How many plants are needed to make enough oxygen for one person for one hour?


Question:
How many plants are needed to make enough oxygen for one person for one hour? We are experimenting with Anacharis plants.


Replies:
The problem can be solved when broken down into smaller questions:

1. How much oxygen does a person need in an hour?

2. How much oxygen does a plant produce in an hour?

3. Based on the above, how many plants will provide the oxygen needs of the person for the hour?

Here is the solution to the first question: A resting, healthy adult on an average, cool day breathes in about 53 liters of oxygen per hour. An average, resting, health adult breathes in about 500 mL of air per breath. This is called the normal tidal volume. Now, 150 mL of this air will go to non- functioning areas of the lung, called the "dead space." The average breath rate for this average person is 12 breaths per minute. So, the amount of air breathed in by the person which is available for use is 12 x (500 mL -150 mL) = 4,200 mL/minute. Multiply by 60 to get 252,000 mL/hour. That is, every hour, the person will breathe in 252 L of air. Now, on an average, cool, clear day, only 21% of that air is oxygen. So, 21% of 252 L is 53 L. So, in an hour, the person breathes in about 53 L of oxygen.

As you might see, there is a lot of assumptions in the above reasoning: average healthy, resting adult on an average, cool, clear day. Any change in activity, health, size, sex, etc. of the person or the temperature, humidity, barometric pressure, etc. of the day will change the figure we calculated. But, let us just work with the 53 L as a nice "average."

Now, to answer the second question, how much oxygen do plants produce in an hour. Actually, I have data for how much oxygen LEAVES produce in an hour: 5 mL. If your average plant has 30 leaves, then that would be 5 ml/leaf x 30 leaves = 150 ml/plant/hour. So, if an average person needs 53,000 ml (53 l) of oxygen per hour, and the average plant produces 150 ml per hour, then 53,000/150 = 353 plants. Since these are round figures, let us just say that between 300 to 400 plants are needed to produce enough oxygen to keep a person alive in an hour. Again, there are many assumptions: average leaf, average plant. Plus you will need to take into accounted oxygen production decreases as carbon dioxide concentration increases Assuming this hypothetical person is in a confined space with all these plants, the CO2 concentration will rise due to the person's expiration. This will inhibit the plant's photosynthetic rate. Oh, yes, do not forget we will need good amount of light and water, too.

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