The incidence of asthma has doubled in the past two decades, though the cause of this rise is difficult to pinpoint. While the hygiene hypothesis has, for years, served as one possible explanation, newer studies show a possible link between asthma and vitamin levels or exposure to certain chemicals or medications. How might this knowledge help us prevent the development of asthma, and what research still needs to be performed to determine the key factors here, and put an end to the increasing incidence of asthma? Dr. Harold Nelson, professor of medicine at the University of Colorado School of Medicine and National Jewish Health hospital in Denver, lays out the evidence for these potential risk factors, from the child's date of birth, to acetaminophen use, to vitamin D levels, to exposure to cleaning sprays and other household chemicals. Dr. Jennifer Shu hosts.