Airways and respiratory system
An adult breathes about 15,000 liters of air in and out every day: about enough air to fill a medium-sized room. For the airways - which start at the nose and end deep in the lungs - this means a constant inflow of dust, toxins and germs. Healthy airways protect themselves with a variety of self-defense mechanisms. But no defense system is perfect. Viruses and bacteria sometimes still get in. If the system is oversensitive, people can develop problems such as allergies and asthma.