Ask Mr. Smarty Plants

All green plants produce oxygen as a byproduct of photosynthesis. How much oxygen a plant produces depends on the size and health of the plant and the amount of light available to it. In general, the more sunlight there is, the more photosynthesis will occur and the greater the amount of oxygen will be produced. However, there are different optimal amounts of light for various plants. Some plants, e.g., tropicals such as philodendrons, produce oxygen very efficiently at low light levels; whereas, other plants require high levels of light to carry out photosynthesis. So, it depends on the light level in the area of your house where your plants are growing and the kind of plants you choose. If you choose plants that require bright light for photosynthesis and you don't have bright light, they will need to use stored energy in order to function. They will be breaking down reserved carbohydrates and fats in a process called respiration. Respiration uses oxygen and releases carbon dioxide. If a plant adapted to efficient photosynthesis at low light levels is subjected to high intensity light, its light-absorbing pigments could be damaged and it may overheat. These effects could interfere with its ability to photosynthesize and produce oxygen. Therefore, the plants that will produce the most oxygen are the ones that will be the most efficient at photosynthesis at the light level in your house.

Your memory of a group of four plants that will increase oxygen in the house might be those plants identified in a study by NASA as plants that clean the air by removing harmful chemicals such as formaldehyde, benzene, carbon monoxide and trichloroethylene. These include philodendron, spider plant, and golden pothos that are effective in removing formaldehyde; gerbera daisy and chrysanthemum for benzene, and peace lily and chrysanthemum for trichloroethylene. You can read more about the effectiveness of houseplants for removing significant amounts of household air pollution.