But pollution is not the main cause of asthma, researchers now say. In fact, asthma rates continued to rise as air pollution improved in the 1990s.
One of the more unusual theories on asthma comes from Dr. Homer Boushey, whose colleagues sometimes roll their eyes when his ideas are mentioned. Boushey, who runs the UCSF asthma lab, says that many cases of asthma can be pinned on a common cold virus that if caught during the first few months after birth upsets the immune system for life, leaving kids vulnerable to allergens that otherwise wouldn’t faze them.
Even Boushey calls himself a borderline heretic. But his research falls into the current prevailing theory of why asthma cases have increased so steadily: the hygiene hypothesis.
Essentially, researchers say, kids are too clean. When children are not exposed to the allergens that they might find on, say, a farm, their immune systems never develop the ability to distinguish safe irritants from unsafe ones. As a result, they become overly sensitive to harmless irritants.
“In most of our history we were exposed to enormous sources of microbes. We had large families, we lived with farm animals, we played in the dirt,” Boushey said. “Now maybe we don’t have enough stimulation of the immune system in our development as infants, and so we’re more vulnerable to things that cause asthma.”
Thinking along those lines, UCSF is running a five-year clinical trial on the relationship between asthma and probiotics, or bacteria that are commonly found in foods like yogurt and that could have health benefits. In the study, babies who have at least one parent with asthma are given daily doses of probiotics; they will be followed for five years to see if they develop asthma.