fig2

Figure 2. Concentration (in ng/g) of nicotine, nicotelline, ∑TSNA, and ∑PAH in indoor SHD in different districts. District classification (number of samples): 1: urban (10); 2: urban background/rural (7); 3: urban/harbor (14); 4: urban background/rural (7); 5: quarries/industrial/trans-shipment hub (16). ○: Outliers; *: extreme values.