Another recent study suggests that promoter variants in NOS2 encoding for the inducible nitric oxide synthase (iNOS), when exposed to particulate matter with aerodynamic diameter ≤2.5 μm (PM2.5), were found to influence iNOS methylation pattern and thus affect the concentration of nitric oxide in exhaled breath (FeNO) levels, FeNO being considered a predictor for the future risk of asthma and wheeze (Salam et al., 2012). Here, NOS2 is linked to asthma.