Heat, light and carbon dioxide (CO²) promote plant growth. Plants grow by converting CO² to carbon through photosynthesis. Air generally contains approximately 350ppm CO². Optimal CO² levels depend on the type of plant and generally lie above 700ppm. With increased artificial lighting, as present in greenhouses, plants absorb even more CO². If the greenhouse atmosphere is enriched with CO², the temperature kept on a constant level and sufficient lighting provided, plant growth and consequently the harvest yield can be increased significantly.