[2] The OPALS allergy scale was first published in Allergy-Free Gardening, by Thomas Leo Ogren, in 2000.
[1] It covers over 3,000 common trees, shrubs, flowers, and grasses.
[3] The allergy scale was updated and extended in 2015 in The Allergy-Fighting Garden.
Additionally, plants that cause contact allergies (such as rashes or itching), or that are highly poisonous when ingested even though their pollen does not cause respiratory allergies, are never given low ratings.
Plants with ratings of 9 or 10 have an extremely high potential to cause allergic reactions.