# Reality Check: Experimental Studies on Microplastics Lack Realism
## 4 Concentration
### Human Effects  
Although it is likely to have major impacts on human health, the concentration of MPs used during experimental studies is not often considered, and is sometimes critiqued. Evidence regarding the concentration of MPs in human tissues does exist in some cases, albeit in a nascent form. In a screening of human colon samples, 28.1 (\(\pm\)15.4) particles/gram of tissue were found [20]. Yet, as with size and shape, the concentration of MPs in human tissue appears to vary in an organ-dependent manner. In human lung tissue, recent research estimated 0.56 MP particles/gram of lung tissue (equaling about 470 particles in an average set of lungs). The authors indicate that this suggests most particles inhaled are retained in airways [19], an observation in line with previous characterizations.  
Many experimental studies focusing on MPs in the context of human health either use a single concentration of MPs or use a range that is not environmentally relevant. One recent report studying human cells (HUVECs) used a range of MPs at 0-100 \(\mu\)g/mL [36]. Based on previous findings, these authors argued that this would be a good estimate for the actual concentration of MPs in human blood (\(\sim\)99.4 \(\mu\)g/mL). They suggest that the actual concentration might be higher if there are low turnover rates. Indeed, the evidence mentioned here indicates that only a fraction of MPs that enter our body are excreted--with some likely remaining in the blood or getting embedded in other organs [37]. For 0.5 \(\mu\)m sized particles, regardless of concentration cell viability decreases drastically at 72 h after of exposure. For 1 \(\mu\)m sized particles, at 72 h cell viability is impacted in a cell concentration-dependent manner, with 100 \(\mu\)g/mL having the strongest effect. Regardless of concentration, 5 \(\mu\)m particles did not significantly alter cell viability [36]. Although the authors proposed that MPs induce autophagy-dependent cell death and negatively interact with cell-surface receptors, the reasoning for the differences observed between 0.5 \(\mu\)m and 1.0 \(\mu\)m remain unclear.  
In another study using human lung epithelial cells (BEAS-2B), two concentrations of polystyrene MPs were used. At low doses (10 \(\mu\)g/mL), cytotoxicity, an inflammatory response, and barrier dysfunction were observed. High doses (1000 \(\mu\)g/mL) exacerbated the same effects, but also reduced alpha1-antitrypsin levels, increasing the risk for COPD [57].  
These results elicit a more thorough characterization of the concentrations of MPs human cells are exposed to in different tissues. First it is necessary to recognize that there will be concentrations that persist over prolonged periods (those MPs embedded in tissues), and more transient concentrations (MPs that move through the body). These two categories might be regional or tissue-specific, depending on how MPs translocate and are circulated in different organs. Once this improved understanding is reached, similar concentrations should be used in experimental studies to properly model the prolonged and transient scenarios.