Moss and air pollution come up together a lot, partly for good reasons and partly for hype. Here is what moss genuinely does, and where the claims run ahead of the evidence.
Moss as a pollution monitor
This part is solid. Because moss has no roots and feeds straight from the air and rain, it accumulates whatever is in them, including heavy metals and nitrogen compounds. Scientists exploit this directly, sampling moss across a region to map airborne pollution cheaply and over wide areas, a technique called biomonitoring. As a sensor of air quality, moss is genuinely useful.
The moss wall claims
You will have seen installations marketed as moss walls that "clean the air of a city" or do the work of many trees. Treat these carefully. Moss does take up some particulates and gases, and a large damp moss surface has a real if modest effect on its immediate surroundings. But the headline figures are often generous, the units do not always survive scrutiny, and the walls usually need careful watering and upkeep to stay alive and effective at all. The honest position is that moss helps a little, locally, and is not a substitute for cutting pollution at source.
What moss can really offer in a city
Set aside the strongest claims and there is still a genuine case: moss surfaces cool their surroundings, hold rainwater, add habitat, and yes, monitor and modestly absorb pollution, all with very little weight or upkeep compared with planting. As one tool among many for greener, cooler streets, moss earns its place. As a magic air filter, it does not. See what moss is good for for the wider picture.