Objective To understand the pollution characteristics of metal elements in ambient PM_2.5 in Xining, China, from 2018 to 2020, and to evaluate the health risk of people exposed through respiratory routes.

Methods From 2018 to 2020, PM_2.5 samples were continuously collected from the north and east districts of Xining on the 10th to 16th of each month and when haze pollution occurred. The pollution characteristics of PM_2.5 and its chromium, arsenic, cadmium, nickel, lead, manganese and mercury were analyzed. The source of metal elements was identified by the enrichment factor (EF) method, and a health risk assessment model was used to evaluate the health risk of population with respiratory exposure.

Results In 2018, the average annual mass concentration of PM_2.5 in the north and east districts of Xining was higher than the second-level standard limit (35 μg/m³) according to the Ambient Air Quality Standard (GB 3095-2012), and was lower than 35 μg/m³ in both urban areas in 2019 and 2020. The mass concentrations of metals in PM_2.5 in the two urban areas were in the order of lead > manganese > nickel > arsenic > chromium > cadmium > mercury from 2018 to 2020. The EF analysis showed that the EF of the seven metals was all less than 1. The average annual excess non-carcinogenic risk of the population exposed through respiratory route from high to low was lead > manganese > mercury, and was chromium > arsenic > cadmium > nickel with non-carcinogenic risk. The per capita excess carcinogenic risk of arsenic, cadmium, and nickel was still at an acceptable risk level for the population, whereas the average annual excess carcinogenic risk of adult males and children exposed to chromium in the northern part of the city was slightly higher than 10^-6.

Conclusion The metal pollution in PM_2.5 in Xining is mainly derived from natural factors (rock weathering or soil dust), and chromium pollution should be focused on.