Validity of results
This study was based on a prospective follow-up of a large
population-based cohort from Finland from early childhood to the ages of
20-27 years. The response rates were relatively high (80.3% at baseline
and 77.3% and 63.2% at follow-ups) and no substantial differences were
detected between the baseline and follow-up study populations,
suggesting that any major selection bias was unlikely.
One major advantage of the present study is that we assessed exposure to
green spaces at the individual level, including both the prenatal and
early-life residential addresses, which is likely to improve the
exposure assessment. Furthermore, we estimated the cumulative exposure,
which took into account the duration of exposure in different places of
residence from the estimated conception to the end of the follow-up
period. In addition, we assessed the season-specific cumulative exposure
to green spaces separately for the spring and summer seasons, whereas
most of the previous studies have used annual averages of greenness
exposure based on one time-point in the summer, when the vegetation
growth is at its maximum. It has been suggested that seasonal
differences in timing of exposure assessment could explain some of the
heterogeneity detected in the effects estimates in previous studies.
Taking into account the abundance of the season-specific vegetation
during pregnancy enabled an accurate assessment of trimesters-specific
effects of greenness in different seasons.
NDVI is an objective measure to assess green vegetation density, but its
disadvantage is that it does not provide information on plant species,
biodiversity, or accessibility to these green spaces. Season affects the
type, distribution, and abundance of vegetation, as well as the levels
of allergenic material, such as pollen grains, which have been shown to
play a role in the development of allergic diseases. We did not measure
exposure to pollen, but our season-specific exposure assessment
indirectly provides information on the temporal succession and changes
in the abundance of allergologically potent vegetation.
The definition of asthma was based on parental- and self-administered
questionnaire information on the presence of asthma and the age of
onset. We evaluated possible misclassification by contacting those
parents who reported physician-diagnosed asthma, and all the parents
were found to be well-informed about the presence of asthma in their
children. It is also reasonable to assume that young adults at the
20-year follow-up were well aware of the asthma diagnosis. The Finnish
healthcare system provides rather good reimbursement for the costs of
asthma medications when the diagnosis is made according to the criteria
approved by the Finnish Social Insurance Institution, which is a strong
economic incentive for people to have their asthma clinically diagnosed.
Moreover, a study that used Finnish version of ISAAC questionnaire on
parental reports for physician-diagnosed asthma in children showed that
questionnaires provide a reliable method for health outcome assessment
in epidemiological studies.