8 April 2024

Forests keep us from breathing in microplastics


Airborne microplastics latch onto tree leaves, explains a Japanese study. In this way, forests act as sinks thereby limiting their uptake by humans

by Matteo Cavallito


Forests could function as actual reservoirs of airborne microplastics. This is claimed by a study from Japan Women’s University. The investigation, in particular, used a new technique to measure the levels of these materials adhering to tree leaves. And therefore shed a light on a still little-known phenomenon.

“Airborne microplastics are tiny plastic particulates (less than 0,1 millimeter) that become suspended in the atmosphere and dispersed throughout the environment, but it has been unclear where they end up,” says a statement released by the Japanese university. Moreover, “Forests have been known to accumulate airborne pollutants, but their ability to capture airborne microplastics has been poorly understood.”

The study

In the course of the investigation, the results of which were published in the journal Environmental Chemistry Letters, the authors developed a method for analyzing the levels of these microplastics on leaf surfaces and understanding how the leaves manage to trap them.  Experiments, conducted in a small area near Tokyo, focused on plants of Quercus serrata, an oak species typical of Japanese forests.

“Leaves were water-washed to yield a first extract, sonicated in water to yield a second extract and then extracted with 10%w potassium hydroxide to yield a third extract,” the study explains. The authors then used infrared spectroscopy to identify and quantify the concentration of microplastics.

The results, they explain, “show that the average number of microplastics in leaf were 0.01 piece/cm2 in the water extract (7.6%), 0.05 piece/cm2 by sonication (38.4%), and 0.07 piece/cm2 in the potassium hydroxide extract (53.8%).” The findings, therefore, “suggest that canopy leaves could be a long-term sink for airborne microplastics, rather than merely temporary accumulators.”

Canopies trap 420 trillion microparticles each year

According to Akane Miyazaki, a researcher and co-author of the study, microparticles would tend to accumulate “when they stick to the waxy surface coating of leaves.” Based on the results, “Based on our findings, we estimate that the Quercus serrata forests of Japan (~32,500 km2) trap approximately 420 trillion airborne microplastics per year in their canopies,” says her colleague Natsu Sunaga. “This shows that forests can act as terrestrial repositories for microplastics.”

It is still unknown how the accumulation of microplastics will affect these environments, the authors note, nor how this phenomenon will impact ecosystem functions and soil health. What is certain, the scientists conclude, is that forests and even roadside trees could reduce the amount of plastic absorbed by humans.

Stima delle quantità annue di plastica utilizzata nei terreni agricoli mondiali divise per tipologie di prodotto. FONTE: Assessment of Agricultural plastics and their sustainability. FAO, 2021

Estimated annual amounts of plastic used in global agricultural land divided by product type. SOURCE: Assessment of Agricultural plastics and their sustainability. FAO, 2021

The problem with microplastics

The issue of microplastic contamination has been the focus of attention for some time. Much has been discussed about the impact of their dispersion in the oceans, while less, perhaps, has been said about their diffusion into the air and soil. This latter phenomenon is also linked to the significant use of plastics in agricultural supply chains quantified at 12.5 million tons of products each year according to a report released in 2021 by the FAO, which, not surprisingly, recommended replacing synthetic mulch films with biodegradable and compostable specimens.

The same UN agency estimated that 37.3 million tons of plastic are used in food packaging. The largest users are the various segments of agricultural and livestock production, with 10.2 million tons per year combined. This is followed by fisheries and aquaculture with 2.1 million tons and forestry with 200 thousand tons.