Plants As Air Purifiers

Many years ago, in the world, when there were no industries, no cars, and no human-made polluting processes, the air and the plants coexisted with the emissions of natural pollutants, and there was no problem. 

The plants had the capacity for adequate development and growth; the air was clean. Some plants can emit chemicals that can be pollutants. Nevertheless, these compounds are compatible with nature. So they can be used by other living organisms. 

In the natural environment, these natural air pollutants are not a big deal. After the industry started, the damage effects began to show and arise to giant levels of massive polluting industrialization. As I have said before, we needed to adapt the industries to biocompatible and safe processes, necessary to realize the change for the environment. Nowadays, those processes are so expensive or have not yet been developed. 

The implications of making this change would be to have clean nature, seeking to economize and respect nature. But also continue with the industries and its products that are so useful to humans. 

Environmental pollution and its large number of components are a problem for plants. The polluting gases and other chemical compounds in the air generate a wide variety of damage to their morphology. 

Science has strongly confirmed that the plants can absorb chemicals such as Sulfur Dioxide (SO2), nitrogen oxides (NOx, NO2- and NO3-), CO2, SO2, O3, Hydrofluoric Acid (HF), Hydrochloric Acid (HCl), Chlorine (Cl-), Ammonia (NH3), Ethylene (C2H4), photoxidants and other organic substances that enter the plant and are used in the metabolism. 

Image 1 shows a dense cloud of pollution that obstructs the vision; in winter, it tends to worse due to the increased permanence of these compounds in the air. You should also note that leaves are absent, so the ability of plants to clean the air is so reduced.

Image 1: Plants and nature often interact with pollutants such as smoke.

On the other hand, air pollution has many components, depending on the place. The amounts of some compounds may be higher than others. The uncontaminated rain has a pH close to 5.6 because the CO2 dissolved in it produces H2CO3, which is a weak acid, so it is not very dangerous; is not very concentrated; skin pH is among 4.7 to 5.75, while eye fluid or aqueous humor is around a pH value of 7. 

When other chemical compounds enter the composition of air such as NO and SO2 in the drops of water in the atmosphere, it causes the pH of the rain to drop to a value of 3 to 4. For example, in Southern California, Contaminated droplets in polluted air can be as acidic as a pH of 1.7 making the polluting mist too dangerous to be exposed to; this could cause burns in eyes and skin. 

All of this acid rain can remove mineral nutrients from the leaves, damaging the plant; the pollution and the acid can result in the release of aluminum ions and other metals from soil minerals, causing aluminum and metal toxicity. The rain also precipitates airborne compounds causing them to fall to the ground. 

Likewise, high absorption and accumulation of metals such as Iron, Lead, Zinc, Nickel, and Manganese in plants have been shown, this may cause the death of the plants, alter their structures, damage our food, make the plant toxic for use, among others. In Image 2, a maple leaf is discolored from a red to a dull hue, caused by air pollution intake; in this case, the leaf pigments are reactive towards air pollutants, which generates a loss of color when reacting.

Image 2. Pollution causes black spots on leaves, different from natural aging.

Has been demonstrated that placing plants inside the home generates a continuous decrease in indoor concentrations of volatile organic compounds (VOCs), with the benefit of not contaminating other locations due to energy production, such as air purifiers (which are also very useful in cases of extreme pollution). Plants have the ability to capture indoor concentrations of VOCs, like Formaldehyde, Toluene, Benzene, Toluene, Ethylbenzene, and many more. 

It depends on the species of the plant, it can have a higher and broader absorption of pollutants and certain specific compounds. Therefore, it has been proposed to position plant barriers in places of high contamination, such as next to polluting roads or factories, which is significant in reducing pollution. For example, Conifers offer one of the best particle reduction due to their leaf type, trees such as Pines and Cypresses and Evergreens, some Shrubs, Mosses, Aloe, and many others plats have been reported to be helpful to clean the air.

In Image 3 we see the high prevalence of constructions in certain cities, which is accompanied by an industrial environment that generates remarkable pollution; note the little number of trees, green areas, and plants compared to human constructions.

Image 3. The low presence of plants in city areas becomes a predominant factor to favor air pollution.

It should be noted that when plants receive the status of "air purifiers", does not mean that plants are adapted to that polluted environment or that they have an ideal response to this environmental condition; it means that the plants take the shot for us and absorb all these toxins, which causes great damage to their structures and possibly the death of the plant. 

For this reason, if you want to grown plants for this purpose, it must also be taken into account that these plants will most likely suffer significant damage when they are placed in excessively contaminated sites, so they probably need maintenance or renovation as their basic needs are obstructed. This is another consideration when looking for plant-based strategies to clean up air pollution.

However, if you live in a highly polluted place like being the neighbor of a smoke factory; don't risk it and buy a high-tech air purifier. Although having some type of plants inside your house is an aesthetic, pleasant, and a great alternative to having cleaner air to breathe every day.

References

• Gheorghe, Iuliana & Barbu, Ion. (2011). The Effects of Air Pollutants on Vegetation and the Role of Vegetation in Reducing Atmospheric Pollution.

• Citation: Azzazy MF (2020) Plant bioindicators of pollution in Sadat City, Western Nile Delta, Egypt. PLoS ONE 15(3): e0226315. https://doi.org/10.1371/journal.pone.0226315

• Vittoria Traverso. Urban trees can help cut air pollution from New York to Beijing, but which trees do the best job? Future Planet weighs up the options. BBC. 4th May 2020.

• Barwise, Y. and P. Kumar. “Designing vegetation barriers for urban air pollution abatement: a practical review for appropriate plant species selection.” npj Climate and Atmospheric Science 3 (2020): 1-19.