Climate change, resulting in more frost-free days and warmer seasonal air temperatures, can contribute to shifts in flowering time and pollen initiation from allergenic plant species. Increased CO2 by itself can elevate production of plant-based allergens. Higher pollen concentrations and longer pollen seasons can increase allergic sensitizations and asthma episodes and diminish productive work and school days. Simultaneous exposure to toxic air pollutants can worsen allergic responses. Extreme rainfall and rising temperatures can also contribute to indoor air quality problems, including the growth of indoor fungi and molds, with increases in respiratory and asthma-related conditions. As pollen exposures increase, patients and their physicians will face increased challenges in maintaining adequate asthma control.