You often hear a variation on the idea that plants "absorb" air pollution. For example, in a Yahoo News article on Beijing smog, the reporter writes:

The China Daily said there are also not enough 'green areas' in the city 'to help soak up the fumes discharged by vehicles and industries'.

The Hindu Times writes:

plant trees that purify the air in and around your home

Now, I understand that plants consume some CO2, but to what extent do they "filter" the air from other toxins? What mechanical, chemical, or biological processes are involved? Do all plants have this effect or just some? Which toxins get filtered and which do not? How effective is plant-based air filtration in the home?

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    Fits better on Biology? Or do you have a specific reason to question this claim? – gerrit Dec 8 '15 at 15:25
  • Not 'around your home' but interesting nonetheless: spinoff.nasa.gov/Spinoff2007/ps_3.html – stijn Dec 8 '15 at 20:47
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    @gerrit I think skeptics is a good place for the question. do we need a reason to doubt? this is a classic case of "is it true" or "does it just make you feel warm and fuzzy?" – denten Dec 9 '15 at 14:22

According to the US Department of Agriculture, Forest Service publication THE EFFECTS OF URBAN TREES ON AIR QUALITY

Trees remove gaseous air pollution primarily by uptake via leaf stomata, though some gases are removed by the plant surface. Once inside the leaf, gases diffuse into intercellular spaces and may be absorbed by water films to form acids or react with inner-leaf surfaces. Trees also remove pollution by intercepting airborne particles. Some particles can be absorbed into the tree, though most particles that are intercepted are retained on the plant surface. The intercepted particle often is resuspended to the atmosphere, washed off by rain, or dropped to the ground with leaf and twig fall. Consequently, vegetation is only a temporary retention site for many atmospheric particles.

Pollutants removed by trees include ozone, sulfur dioxide, particulate matter, nitrogen dioxide, and carbon monoxide.

However, "Emissions of volatile organic compounds by trees can contribute to the formation of ozone and carbon monoxide."

VOC emission rates also vary by species. Nine genera that have the highest standardized isoprene emission rate and therefore the greatest relative effect among genera on increasing ozone, are: beefwood (Casuarina spp.), Eucalyptus spp., sweetgum (Liquidambar spp.), black gum (Nyssa spp.), sycamore (Platanus spp.), poplar (Populus spp.), oak (Quercus spp.), black locust (Robinia spp.), and willow (Salix spp.). However, due to the high degree of uncertainty in atmospheric modeling, results are currently inconclusive as to whether these genera will contribute to an overall net formation of ozone in cities (i.e., ozone formation from VOC emissions are greater than ozone removal). Some common genera in Brooklyn, NY, with the greatest relative effect on lowering ozone were mulberry (Morus spp.), cherry (Prunus spp.), linden (Tilia spp.) and honey locust (Gleditsia sp.)

See also: Tree and forest effects on air quality and human health in the United States Environmental Pollution (2014) vol. 193, pages 119-129:

The average annual percent air quality improvement due to trees varied among pollutants and ranged from a low of 0.13% in urban areas for PM2.5 to a high of 0.51% in rural areas for O3

See also: Modeled PM2.5 removal by trees in ten U.S. cities and associated health effects Environmental Pollution, 178 (2013), pp. 395–402

Average annual percent air quality improvement ranged between 0.05% in San Francisco and 0.24% in Atlanta.

Atlanta is 52.1% covered in trees, the most of the cities studied.

So even if a city is more than half covered with trees, the reduction in pollution is less than 1%.

  • What about particulate? – Ravenstine Dec 8 '15 at 17:40
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    @user10800 the reference says "Air quality improvement in New York City due to pollution removal by trees during daytime of the in-leaf season averaged 0.47% for particulate matter" – DavePhD Dec 8 '15 at 17:45
  • Ah thanks, I didn't catch that for some reason. 0.47% sounds very low; I know a lot of people who believe that trees have a significant impact on smog. – Ravenstine Dec 8 '15 at 18:15
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    Wow, so not only do cherry trees look absolutely beautiful and produce delicious fruits, they're also among the best at reducing pollution? Nice! – Mason Wheeler Dec 8 '15 at 18:34
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    @gerrit the first reference says "In urban areas with 100% tree cover (i.e., contiguous forest stands), short-term improvements in air quality (one hour) from pollution removal by trees were as high as 15% for ozone, 14% for sulfur dioxide, 13% for particulate matter, 8% for nitrogen dioxide, and 0.05% for carbon monoxide" citing to nrs.fs.fed.us/pubs/8420 – DavePhD Dec 9 '15 at 17:37

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