1. Department of Environmental Science, School of Geography and Tourism, Shaanxi Normal University, Xi’an, Shaanxi 710062, PR China
2. International Joint Research Centre of Shaanxi Province for Pollutant Exposure and Eco-environmental Health, Xi’an, Shaanxi 710062, PR China
3. Environmental Research Group, School of Public Health, Imperial College London, 80 Wood Lane, London W12 0BZ, U.K.