Strengths and limitations
This study utilized a well-defined population with detailed observations
and personal histories covering a wide range of variables at clearly
defined time points. However, by being restricted to a single center the
findings are not as applicable in a wider context. Moreover, as we did
not have information on the residential addresses of the participants,
we used data from one air quality monitoring station as a proxy for
personal exposure to PM2.5. Furthermore, confirmed viral test was not
available for all participants. Also, diagnosing asthma in young
children is challening as it is difficult to instruct this patient group
to do a spirometry test. Finally, given the retrospective nature of this
study, we had limited information regarding some details of specific
variables. For example, we were not able to obtain detailed information
on smoking status, e.g. whether the parents of the children smoke on a
daily basis or occasionally.