A U.S. Department of Agriculture study published in March in the journal Proceedings of the National Academy of Sciences has found that ragweed season is almost 16 days longer than it was in 1995 due to changes in the first frost line of the fall in North America. The first frost steadily has been creeping northward and later into the year, lead researcher Lewis Ziska wrote. That’s 16 more days of allergy relief sales — especially good news for Chattem as it shepherds its recently switched Allegra antihistamine through its first year.