Microplastics are generally defined as plastic particles smaller than 5 millimeters but larger than 1 micrometer, while nanoplastics range from 1 nanometer to 1 micrometer in size.

The size distinction between microplastics and nanoplastics is fundamental, as it determines their environmental behavior, detection methods, and biological interactions. Microplastics are visible under light microscopy, whereas nanoplastics require electron microscopy due to their submicron scale. This size range affects their aggregation, transport, and bioavailability in ecosystems, influencing exposure risks and toxicity profiles.