Is Rainfall Depositing Microplastics Into Forests Worldwide?

Take a walk through a forest in central Germany. Breathe the clean air, listen to the silence. Now picture this: every square metre of soil beneath your feet holds, on average, nearly 400,000 tiny plastic particles — some no wider than a human hair. That is what Technical University Darmstadt researchers Collin J. Weber and Moritz Bigalke found, publishing their results in Communications Earth & Environment in August 2025. The source, their data suggests, is not a nearby factory or a careless hiker. It is the rain.

Forests Were the Last Place Scientists Looked

Microplastic research has spent years focused on the obvious problem areas — agricultural fields dosed with sewage sludge, coastal sediments, urban parks. Forests, which cover roughly 31% of the global land surface, were largely passed over. The assumption was that without direct plastic sources nearby, forest soils would stay relatively clean. A few earlier studies in South Korea, China, and Mexico did find plastic in forest soil — but their equipment could only detect larger fragments. Anything smaller than about 40 micrometres, roughly half the width of a fine human hair, slipped through undetected. And none of those studies sampled the leaf litter layers sitting right at the surface, which is exactly where airborne particles land first.

Why Earlier Studies Missed Most of the Plastic

Weber and Bigalke sampled four mixed forest sites east of Darmstadt through late 2023. They collected material from every layer of the soil profile — fresh leaf litter at the top, two stages of decomposing organic matter in the middle, and two deeper mineral layers below that. They also set up glass funnel collectors to catch rainfall passing through the canopy over a two-month window. The key difference from earlier work was sensitivity. Their extraction method detected particles down to about 20 micrometres, and they used a chemical imaging technique to confirm exactly what type of plastic each particle was — rather than estimating visually under a microscope. The result: particle counts roughly 20 times higher than anything previously reported for forest soils.

How Does Rain Carry Microplastics Into Forest Soil?

Here is the part that should stop you. The plastic found in the rainfall water and in the soil matched almost perfectly — same types of polymer, same average particle size of around 62–65 micrometres, same shape. That is hard to explain any other way than a shared origin. Lead author Dr. Collin J. Weber describes the process directly: microplastics drift in through the air and settle on tree leaves — what scientists call the "comb-out effect." Then rain washes them downward, or autumn leaf fall carries them to the ground. Once on the forest floor, the leaves break down, and the plastic fragments migrate deeper into the soil, layer by layer, year by year. The team ran the numbers back to 1950, matching deposition rates to the rise of European plastic production, and found that the plastic levels in today's soils could plausibly have built up this way — though only under the higher-end estimates.

What Forests With This Much Plastic Actually Mean

The concentrations found here are not small. These forest soils came in at levels matching — or beating — agricultural and wetland soils in global comparisons, landing squarely in the same range as urban soils. Think about that. Cities accumulate plastic through traffic, construction, and constant human activity. These forests had none of that. Just rain, falling quietly for decades. There is also a human angle worth sitting with: if microplastics travel through the air and settle in remote forests far from any city, they are almost certainly present in the air people breathe every day. That raises harder questions about long-term exposure that this study does not answer but clearly implies. For now, the finding matters most for how scientists define pollution baselines — plastic arriving through the air, the authors argue, should be counted as a constant background before any specific local source is even added.

The Questions the Data Can't Answer Yet

The study has real limits, and Weber and Bigalke are upfront about them. The rainfall sampling ran for only two months, entirely outside the growing season — meaning the canopy was bare and trapping far less than it would in summer. Fibre-shaped plastic particles were also likely undercounted, since thin fibres can slip through the fine filters used during extraction regardless of how long they are. And the team measured rainfall after it passed through the canopy, not clean air above the trees, so it is hard to separate what came from the sky from what had already been cycling inside the forest. Year-round sampling, above and below the canopy, is the obvious next step. What this study does establish, clearly, is that forests across the world have been quietly stockpiling airborne plastic since at least the 1950s — and the accumulation has not stopped.

• Forests aren't clean by default — atmospheric deposition alone can push soil microplastic concentrations to levels associated with urban environments, even far from direct human activity.
• Litter layers matter — organic horizons that previous studies discarded are where incoming plastic first accumulates; skipping them means missing the story of how particles enter the system.
• Background contamination needs a new baseline — any future study assessing a specific plastic source in forest or rural soils must now account for decades of diffuse atmospheric input already baked into the soil.

"Deposition should be considered as a significant background when specific sources of MPs are discussed, and their cumulative effects might even be higher than those of well-known high MP sources because MPs from deposition accumulate constantly over long timescales." — Weber & Bigalke, Communications Earth & Environment, 2025.