Investigating the microwave degradation of polypropylene microplastics and their impact on human intestinal cell models

Humans are inevitably exposed to microplastics (MPs) through various pathways, with ingestion being a primary route. Polypropylene (PP), commonly used for food storage and microwave (MW) heating, has the potential to release MPs when exposed to heat or prolonged use. Despite their detection in human tissues, including the gut, the health impacts of degraded PP particles remain poorly understood. This study investigates the characteristics of MW-treated PP particles and their effects on human intestinal Caco-2 cells, focusing on cell metabolic activity, membrane damage, and oxidative stress. Caco-2 cells were exposed to PP-MPs with a concentration of 200 µg/mL subjected to various MW degradation cycles: 3-minute cycles (repeated 1, 5, and 10 times) and a 30-minute continuous cycle at high power (1000 W), for 24 and 48 h. Surface charge and infrared spectral analyses were employed for chemical characterization of the MPs. Toxicological assays were used to evaluate cell metabolism, membrane integrity, and oxidative stress. Results indicated that prolonged MW exposure led to oxidative degradation in the PP-MPs. After 24 h of exposure to MW-treated PP-MPs, an increase in the metabolic activity of Caco-2 cells ranging from 14% to 35% was observed. However, after 48 h, a statistically significant decrease in metabolic activity, ranging from 10% to 14% was observed for the cells treated with PP-MPs subjected to short MW cycles, while a persistent upregulation was observed for the 30 min continuous MW MPs. No membrane damage was detected in Caco-2 cells under any experimental condition. In contrast, oxidative stress (OS) levels surged by at least 74% in all MW cycle treatments after 24 h exposure and remained elevated compared to untreated controls after 48 h, suggesting OS as a key mechanism influencing cytotoxicity. Cell imaging suggested that MW-degraded PP-MPs induced a ROS-driven intracellular vacuolization consistent with early apoptotic signaling in Caco-2 cells. This study enhances our understanding of the biological effects of PP-MPs exposed to MW degradation, highlighting the need for further research into the broader health implications of plastic usage.