Polycyclic aromatic hydrocarbons in the urban atmosphere of Nepal: Distribution, sources, seasonal trends, and cancer risk.

Atmospheric polycyclic aromatic hydrocarbons (PAHs) in urban areas have always been a global concern, as these areas are considered to be the source region. Despite studies on the concentrations of PAHs in water, soils and sediments, knowledge of the distribution patterns, seasonality and sources of PAHs in urban areas of Nepal remains limited. In this study, polyurethane foam passive air samplers were used to measure gas-phase PAH concentrations over different land types in three major cities of Nepal-namely, Kathmandu (the capital) and Pokhara (both densely populated cities), and Hetauda (an agricultural city). The average concentrations of ∑15PAHs in ng/m3 were 16.1±7.0 (6.4-28.6), 14.1±6.2 (6.8-29.4) and 11.1±9.0 (4.1-38.0) in Kathmandu, Pokhara and Hetauda, respectively. Molecular diagnostic ratio analysis suggested that fossil fuel combustion was a common PAH source for all three cities. In addition to this, coal combustion in Kathmandu, vehicle emissions in Pokhara, and grass/wood combustion in Hetauda were also possible sources of PAHs. In terms of cancer risk from PAH inhalation, a religious site with intense incense burning, a brick production area where extensive coal combustion is common, and a market place with heavy traffic emission, were associated with a higher risk than other areas. There were no clear seasonal trends in atmospheric PAHs. The estimated cancer risk due to inhalation of gas-phase PAHs exceeded the USEPA standard at >90% of the sites.