Do Plants “Exhale” at Night?

Plants reverse the process of photosynthesis at night, instead producing carbon dioxide.
Image: Audrey / Flickr CC

We all know that plants and trees produce oxygen during the day while photosynthesis is occurring.  However, not many people know that they actually go through an opposite reaction overnight.  As strange as it sounds, plants actually reverse the process and produce carbon dioxide while there is a lack of sunlight.

Some people think that plants breathe in carbon dioxide during the day and breathe out oxygen.  However, plants do not really “breathe” at all.  They don’t have lungs! They absorb CO2 through their stomates, or small cells, in their leaves and they convert sunlight into energy and release oxygen.

Part of the confusion about whether plants “breathe” can be due to the process in which they take in oxygen and produce carbon dioxide.  It is called “respiration.”

Photosynthesis process. Image:
Photosynthesis process.

While a tree is growing, it must take in more CO2 than oxygen it emits.  So, although trees do give off carbon dioxide at night, it is not enough to worry about.

There is an old rumor that it is dangerous to sleep in a room with too many trees.  Some worried that the CO2 they emit at night could kill you.  At one point flowers were even eliminated from some hospital rooms.  What was the real danger?  People would often knock them over at night.

Even today, in India, some people tell you never to sleep under a Banyan Tree.  They say the overhanging branches form a canopy and the CO2 emitted at night is dangerous enough to suffocate you.

Yet, cows, goats and other animals sleep under trees all the time.  We would see a lot more dead animals if those rumors were true.  Also, what about the people whose homes are in the middle of jungles completely surrounded by plants and trees?  They are just fine.

Wind may play a factor, but the truth is, while plants do emit CO2 at night, they just don’t emit enough to harm anyone.

Is it safe to sleep under a Banyan tree? Some people don't think so. Image:
Is it safe to sleep under a Banyan tree? Some people don’t think so.

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