Children’s vulnerability to ozone’s ill effects provokes great concern among health professionals for several reasons. One reason is that children’s respiratory defenses have not reached their full capability. Another is that children breathe more air per pound of body weight than adults so they take in more ozone per pound of body weight than adults do. Children are also in a phase of rapid growth, and their metabolic rates are higher than adult rates tend to be. Furthermore, children generally exercise outdoors more than adults do. According to a recent study conducted by researchers at the University of Southern California Keck School of Medicine with fourth grade school children, each increase of 20 ppb in ozone is associated with a 63 percent school absence rate increase for illness. The same increase in ozone concentration is associated with an 83 percent increase for respiratory illness. (Gilliland 2001) 20 ppb represents a relatively small and common increase in ozone concentration.