For each interquartile increment ( 3.49 g/m ( 3 ) ) of cumulative PM2 .5 exposure , the average WM volume ( WMV ; 95 % confidence interval ) was 6.23 cm ( 3 ) ( 3.72-8 .74 ) smaller in the total brain and 4.47 cm ( 3 ) ( 2.27-6 .67 ) lower in the association areas , equivalent to 1 to 2 years of brain aging .