Answer: Ragweed Pollen

Millions upon millions of people around the world suffer from what is commonly called “hay fever”. Hay fever, formally allergic rhinitis, is a type of inflammation of the nose resulting from an immune system overreaction to airborne allergens and typically involves a stuffy or runny nose, sneezing, red, itchy, and watery eyes, and swelling around the eyes.

There are a wide range of allergens that can cause hay fever, but the primary cause around the world is pollen, typically the small and light pollens of trees, grasses, and weeds that are pollinated via air movement and not the heavier pollens found in insect-pollinated plants like, say, magnolia trees.

Among all the allergen sources that trigger hay fever, however, there isn’t a source as prolific as ragweed. Ragweed is a flowering plant in the genus Ambrosia of the Asteraceae family and you’ll find it all throughout the tropical and subtropical regions of North, Central, and South America (as well as several species that have hitched a ride to Europe and become naturalized there).

Despite its smaller size compared to the vast reach of pollen shedding trees, ragweed is an astoundingly prolific pollen producer. A single ragweed plant can produce around a billion grains of pollen per season. Further, these particles are so light that they can stay airborne for days and travel on the wind for hundreds of miles. As such, a single stiff wind can loft untold billions of ragweed pollen particles high into the air and rain them down over hundreds of square miles of the surrounding countryside.

Given the sheer number of pollen grains and the ease with which they travel, it should come as no surprise that up to half of the hay fever cases in North America—where the plant is most prolific—can be traced back to ragweed pollen.