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Author of Allergy-Free Gardening, Thomas Leo Ogren is also creator of the Ogren Plant Allergy Scale (OPALSTM), the first and only numerical plant-allergy ranking system. He rated species of plants on a scale of 1 to 10, with 10 being severely allergenic. According to his findings, "A few examples of trees with a rating of 1-3 are firs, cedars, and strawberry tree (Arbutus unedo), as well as the fruit trees like apricots, avocados, peaches, plums, nectarines, persimmon, pineapple, guava, but also almonds and flowering red leaf plums, bronze loquat, and flowering pears. Also the female trees of ash, Chinese fringe tree, Chinese pistache, junipers, some named maples, palms [pictured], podocarpus, poplars, sour gum or tupelo, and willows. Some of the worst trees are acacia, alder, beech, birch, buckeye, CA pepper, camphor, catalpa, Chinese evergreen elm, cypress, liquidambar, mimosa, oak, olive, pecan, sycamore, walnut, and zelkova."