Although the impact of increasing atmospheric carbon dioxide concentration ([CO₂]) on production of common ragweed (Ambrosia artemisiifolia L.) pollen has been examined in both indoor and outdoor experiments, the relationship between allergen expression and [CO₂] is not known. An enzyme-linked immunosorbent assay (ELISA) was used to quantify Amb a 1, ragweed’s major allergen, in protein extracted from pollen of A. artemisiifolia grown at different [CO₂] values in a previous experiment. The concentrations used approximated atmospheric pre-industrial conditions (i.e. at the end of the 19th century), current conditions, and the CO₂ concentration projected for the middle of the 21st century (280, 370 and 600 μmol mol^–1 CO₂, respectively). Although total pollen protein remained unchanged, significant increases in Amb a 1 allergen were observed between pre-industrial and projected future [CO₂] and between current and projected future [CO₂] (1.8 and 1.6 times, respectively). These data suggest that recent and projected increases in [CO₂] could directly increase the allergenicity of ragweed pollen and consequently the prevalence and / or severity of seasonal allergic disease. However, genetic and abiotic factors governing allergen expression will need to be better established to fully understand these data and their implications for public health.