The prevalence of allergic disease is expected to increase with climate change. Grasses, which have highly allergenic pollen, are widely distributed across the globe. Changes in production and allergen content of grass pollen have not been specifically investigated. We tested the effects of elevated carbon dioxide and ozone on growth, pollen and allergen production of Timothy grass (Phleum pratense L.). Timothy is also used as an agricultural forage crop so changes in plant productivity can also affect humans indirectly. Plants were fumigated in eight chambers at two concentrations of ozone (O3; 30 and 80 ppb) and carbon dioxide (CO2; 400 and 800 ppm) to simulate present and future projected levels. Destructive harvests were completed every three weeks to measure productivity. Pollen was collected in polyethylene bags placed around flowers and assessed for pollen number and concentration of the allergenic protein, Phl p 5. We found that elevated CO2 significantly increased the amount of pollen produced per flower regardless of O3 level. In addition, the amount of Phl p 5 allergen per flower was significantly increased in plants grown at elevated CO2 / low O3 conditions. We also found that plants grown in both elevated CO2 and elevated O3 increased the amount of pollen produced per weight of flower. The Phl p 5 allergen content per pollen grain was significantly reduced by elevated O3, as was flower length and weight. However, this was partially ameliorated by elevated CO2. Productivity was affected negatively by elevated O3 throughout the life cycle. CO2 increased shoot productivity during the intermediate stages of life and also ameliorated the negative impacts of elevated O3. We conclude that increasing levels of CO2 will cause a 2.5 times increase in Timothy grass pollen production thus increasing human airborne pollen exposure. Increases in pollen were likely a result of increased shoot biomass in the stages leading up to reproduction. If Timothy grass is a good model for other grasses, this portends for increased allergy suffering worldwide and an important health impact of global climate change.