Pollution is anything that contaminates the natural environment and causes it to get worse. In one sense then, litter doesn’t create pollution – it IS pollution.
Litter might hurt animals, and chemicals in some pieces of litter might get washed out by rain and pollute the soil. Tiny pieces of litter can even collect in the pacific ocean, polluting the sea: https://en.wikipedia.org/wiki/Great_Pacific_garbage_patch
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