Children in the study who entered day care between the ages of 6 and 12 months were found to have a 75% reduction in the risk of wheezing, which is considered an early symptom of asthma.
Those who began day care after their first birthday were found to have a 35% reduction in risk, compared with children who did not attend day care at all.
"Our findings suggest that this 6-to-12 month window of day care entry is especially protective, but this needs to be confirmed in other studies," study researcher Nicolaos C. Nicolaou, MD, tells WebMD.
Hygiene Hypothesis Explored
Proponents of the hygiene hypothesis say it explains, in large part, why allergy and asthma rates have skyrocketed in industrialized countries as exposure to infectious agents and germs has declined.
As our homes and public spaces have become cleaner, the theory goes, young children are being exposed to fewer of the germs and infectious agents that may help their developing immune systems recognize and fight allergic disease.
Asthma is now the most common chronic disorder in childhood, affecting an estimated 6.2 million children in the U.S., according to the American Lung Association.
Several earlier studies designed to explore the hygiene hypothesis have focused on day care attendance because children who go to day care tend to be exposed to more children -- and therefore more germs and infections.
Many of these studies have supported the theory, Nicolaou says, but others have not. "It is fair to say that the findings have been conflicting."
Impact of Day Care
Nicolaou and colleagues from England's University of Manchester examined the impact of day care attendance and other environmental exposures on asthma symptoms in more than 900 children followed from birth though age 5.
Being male, testing positive for allergic sensitivity, having a mother with asthma, and having a mother who smoked during pregnancy were all associated with an increased risk for wheezing at age 5.
Day care attendance was found to protect against wheezing, but having older siblings was not.
"This is probably because the size of the exposure matters," Nicolaou says. "Being exposed to a lot of children very early appears to be more protective than being exposed to just a few."
He adds that day care attendance is probably most protective for children who have a genetic risk for asthma.
"We don't want to send the wrong message that day care is definitely protective for every kid," he says. "We just can't say that."
The new study supports, but does not prove, the hygiene hypothesis, asthma and allergy specialist Neil Kao, MD, tells WebMD.
Kao says he would like to see larger studies that follow children into adulthood.
He is an assistant professor at the University of South Carolina, Greenville, and a spokesman for the American Academy of Allergy, Asthma and Immunology,
"This study is a very good effort, but for me the hygiene hypothesis has not been proven," Kao says.