- Natural forests composition can have a substantial impact on pollen allergenicity in cities.
- Cities developed within Pinaceae forests tend to have lower allergenic potential.
- Forests/open spaces had slightly higher allergenic potential than developed areas.
- On average, less than 10 % of city leaf area is within the lowest allergenic class.
Tree pollen with allergenic potential in cities triggers nasal, skin, eye, and asthmatic allergic reactions in humans. Pollen is one of the most common allergy-causing inhaled substances. Tree species composition, cultivar selection and the proximity of certain trees to humans can influence allergic responses. Data from 53 cities or states from various parts of the world were used to assess the magnitude and differences in an allergy index (AI) among the sampled locations and among land uses within cities. Index values for species ranged from one (most allergy-free species) to 10 (highest allergy potential). The average index score among the cities and states was 6.3, with values ranging from 4.2 (Halifax, Nova Scotia) to 8.3 (Austin, Texas). On average, forest/open space areas had a slightly higher average index score (6.4) than commercial/industrial/transportation (6.2) and residential areas (6.1). About 2/3 of the analyzed cities had higher index scores in forest/open space than in residential areas. Forest/open space areas contributed over 40 percent of total leaf area and may influence allergenicity in cities. Cities developed within areas with naturally occurring Pinaceae (pine, spruce, fir, cedar, Douglas fir, larch) forests and/or have more Pinaceae species tended to have the lowest pollen index scores. Most leaf area tended to come from trees in index class 8. While pollen production is essential for natural regeneration and potential seed/food production, limiting tree species with high allergenic potential in areas near people can help reduce tree-related allergies. Through better understanding and quantification of urban forest allergenicity, managers can create sustainable local-scale landscapes that limit pollen exposure to humans, but also provide numerous ecosystem services and values to residents.