Microbiologists agree that frequent mask wearing creates a moist environment in which microbes are allowed to grow and proliferate before entering the lungs. Those foreign microbes then travel down the trachea and into two tubes called the bronchi until they reach small air sacks covered in blood vessels called alveoli.
"The lungs were long thought to be sterile, but we now know that oral commensals–microbes normally found in the mouth–frequently enter the lungs due to unconscious aspirations." – Leopoldo Segal, Study Author and Director of the Lung Microbiome Program and Associate Professor of Medicine at New York University Grossman School of Medicine
According to the study, after invading the lungs these microbes cause an inflammatory response in proteins known as cytokine IL-17.