Now a friend tells me that houseplants and cut flowers do the opposite and should not be place in a room with a sick person. Where there is sufficient sunlight, houseplants produce oxygen and absorb carbon dioxide from the air. The carbon dioxide is used to create carbohydrates and other materials for the plant. The oxygen is a by-product of the light reaction of photosynthesis in which water is split into hydrogen, used to make the carbohydrates, and oxygen, which is released into the air. Since photosynthesis, especially the part producing the oxygen, requires light, the release of oxygen ceases in the dark or in dim light.
Plants also respire, as do animals.
In respiration, oxygen is consumed and carbon dioxide is released. This occurs in the dark and the light. During the day, more carbon dioxide is consumed by photosynthesis than is produced by respiration. At night the production of carbon dioxide is greater. However, the production of carbon dioxide is not the reason for concern regarding the placing of plants in a room with an ill person.