Carbon dioxide fertilization involves increasing carbon dioxide levels in greenhouses to improve plant growth and yields. Higher CO2 levels shift the activity of the enzyme rubisco to favor carbon fixation during photosynthesis. On average, yields increase 33% when CO2 is doubled. Commercial greenhouse operations commonly use natural gas, propane, or boiler flue gases to maintain CO2 levels between 1000 ppm for sunny conditions to 700 ppm for young plants or lower for cloudy days with more ventilation. Higher CO2 leads to increased sugar levels in fruits and vegetables, improving taste, growth rates, and time to flowering for some ornamental crops.