Humidifying the air

It is true that plants can improve air quality. But it is not their oxygen-production that really helps – it is rather ensuring sufficient humidity and clearing the air of certain pollutants.
To increase humidity, buy plants that have big leaf-surfaces or have their roots standing in water (like papyrus cane). You can not solve the problem only with plants, though, as you simply would not find enough place for the necessary amount in one room, so if it is possible, use evaporating bowls as well.
Plants are better than bowls when it comes to the need for “automatic” balancing of humidity-fluctuation. In drier air (e.g. in summer, or the heating season), they evaporate more, and if it is humid, they evaporate less. We should not forget that the amount of evaporated water depends on the size of the leaf- or water-surface. In a big office 1-2 plants and small evaporating bowls would not be enough to achieve real changes.