 Climate change could push the cost of US allergies and asthma beyond the current $32 billion annual price tag, conservation and health groups reported on Wednesday. A warming planet makes for longer growing seasons that would produce more allergy-provoking pollen in much of the heavily populated eastern two-thirds of the United States, the National Wildlife Federation and the Asthma and Allergy Foundation of America said in their report. The cost of coping with allergies and allergen-driven asthma in the United States is at $32 billion in direct medical costs, lost work days and lower productivity, the report said. "Climate change could allow highly allergenic trees like oaks and hickories to start replacing pines, spruces and firs that generally don't cause allergies, exposing many more people to springtime allergy triggers," said Amanda Staudt, a climate scientist at the wildlife federation. Spring-like conditions are already arriving 14 days earlier than 20 years ago, Staudt said. In the fall, ragweed plants will grow larger and more loaded with pollen over a longer growing season, Staudt said in a telephone interview. There is also evidence that ragweed, the biggest US allergy trigger, grows faster as carbon dioxide increases in the atmosphere. Carbon dioxide, a greenhouse gas that spurs climate change, is emitted by human-made sources like fossil-fuelled vehicles and coal-fired power plants as well as natural sources including human breath. CARBON DIOXIDE CONNECTION "With more carbon dioxide, each ragweed plant can produce more pollen and can even produce more allergenic pollen, so fall allergies are going to get a pretty big hit," Staudt said. The average global temperature last year tied for the second highest year on record and the decade from 2000-2009 was the hottest on record, according to NASA's Goddard Institute for Space Studies. This means agricultural and natural growing zones are shifting northward, allowing pollen-bearing trees to survive over a wider range than they have historically, the report said. About 10 million US residents have so-called allergic asthma, in which asthma attacks are triggered by pollen or other airborne allergens. These attacks are likely to increase as global warming causes these allergens to become more widespread, numerous and potent, the report said. Poison ivy, one of the top 10 medically problematic plants in the United States with more than 350,000 cases of contact dermatitis reported annually, would become more toxic and more widespread as the climate changes. When exposed to more carbon dioxide, poison ivy plants produce a more allergenic form of urushiol, the substance that makes skin itch.