Allergy epidemiology in the St. Louis, Missouri, area. IV. weeds.

Weeds elicit two levels of allergenicity, a high level by the Ragweeds and other members of the Asteraceae, and a much lower level by members of the families Amaranthaceae (Pigweed, Western Water Hemp), Chenopodiaceae (Lamb's Quarters, Kochia, and Russian Thistle), and Plantaginaceae (Plantain) in the St. Louis, Missouri area. The frequency of reaction among both adults and children as well as the cross-reactivity values are similarly divided.