Some people are more susceptible to illnesses when the air is dry, although the direct impact of low humidity on getting sick remains unknown. One proposed mechanism is that virions (infectious particles) become airborne more easily and stay viable in low humidity conditions. Low humidity could also increase susceptibility due to drying out of mucous membranes, making it easier for virions to penetrate. An accompanying decrease in ciliary action (tiny hairlike projections that help remove bacteria and debris from the lungs) could further contribute to an increased susceptibility to infection.