A review of research advances in the distributions and risk assessments of metal elements in atmospheric particles with different particle sizes

Metal elements (MEs) are important compositions of atmospheric particles. The research progress of distributions and ecological and human health risk assessments of MEs in atmospheric particles with different particle sizes were reviewed. In atmospheric particles, MEs mainly presented three particle size distributions: unimodal distribution with the peak in fine or coarse mode particles, and bimodal distribution, corresponding to different pollution sources. Moreover, enrichment factor, geo-accumulation index and potential ecological risk index were the common three methods for ecological risk assessment of MEs. The degrees of enrichment of fine mode particles were significantly higher than that of coarse mode particles for most MEs. Atmospheric particles in Chinese cities all showed a very high comprehensive potential ecological risks (Risk Index > 1 000). Cd, Pb, As were the main polluting elements at present, which should be paid special attention to. In addition, particles of different sizes would be deposited in different parts of the respiratory system. The health risk assessments showed that heavy metals had carcinogenic and non-carcinogenic risks to the human body in most Chinese cities. Cr, As and Cd showed carcinogenic risks to human health, and Mn may cause non-cancer risks. Thus, combining the distribution characteristics of MEs in particles with different particle sizes with their risk assessment was the key research direction in this field in the future.