It causes about half of all cases of pollen-associated allergic rhinitis in North America, where ragweeds are most abundant and diverse.

[13] Concentration of ragweed pollen—in the absence of significant rainfall, which removes pollen from the air- is the lowest in the early morning hours (6:00 AM), when emissions starts.

[14] Ragweed pollen can remain airborne for days and travel great distances, and can even be carried 300–400 miles (500–600 km) out to sea.

[12] Ragweeds native to the Americas have been introduced to Europe starting in the nineteenth century and especially during World War I, and have spread rapidly since the 1950s.

[16] The major allergenic compound in the pollen has been identified as Amb a 1, a 38 kDa nonglycosylated protein composed of two subunits.

[18] Foods commonly involved include beans, celery, cumin, hazelnuts, kiwifruit, parsley, potatoes, bananas, melons, cucumbers, and zucchini.

Signs of reaction can include itching, burning, and swelling of the mouth and throat, runny eyes and nose, hives, and, less commonly, vomiting, diarrhea, asthma, and anaphylaxis.

When the ragweed antigen then attaches to these antibodies the mast cells release histamine and other symptom-evoking chemicals.

[citation needed] As of 2006, research into allergy immunotherapy treatment involved administering doses of the allergen to accustom the body to induce specific long-term tolerance.

There is evidence that mechanical and chemical control methods are actually no more effective in the long run than leaving the weed in place.