**4. Discussion**

The present report further confirms the detrimental impact of PM2.5 air pollution on atherogenic processes in modernizing China, independent of traditional atherosclerotic risk factors [12,18,30]. Specifically, Northern Chinese were more prone to higher carotid IMT and worse arterial endothelial dysfunction, compared with Southern Chinese. This may be attributed to more smoking, higher SBP, DBP, homocysteine, PM2.5 exposure and metabolic syndrome rates, and lower (unfavorable) vitamin B12 and folate levels. Metabolic syndrome includes the impact of several atherosclerotic risk factors i.e., blood pressure, waist circumference, HDL and LDL-cholesterol and fasting glucose. We and others have documented its detrimental impact on atherogenesis, independent of PM2.5 [18,31].

Multivariate regression of the overall cohort suggested that PM2.5 exposure and location were important determinants of carotid IMT, independent of homocysteine and other traditional vascular risk factors. Higher homocysteine presumably could be related to unique Northern dietary pattern of low folate and vitamin B12 intakes, the formal documentation of which is awaited with interest. On this issue, we have previously confirmed the beneficial effects of vitamin B12 and folate supplementations on atherogenic process (FMD and IMT) in 207 Northern Chinese adult subjects with subnormal nutritional status [32].

Greater carotid IMT is an important prognostic atherosclerosis surrogate related to later risk of stroke and cardiovascular diseases [33]. We have previously shown that increased carotid IMT is a marker of subclinical atherosclerosis in westernized as compared with rural Southern Chinese [15]. To contextualize the magnitude of the IMT difference (19.3%, 0.11 mm), a 0.16 mm increase in carotid IMT has been associated with 41% increase in stroke and 43% increase in acute myocardial infarction over a follow up period of 2–7 years [33]. The 19.3% difference in carotid IMT in the present study was far greater than the kind of difference between diabetic and non-diabetic Chinese adults [17].
