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The researchers began by analysing hospital records of the 56.5 million US Medicare patients. They looked at those who were admitted for the first time between 2000 and 2014 with the protein damage. Armed with the patients' zip codes, the scientists estimated their long-term exposure to PM2.5 pollution, airborne particles that are smaller than 2.5 thousandths of a millimetre. These can be inhaled deep into the lungs and are found in the bloodstream, brain and other organs.

They found that long-term exposure to PM2.5 raised the risk of Lewy body dementia, but had less of an impact on rates of another neurodegenerative brain disease that is not driven by the toxic proteins.