Exposure to airborne pollen has well known effects on human health. Over the past several decades, there has been a shift in allergenic pollen production in plants such as ragweed, oak, alder, and birch, as well as changes in timing and seasonality of pollen release for some species. Climatic factors, including precipitation patterns and temperature, have a direct impact on plant phenology and pollen production. The net impact of recent global climatic changes on pollen production and seasonality has not been summarized. We performed a literature review and analysis to find peer-reviewed research linking shifts in pollen to shifts in climate factors. We included studies that tracked pollen counts over time (at least 5 years of measured airborne pollen data), reported the presence or absence of a trend in pollen seasonality (length, start, end, or peak pollen date) or intensity (total annual pollen, daily concentration, or peak concentration), and reported a relationship between the measured pollen and locally/regionally relevant meteorological data (temperature, precipitation, humidity, or wind). Research published through January 2016 was included. In addition, we only included studies that reported quantitative pollen or trend data.
After creating and reviewing a database of over 15,000 potentially relevant articles, we determined that 62 articles fit all of our criteria and explicitly linked temperature or precipitation to quantitative changes in pollen over time. The majority of articles, which described more than 25 taxa, showed an advancing start date of pollen season, delayed pollen season end date, increased pollen season length, increased total annual pollen production, increased mean daily pollen counts, and increased pollen concentration on the peak pollen date. Trends for specific species and geographic areas were analyzed, as were the net impact of temperature and precipitation. While many different factors can affect pollen, the studies examined in this review add to the emerging evidence that climatic factors play a strong role in various aspects of pollen production and distribution. This is the first comprehensive literature review on the effects of temperature and precipitation on pollen trends.