From Metaefficient.com:
An easy way to add humidity to a room, is to put a few areca palms in it. These palms release copious amounts of water into the air, and remove chemical toxins too. They are consistently rated among the best houseplants for removing all indoor toxins tested. According to B. C. Wolverton, author of How To Grow Fresh Air, a six foot (1.8 m) areca palm transpires approximately 1 quart (1 litre) of water every 24 hours. So if you put three in your bedroom, the palms will be release three quarts of water per day — not bad!

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