potential confounders and the interaction with smoke exposure.
Also, earlier studies did not use individual exposure
levels [27], took only the birth addresses into account or
were not able to adjust for home movement [9,14,25]. Children
in our study were exposed to a smaller range of NO2
exposure (range 28.8-56.1 μg/m3) as compared with another
Dutch study (NO2 range 12.6-58.4 μg/m3) which
might have led to smaller effect estimates [14]. By using
long term exposure averages, the potential short term high
risk exposure levels may be missed. At the age of 1 year
only, we obtained information about wheezing in the last
month and the average exposure to air pollutants during
that month. Increased levels of air pollutants exposure during
the previous 1 month were associated with increased
risks of wheezing. We were not able to assess this short
time interval at older ages.