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I usually hear that plants should be placed in working areas to supplement the higher oxygen needs of working humans, or on the other hand, that should never be grown in bedrooms because they drain oxygen for their own respiration during night.

I somewhat doubt this is significant in comparison to natural variation in oxygen flow from the outside...

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    I was told this in the primary school by the biology teacher, when taught about photosynthesis and respiration of plants. It is a common myth in our country (mentioned as false e.g. in tyden.cz/rubriky/zdravi/…).
    – Suma
    Sep 7, 2011 at 7:20
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    Another (English language) reference to prove the claim is notable: ca.answers.yahoo.com/question/index?qid=20070612124148AAZHiWZ
    – Suma
    Sep 7, 2011 at 7:22
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    @xiaohouzi79: I disagree, this isn't a duplicate. The question here is about oxygen drain in the night, which isn't dealt with in that other question.
    – Hendrik Vogt
    Sep 7, 2011 at 9:47
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    The other question is about removing pollutants from air, it is not about oxygen production of plants. It's not an exact duplicate so I'm reopening.
    – Mad Scientist ♦
    Sep 7, 2011 at 11:32
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    In an upcoming stunt for a tourist attraction and television show, a man will spend 48 hours locked in a small, sealed room with only "dozens of specially chosen plants" to replenish the oxygen. Lights will ensure that the plants remain in 'day mode' consuming carbon dioxide. Source
    – Oddthinking ♦
    Sep 11, 2011 at 6:26

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You are correct that the effect on oxygen levels are not significant. There is a much greater impact is on CO2 levels. Still, with respect to the effect on indoor air quality, the most important influence of plants is the removal of a class of pollutants known as volatile organic compounds (a.k.a. 'fumes') that include formaldehyde (Kim et al, 2010, Tarran et al 2007, Aydogan et al 2011. Plants also substantially reduce CO (carbon monoxide) (Tarran et al 2007).

Regarding the effect of plant respiration on air composition, it is also important to note that plant respiration is primarily affected by temperature, and occurs all day and night. However, during the day, the CO2 released by respiration is much less than the CO2 taken up by photosynthesis - otherwise the plant would have a negative carbon balance and eventually die.

For each unit of CO2 a plant takes up, it releases one unit of O2. However, the baseline concentration of O2 in the atmosphere is 20.95%, whereas the baseline concentration of CO2 is about 0.04% (NASA Fact Sheet).

Therefore, even if the plant converted all of the CO2 in the room to O2, the change in the percent would be from 20.95% to 21%. This would be difficult to detect. (Thanks to Oddthinking for pointing this out). Plant respiration rate is much lower than photosynthesis rate (otherwise they would have a carbon deficit and die) so the effect on O2 at night would be less than this.

Indeed, the presence of a person in the room would has a much larger effect on the O2 and CO2 in the room, and ventilation rate is the most important factor in maintaining a habitable atmosphere.

The most relevant citation:

Related reading not cited above:

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    It seems to me the critical fact in here is "For each unit of CO2 a plant takes up, it releases one unit of O2. However, the baseline concentration of O2 in the atmosphere is 21%, whereas the baseline concentration of CO2 is about 0.04%" If the plant successfully turned converted ALL of the CO2 into O2 (+ solids) - a gross exaggeration - the O2 levels would be 21.04% - hardly a big difference. (I am trying to remember enough Year 11 science to find any contradictions here, but I can't see any.)
    – Oddthinking ♦
    Sep 7, 2011 at 3:49
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    @Oddthinking Well, this is obvious that neither human nor plant will dramatically change air composition, the problem is how much plant biomass will balance CO_2 flux from a single person and whether it is totally overwhelmed by the exchange with atmosphere (as I suspect).
    – mbq
    Sep 7, 2011 at 9:07
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    "presence of a person in the room would has a much larger effect on the O2" this should be possible to reference (e.g. how much CO2 does an average person produce while sleeping?, and provide some estimation of CO2 level impact for a very small bedroom, e.g. 25 m3).
    – Suma
    Sep 7, 2011 at 12:36
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    @suma, this is addressed in the Tarran et al reference, and is byond the scope of the original question.
    – David LeBauer
    Sep 7, 2011 at 13:33
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    @mbq: If the question was a (more detailed) variant on "How many plants does it take to counteract one person?", I would agree. However the question is whether the plant has much affect, and I think this shows that even a roomful of plants has a smaller effect than 0.04%.
    – Oddthinking ♦
    Sep 7, 2011 at 14:16

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