Microplastics effects on human health

Microplastics effects on human health are a subject of growing concern and an area of research. The tiny particles known as microplastics (MPs), have been found in various environmental and biological matrices, including air, water, food, and human tissues. Microplastics, defined as plastic fragments smaller than 5 mm, and even smaller particles such as nanoplastics (NP), particles smaller than 1000 nm in diameter (0.001 mm or 1 μm), have raised concerns impacting human health. In scientific literature, combined microplastics and nanoplastics are referred to as MNPs or NMPs or NMPPs for nano-and microplastic particles.

Routes of exposure and bioaccumulation
The major routes of exposure include ingestion, skin contact, and inhalation. MNPs can remain in the organ of entry or enter systemic circulation to bioaccumulate in various tissues depending on size. MNPs above 150 μm or 10 μm in diameter do not enter the blood and remain in tissues whereas particles below 200 nm pass through intestinal barriers and reach extracellular spaces.

Ingestion
Direct ingestion includes drinking water, beer, honey and sugar, table salt,  and indoor airborne particulates falling on open meals. Indirect ingestion includes toothpaste, face wash, scrubs, and soap  and enter systemic circulation.

Maternally
Recent studies have shown the presence of microplastics 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 microplastics is limited in comparison. A study in 2022 found plastics smaller than five millimeters in diameter in 75% of breast milk samples examined. It has been suggested that these plastics are especially dangerous for young children due to their role as hormone disruptors. 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 microplastic 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 microplastics and other potentially harmful chemicals. It has been suggested that mothers should directly breast feed 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 microplastics in the milk. Similar results have been seen from heating plastic reusable food containers in a microwave, showing the release of both microplastics and nanoplastics. It has been suggested that mothers try to avoid ingesting microplastics 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.

Contact
This is skin penetration through wounds and pores such as sweat glands and hair follicles as the skin interacts with MNP-contaminated media such as soil or water and cosmetics mentioned above and enter systemic circulation.

Inhalation
This is indoor and outdoor airborne entry into the respiratory system from upholstery and household furniture to urban dust, rubber tires and synthetic fibers. MNPs can remain in the lungs or be ingested via mucociliary clearance to enter the systemic circulation.

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.

Acute inhalation is 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., 4×1010 particles per m3 from extrusion 3D printers versus 50 particles per m3 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 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 polyvinyl chloride 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 to 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  and recycling facilities  and landfills serve as a reservoirs of particulates workers may potentially be exposed to.

Medical plastic

 * Face masks and respirators: globally up to 7 billion facemasks which amounts 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 microplastics 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 suggested that microplastics and nanoplastics exposure may have adverse effects on human health, such as:


 * Inflammation
 * Inflammation in the lungs from inhalation
 * Oxidative stress
 * Cardiovascular
 * Cerebrovascular
 * Respiratory see above for more
 * Genotoxicity
 * Mutagenicity
 * Cytotoxicity
 * Disruption of hypothalamic-pituitary axis (HPA), including the:
 * Hypothalamic-pituitary-thyroid
 * Hypothalamic-pituitary-adrenal
 * Hypothalamic-pituitary-testicular
 * Hypothalamic-pituitary-ovarian axis
 * Reproductive toxicity
 * Decreased reproductive health (decreased sperm quality )
 * Developmental abnormalities
 * Immunotoxicity
 * Endocrine disruption
 * Neurotoxicity
 * Carcinogenicity
 * Metabolic disturbances
 * Disrupted gut-liver axis resulting in increased risk of insulin resistance

Laboratory investigations demonstrate that microplastics can damage human cells, triggering allergic reactions and cell death. MPs may also disrupt 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 microplastics to adverse health effects in humans remain yet limited and research is ongoing to determine the full extent of potential harm caused by microplastics and their long-term impact on human health.

Clinical studies
In a cohort study involving 304 patients who were undergoing carotid endarterectomy for asymptomatic carotid artery disease in 3 Italian hospitals, polyethylene was detected in carotid artery plaque of 150 patients (58.4%) with a mean level of 21.7±24.5 μg per milligram of plaque; 31 patients (12.1%) also had measurable amounts of polyvinyl chloride, with a mean level of 5.2±2.4 μg per milligram of plaque. Those with carotid artery plaque in which MNPs were detected had a higher risk of a composite of myocardial infarction, stroke, or death from any cause at 34 months of follow-up than those in whom MNPs were not detected.

Mitigating inhalation exposure to MNPs
Also see Health and safety hazards of nanomaterials.

As 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. And 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 nonventilating 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.