In the past ten years, allergy and asthma rates have continued to increase dramatically around the world. An estimated 50 million Americans (1 in 5) suffer from asthma and allergies--more than diabetes, heart disease, cancer, and Alzheimer's. Thomas Ogren's innovative system is based on the fact that only male plants, which are over-planted in gardens and civic landscapes, produce pollen. By raising public awareness and by removing or replacing troublesome plants in your yard with pollen-blocking females and other pollen screens, allergy-sufferers can reduce or eliminate their symptoms. More than 3,000 plant listings are included and graded 1 to 10 according to OPALS, the plant-allergy ranking system created by Ogren and now used by the United States Department of Agriculture.