Heavy metals in PM_2.5 in four metropolitan cities in Northwest China: pollution characteristics and health risk assessment

  Objective  To examine distribution characteristics of 7 heavy metal elements in particulate matter less than 2.5 μm in aerodynamic diameter (PM_2.5) and to evaluate health risk of the heavy metals in PM_2.5 in four metropolitan cities in Northwest China.

  Methods  Through literature retrieval, we extracted the data on heavy metals in PM_2.5 samples collected under normal weather conditions in Urumqi, Xi′an, Lanzhou and Yinchuan municipalities between 2012 and 2017. Exposures to the heavy metals were estimated based on calculations of average daily dose (ADD) and life time average daily dose (LADD). Health risk assessment models were used to evaluate health risks of exposures to heavy metals in PM_2.5 among different populations in the four cities.

  Results  There were regional differences in the concentrations of the heavy metals, with the highest concentration of zinc (Zn) among the 7 heavy metal elements for the four cities. Both the annual mean values of arsenic (As) and hexavalent chromium Cr(VI) were higher than the reference limits for all the four cities and the value of cadmium (Cd) was higher than the reference limit only for Urumqi city. In terms of the four cities as a whole, the main contributor of non-carcinogenic risk was lead (Pb) among the heavy metals but its hazard risk (HR) was less than one with the highest HR for Xi′an city; while the contributor of carcinogenic risk was As. The values of lifetime cancer risk (LCR) associated with the exposure to As and Cr(VI) in PM_2.5 were close to 10^–4 for the residents in Urumqi city but the LCR values associated to the As and Cr(VI) exposure were both greater than 10^–4 in Xi′an city; also for the residents of the two cities, the integrated lifetime cancer risk (ILCR) attributed to the exposures to the heavy metals exceeded the threshold level, with a higher LICR for the residents in Xi′an city.

  Conclusion  There were health risks related to exposures to heavy metals in PM_2.5 for the four metropolitan cities in Northwest China; among the four cities, both the non-carcinogenic and carcinogenic risk due to the exposure to the heavy metals were the highest in Xi′an city, followed by those in Urumqi city, and the risks were within acceptable range in Lanzhou and Yinchuan city.