"The new work [...] analysed language and arithmetic tests conducted as part of the China Family Panel Studies on 20,000 people across the nation between 2010 a

"The new work [...] analysed language and arithmetic tests conducted as part of the China Family Panel Studies on 20,000 people across the nation between 2010 and 2014. The scientists compared the test results with records of nitrogen dioxide and sulphur dioxide pollution." While I admittedly did not read the science paper, the newspaper indicates that they used test scores as the actual number, and the "cognitive drop equivalent to one year of education" phrase is just a way to explain the results, presumably anchored by how people in different school grades score in those tests.