I was cutting a barrel for garden project with electrical saw blade. i think i may have breathed in small microplastics. what happens to these? does the mucus get rid of them eventually and they cause no harm? how long does this usually take?

Pieces of plastics (microplastics) and other items like asbestos or dust from the environment & car exhaust pollutions can all potentially cause Acute & Chronic Lung diseases with Breathing difficulties. Best to use preventive measures by wearing appropriate Masks & Facial Protection whenever possible. Since this is only one incident, there will probably be no consequences. See PCP if symptoms occ

The literature is limited to date. But would advise using N-95 to N-99 mask in future, it will also protect against aerosolized fungi and bacteria https://pubmed.ncbi.nlm.nih.gov/34185251/ https://pubmed.ncbi.nlm.nih.gov/36969097/ I think a one time exposure will not have major effects. Wear a mask when cutting any material ( wood plastic, metal) that can result in microchips. F/U w/ pulmonary