The relative risk for pediatric outpatient visits for asthma during 20-day cold spell in 2008 compared with the same periods in 2007 and 2009a.

aAll the models were adjusted for day of the week, season and long-term trend; and all RRs were statistically significant (PbModel 1 did not controlled for relative humidity and O3;cModel 2 controlled for relative humidity;dModel 3 controlled for O3;eModel 4 controlled for relative humidity and O3;