Are plastics within the air we breathe a threat to human health?

Microplastics are ubiquitous within the air we breathe. Evidence from epidemiological studies suggest a threat to human health, confirmed by in vivo and in vitro studies. However, little information is known about the pulmonary deposition and ultimate fate within the respiratory tract. Additionally, water-soluble polymers (WSPs) were recently identified as an underrepresented form of plastic pollution. Again, limited information regarding pulmonary deposition or pulmonary toxicity is reported. This thesis aims to investigate whether these plastic pollutants could threaten human health.

Microplastics representative of airborne microplastics were produced whilst WSPs commonly found within household items were purchased. The inhalation of each plastic was assessed using in vitro aerosol particle size distribution tests as described within British and US pharmacopeia’s. For WSPs physiochemical properties relating to inhalation were assessed to investigate their influence. Effects of these particles on human health was determined via MTT assay on two cell lines, A549 lung epithelium and THP-1 macrophages.

Inhalation experiments of microplastic showed greatest deposition within upper respiratory tract and conducting airways, suggesting greater exposure in Gastrointestinal tissue (GIT) than lungs. No microplastic induced significant cytotoxicity in THP-1 macrophages, however, cytotoxicity was noted in epithelial cells particularly in fibrous particles. Aerosolization of WSPs varied greatly by polymer with polyacylic acid (PAA) and polyethylene glycol (PEG) showing greatest aerosol production. Most WSPs showed good deposition within respiratory lung regions and good exposure risk. Surface tension and viscosity was shown to have only minimal effects on predicting aerosolization and pulmonary deposition. Some WSPs were found to be highly cytotoxic in both A549 and THP-1 cells, whilst others showed only mild or no cytotoxicity.

The main conclusions from this study is that insoluble microplastics represent a low threat to human pulmonary health, however due to high deposition within the upper respiratory system they could accumulate in the GIT and pose a threat there. Secondly the threat on human health posed by WSPs varies greatly dependant on the polymer, with some polymers such as PAA being high risk to human health, whilst polyvinyl alcohol (PVOH) and polyvinylpyrrolidone (PVP) are a low/medium threat.