Microplastics are very small pieces of plastic, usually less than 5 millimeters long. There is a key distinction in this definition between primary and secondary microplastics. Primary microplastics are manufactured to be extremely small, such as microbeads used in personal care products or shed synthetic textiles during washing. Secondary microplastics, on the other hand, are the result of bigger pieces of plastic breaking down through wind, water or sun exposure.