Greenness, trees and allergenic trees
The Normalized Difference Vegetation Index (NDVI) 18was used as an indicator of greenness (i.e. vegetation degree). NDVI is constructed from the ratio of reflected light to absorbed light in two vegetation-informative bands of the electromagnetic spectrum: the red band (RED) and the near infrared band (NIR). NDVI is calculated as NDVI = (NIR − RED) / (NIR + RED). Thus, NDVI ranges from -1 (water) through 0 (barren areas) to +1 (areas completely covered by vegetation).
For our NDVI calculations, we used a single cloud-free Landsat 5 TM satellite image at a resolution of 30 by 30 m taken on June 6, 1997 (https://earthexplorer.usgs.gov/) which is about the time when recruitment for the study started. Summer images enable maximum spatial contrasts of greenness. 19
Tree registry data of the city of Leipzig were utilized to derive information on trees. Briefly, data are available for street trees and trees in green spaces and include tree geocoordinates, tree species in binomial and German nomenclature and year of planting. Only trees planted before 1998 were considered for this analysis: 63,579 trees from at least 82 genera. Figure E2 shows the spatial distribution of trees superimposed over an NDVI map.
We used two definitions to classify tree genera into allergenic and non-allergenic. Both definitions are based on the same three criteria. For a tree genus to be classified as allergenic according to definition 1, it had to satisfy two out of the three criteria. The stricter definition 2 required that all three criteria be satisfied. The first criterion is satisfied by tree genera whose pollen are being routinely monitored in Germany (http://www.pollenstiftung.de/pollenvorhersage/pollenflug-kalender). The second and third criteria relate to tree species and are satisfied by a genus if among the trees of that genus that were planted in Leipzig before 1998 there is at least one species that satisfies the criterion. The second criterion is satisfied by a species if its pollen allergens were characterized in a published study, or if it was described by a study as causing allergenic reactions in people that came into contact with its pollen (e.g. measured by skin prick test), or if there are reports about its sensitization/allergenicity rates. Literature search was done via Google Scholar in combination with the library access service of Technical University Munich (TUM OPAC plus). The third criterion is satisfied by a species if www.allergen.org, the most complete database of allergens, had at least one aeroallergen listed for it as of September 2, 2019. The details on all tree genera of the city of Leipzig planted before 1998, together with their abundances, are provided in Table E1. According to definition 1, 21,324 trees from the following 16 genera (sorted by tree count in descending order) were classified as allergenic: Fraxinus (ash), Quercus (oak), Platanus(plane), Prunus (prunus), Betula (birch), Populus(poplar), Carpinus (hornbeam), Corylus (hazel), Salix (willow), Pinus (pine), Fagus (beech), Alnus(alder), Sambucus (elder), Castanea (chestnut), Syringa (lilac) and Juniperus (juniper). According to definition 2, there were 9,453 allergenic trees from 7 genera: Fraxinus, Betula, Carpinus, Corylus, Fagus, Alnus and Syringa .
Mean NDVI, total number of trees and number of allergenic trees according to the two definitions were calculated for circular buffers of 100, 500 and 1,000 m around each participant’s home address at birth.
Geographic data pre-processing and calculations were conducted with ArcMap 10.4 and ArcGIS Pro 2.2 Geographical Information System (ESRI, Redlands, CA, USA).