Can the Indian national ambient air quality standard protect against the hazardous constituents of PM2.5?

Globally, exposure to ambient fine particulate matter (PM2.5) pollution claims ~9 million lives, yearly, and a quarter of this deaths occurs in India. Regulation of PM2.5 pollution in India is based on compliance with its National Ambient Air Quality Standard (NAAQS) of 40 mu g/m3, which is eight times the revised global air quality guideline (AQG) of 5 mu g/m3. But, whether the NAAQS provides adequate protection against the hazardous components in PM2.5 is still not clear. Here, we examined the risk to health associated with exposure to PM2.5-bound polychlorinated biphenyls (PCB), heavy metals and polycyclic aromatic hydrocarbons (PAHs) in an Indian district averaging below the NAAQS. The annual average concentrations of PM2.5 mass, Sigma 28PCB and Sigma 13PAHs were 34 +/- 17 mu g/m3, 21 +/- 12 ng/m3 and 458 +/- 246 ng/m3, respectively. Concentrations of As, Cr, Mn and Ni in PM2.5 surpassed the screening levels for residential air. Substantial level of risks to health were associated with exposure to dioxin-like PCBs (Sigma 12dlPCB), PAHs, As, Cr and Ni. The hazard index or lifetime cancer risk were 240, or 9 cases per 1000 population, respectively. The estimated risks to health through exposure to hazardous components, except Ni, were greatest in rural areas, having a lower average PM2.5 concentration, than urban or peri-urban areas, suggesting higher toxicity potential of rural combustion sources. The large disparity between the estimated risk values and the acceptable risk level suggests that it would take a more stringent standard, such as the global AQG, to protect vulnerable populations in India from hazardous components in PM2.5.