The air you breathe is full of chemicals and harmful substances, including dust, soot, mold, fungi, bacteria, viruses and pollutants. However, your body's respirator system has several ways to protect you against most harmful airborne particles. First off, only very small particles -- about three to five microns in size -- can actually reach your deep lungs [source: Merk Manuals].
Before any particles reach your lungs, however, they must first travel through your mouth and down your airway. There are tiny hairs, called cilia, located along the walls of your airways and your nasal passages;there is also mucous that coats your airways. The tiny hairs, which are actually muscles, move the mucous along your airway walls. This is no small task, as the cilia cells beat 1,000 times per minute to move the mucous along. Together, they can trap unwanted particles, making the air you breathe cleaner. Eventually, the mucous layer, with its trapped unwanted particles, reaches the back of the mouth and the digestive system. The digestive fluids of the stomach eventually kill off the potentially hazardous particles. This mucous layer acts as a protective barrier between the upper respiratory system and potentially airborne hazardous particles.
If unwanted substances do reach the lungs, there are also little mobile cells, called phagocytes, which defend your respiratory tract. These tiny cells seek, attack, and destroy unwanted particles located on the alveolar surface. These little cells actually eat unwanted particles. In cases when a more serious threat is present, such as a virus or an infection, your body releases more white blood cells, including neutrophils, which can aid in the process of destroying unwanted particles in the lungs [source: Merk Manuals].