CO2 is an odorless, invisible, and non-flammable gas. It is also safe for humans in the maximum concentrations recommended for plant growth. The average level of CO2 in the atmosphere is about 390 PPM (parts per million). If the level decreases down below 200 PPM in an enclosed growing area, ps, plant growth rates and flowering will increase 20-100%. CO2 can be used from seedling right through harvest. It may take a hundred years for CO2 to get up to 1500 PPM. During that time, plant growth will flourish.