Breathe Deep; Scientific American Body; Special Editions; by Staff Editor; 1 Page(s)

The lungs do the vital job of respiration: bringing in the oxygen that the body's tissues need to survive and sending away the carbon dioxide that they produce as a waste product. That role, however, puts them squarely in the path of airborne toxins, bacteria and viruses.

When the muscular diaphragm at the base of the chest cavity contracts, the cavity expands and air rushes into the lungs to equalize pressure. When the diaphragm relaxes, the cavity shrinks again, forcing air back out as an exhalation. An adult at rest breathes on average between 10 and 15 times a minute, although the rate can rise to more than once a second during heavy exercise. Children breathe faster: newborns inhale and exhale between 40 and 50 times a minute.