Answer: Thomas Ogren, author of Allergy-Free Gardening (published by Ten Speed Press) developed a trademarked scale called OPALS(TM) or the Ogren Plant Allergy Scale. With this scale plants are ranked from 1 to 10, with 10 having the greatest potential to cause allergy problems. Various criteria involving 70 factors are used to rank plants. The scale has been reviewed by allergists and botanists and judged to be a useful tool. USDA researchers at the Northeastern Research Station in Syracuse, New York have started using OPALS(TM) for its allergy projections in major U.S. urban areas.
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