Houseplants have a great reputation for purifying the air in our homes. In Air Purifying Plants – Do They Work?, I debunked the idea that houseplants remove VOCs (toxic chemicals) from our home – it is just a well publicized myth. Several people commenting on that post and the post called A Garden Myth is Born – Plants Don’t Purify Air, to make the point that plants do more than remove chemicals – houseplants increase oxygen levels in the air. This increased oxygen contributes a lot to our well being – or so people claim. Do houseplants increase oxygen levels in the home?

Plants and Photosynthesis

Through photosynthesis, plants combine CO2 with water and produce sugars and O2 (oxygen). Everyone accepts this fact. In nature, the production of oxygen is so important that without plants we would soon use it up and die.

Logically it follows that plants in a home would also contribute a significant amount of oxygen. There is no doubt that they produce oxygen, but how much do they produce? Is the amount significant compared to the amount we consume? Does a home with plants have a higher oxygen level than one without?

Good Quality Oxygen

A number of websites suggest that plants produce a good quality of oxygen. There is no such thing. Oxygen is oxygen. It is a simple molecule and the oxygen produced by plants is exactly the same as the one found in air.

How Much Oxygen Do Humans Consume?

The science of oxygen use by humans is well understood (ref 1). An adult uses about 550 L of oxygen per day.

How Much Oxygen Does a Plant Produce?

The amount of oxygen that a plant produces is much more difficult to calculate because it depends on many variables. Plants produce oxygen as a byproduct of making sugars, which is their energy source. Slow growing plants need much less sugar than fast growing plants, and therefore produce much less sugar and oxygen.

Low levels of light affect photosynthesis and result in less oxygen production. Temperature, water levels and available nutrients also impact photosynthesis and in turn oxygen levels.

Photosynthesis in a plant results in the plant getting carbon from the air and adding it to its body – leaves, stems and roots. Each molecule of CO2 absorbed, adds one atom of carbon to the weight of the plant and produces one molecule of O2. We can therefore get an estimate of the amount of oxygen produced by weighing the plant.

Marco Thorn has made this estimate and concluded that “for every 150 grams of plant tissue grown, 32 grams of oxygen are released. This is 22 liters of oxygen under normal temperature and pressure” (ref 2).

Plants Also Produce Carbon Dioxide (CO2)

Photosynthesis converts CO2 to O2, but plants also respire. During respiration they convert sugar and oxygen into CO2 and water. This is the reverse of photosynthesis, and it happens in all cells, all of the time, day and night.

Over time plants get bigger and heavier due to the carbon they accumulate. Therefore we know that the amount of CO2 produced from respiration is less than the CO2 used in photosynthesis – or else they could not grow.

Respiration reduces the net amount of oxygen plants produce, especially at night when there is no photosynthesis.

Plants vs Humans

Humans consume 550 L oxygen per day (ref 1). How much plant growth do we need to produce that amount of oxygen?

Plants produce 22 L for every 150 g of growth (ref 2). They would need to increase in weight by 3.75 Kg (8 pounds), each day, to produce the oxygen used by one person.

Keep in mind that plants grow slowly. Adding 3.75 Kg to your houseplants every day would require a huge number of plants.

In most homes the plants cannot produce oxygen at anywhere near the amounts we consume.

Do Houseplants Increase Oxygen Levels?

So houseplants can’t supply all the oxygen we need, but do they increase the oxygen level?

From the NASA Fact Sheet we know that air contains 20.95% O2 and 0.04% CO2. If you had enough plants in a room to use up all of the CO2 and convert it to oxygen, the oxygen levels would increase from 20.95% to 21% (ref 3). This increase is difficult to detect and would have no effect on humans. Keep in mind that this increase is the maximum increase possible and assumes plants would use all the CO2 available. In real life the increase is even less.

Do Houseplants Increase Oxygen Levels?

Not really. They do add oxygen to the room, but in such small amounts that their contribution is negligible. People have a much larger effect on O2 and CO2 levels in a room than plants. If you want to live in a higher oxygen environment – get rid of the spouse and kids!

The main factor contributing to good oxygen levels is the ventilation rate – the exchange of air with the outdoors.

Grow houseplants because you enjoy them – not because they will improve the air in your home.

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