Included in the review were nearly 2,000 studies on how microplastics affect health. The rapid review used a stepped approach to evaluate the effects of microplastics on human digestive, reproductive, and respiratory systems. CalSPEC evaluated the quality and strength of the evidence for outcomes related to biological changes (e.g., immunologic responses, inflammatory responses, and hormonal changes) and observable outcomes (e.g., colon shortening and sperm damage) measured in studies meeting the search criteria. CalSPEC then characterized the evidence into one of three human health hazard level classifications based the animal data: 1) presumed to be a hazard to humans, 2) suspected to be a hazard to humans, and 3) not classifiable.  

CalSPEC found that exposure to microplastics is suspected to be a digestive hazard to humans, including cancer. Exposure to microplastics is also suspected to be a reproductive and respiratory hazard, although the evidence from the respiratory studies did not undergo as rigorous of an evaluation. Overall, the evaluated evidence was of “moderate quality.” However, CalSPEC recognizes that the conclusions of the rapid review can be an underestimation of the true harm of microplastic exposure, given that it did not evaluate all health outcomes in the digestive and reproductive systems and did not evaluate plastic chemical additives known to increase the risk of negative health effects.

Government actions regarding microplastics were also reviewed by CalSPEC, including mandates for research to understand the environmental and human health impacts of microplastics, bans and regulations of microplastics by source (microbeads, textiles, and tires), and a high-level overview of multinational agreements or treaties on microplastics.

CalSPEC found 51 laws addressing microplastics across various levels of government and jurisdictions. The majority are concentrated in Europe and California, and are focused on banning microbeads or mandating more research. The report notes that policies are generally siloed by environmental compartment and/or microplastic source, often within a specific geographic area, rather than using a cross-boundary, ecosystem-wide approach.

The report draws three main conclusions:

1. Knowledge about microplastics prevalence, distribution, and toxicity to humans is incomplete
2. Existing evidence rasises concerns about the environmental and human health consequences of microplastics pollution
3. Governments worldwide have only just begun to implement policy interventions designed to mitigate microplastics pollution, but the effectiveness of such interventions is unknown.

CalSPEC calls for research is needed to characterize the prevalence and distribution of microplastic contaminants across all environmental compartments, determine acceptable levels of microplastics exposure to humans and the environment, and evaluate effective prevention and mitigation techniques. However, CalSPEC also suggests that California legislature consider advancing policies that limit microplastic exposure based on present evidence, despite knowledge gaps. “Some degree of urgency is warranted” when dealing with microplastics because of the long timeframe required to reduce plastic pollution and the long half-life of plastic pollutants, according to the report.