurs in a wide range of settings from composite material machining, drilling, hand-held grinding, and sanding of nanotube-containing composites, and sanding of dental composites, and cutting PVC piping and plastics.
* PVC and plastic production produces PVC dust with mortality confirmed among vinyl and PVC workers after reanalysis of data, and coronary artery disease and cancer death among vinyl chloride exposed workers.
* Rubber chemical manufacturing impacting mortality among these workers.

### Environmental and mechanical degradation of plastic

* Carpet and synthetic fibers: indoor air contains high concentrations of degraded synthetic fibers with potential exposure by office workers and custodial staff. Settled dust is ingested by adults and particularly children.
* Wastewater management, recycling facilities, and landfills: plastic goods undergo environmental (weathering) and mechanical degradation and wastewater management. Recycling facilities and landfills serve as reservoirs of particulates workers may potentially be exposed to.

### Medical plastic

* Face masks and respirators: globally up to 7 billion facemasks which amount to 21,000 tons of synthetic polymer, were estimated to be used daily during the COVID-19 pandemic, increasing plastic demand and waste. It is yet unknown if respirable NMP debris on the surface of facemasks poses adverse health effects.
* Medical plastics include a wide range of products from bags to pharmaceutical containers that leach and expose patients and healthcare workers to MNPs. Further research is needed to assess toxicology and medical significance of MNPs from medical plastics.

## Potential health risks

The potential health impacts of MPs vary based on factors, such as their particle sizes, shape, exposure time, chemical composition (enriched with heavy metals, polycyclic aromatic hydrocarbons [PAHs], etc.), surface properties, and associated contaminants.
Experimental and observational studies in mammals have shown that MPs and NPs exposure have the following adverse effects:

### On the cellular level

* Inflammation
* Oxidative stress
* Genotoxicity
* Cytotoxicity

### By systems

* Cardiovascular
* Respiratory
** Inflammation in the lungs from inhalation
* Disruption of hypothalamic-pituitary axis (HPA), including the Hypothalamic-pituitary-thyroid, Hypothalamic-pituitary-adrenal, Hypothalamic-pituitary-testicular, and Hypothalamic-pituitary-ovarian axis
* Reproductive toxicity, decreased reproductive health, decreased sperm quality
* Developmental abnormalities
* Immunotoxicity
* Endocrine disruption
* Neurotoxicity
* Metabolic disturbances
** Disrupted gut-liver axis resulting in increased risk of insulin resistance
** Disrupted hormone function, potentially contributing to weight gain

## Epidemiological studies

Despite growing concern and evidence, most epidemiologic studies have focused on characterizing exposures. Epidemiological studies directly linking MPs to adverse health effects in humans remain yet limited and research is ongoing to determine the full extent of potential harm caused by MPs and their long-term impact on human health.

## Prevalence

MPs have been found in blood.

## Mitigating inhalation exposure to MNPs

As of April 2024, there is no established NIOSH Recommended Exposure Limit (REL) for MNPs due to limited data on exposure levels to adverse health effects, the absence of standardization to characterize the heterogeneity of MNPs by chemical composition and morphology, and difficulty in measuring airborne MNPs. Thus, safety measures focus on the hierarchy of controls for nanomaterials with good industrial hygiene to implement source emission control with local exhaust ventilation, air filtration, and non-ventilating engineering controls such as substitution with less hazardous materials, administrative controls, Personal Protective Equipment (PPE) for skin, and respiratory protection.
Research from the U.S. National Institute of Occupational Safety and Health (NIOSH) Nanotechnology Research Center (NTRC) show local exhaust ventilation and High Efficiency Particulate Air (HEPA) filtration to be effective mitigation to theoretically filter 99.97% of nanoparticles down to 0.3 microns.