One of the main sources is our clothing. Minuscule fibres of acrylic, nylon, spandex, and polyester are shed each time we wash our clothes and are carried off to wastewater treatment plants or discharged to the open environment.
According to a recent study cited by Water World, more than 700,000 microscopic plastic fibres could be released into the environment during each cycle of a washing machine. This has not yet been studied in the case of handwashing, which is more common in developing countries, but the effects could be significant there as well.
A study in 2016 commissioned by clothing company Patagonia and conducted by researchers at the University of California, Santa Barbara, found that washing a single synthetic jacket just once released an average of 1.7 grams of microfibres.Source: https://www.unenvironment.org/news-and-stories/story/plastic-planet-how-tiny-plastic-particles-are-polluting-our-soil