London ranks #1 in airborne microplastic pollution among four cities so far assessed, The Guardian reports. The health risks of breathing in plastic are not yet known – not because there are no risks but simply because they’ve not yet been studied. Some public health and medical experts say the need for research is urgent, as it’s likely that every urban area in the world is affected.

The rate of microplastic deposition measured in London was 20 times higher than that found in Dongguan, an industrial city in China’s densely populated Pearl River Delta, and seven times higher than in Paris. (Reasons for these variations have not been established but may be due to methodological differences.)

Sources of these tiny plastic particles are numerous and include our clothing and packaging we handle daily. We knew the particles were polluting some drinking water, waterways and oceans, but understanding how polluted our air may be with microplastics is at the cutting edge of environmental science. How, for example, did microplastics enter Arctic snow and mountain soils, both documented effects? How far did particles travel? What are their sources?

New research just published in the journal Environment International, and reported in The Guardian, looked at microplastics collected on the roof of a none-story building in central London.

The research, published in the journal Environment International, collected the microplastics falling onto the roof of a nine-storey building in central London. Fifteen types of plastics were identified, with most made of acrylic, likely from clothing. Some eight percent were likely from plastics used in food packaging.

We’ve worked hard to limit (and, ideally, end) traffic and industrial sources of pollution in recent decades because they’re known to cause serious health issues. The evidence is already showing we need to the same for plastics.