In order to assess the pollution levels and health risks of PM_2.5-bound metals in Baoding City before and after the heating period, samples were collected in 2016 at Hebei University from September 25th to November 14th during the non-heating period, and November 15th to December 26th during the heating period, respectively. ICP-MS was applied to analyze seven heavy metals (Cr, Zn, Cu, Pb, Ni, Cd and Fe). The statistical analysis, enrichment factor (EF), pollution load index method, and Risk Assessment Method proposed by U.S. EPA were used to evaluate the non-carcinogenic risks of six of these heavy metals (Cr, Zn, Cu, Pb, Ni and Cd) and carcinogenic risks of three of these heavy metals (Cr, Ni and Cd). The results showed three main results. First, the average daily PM_2.5 concentrations of the national air monitoring stations was 155.66 μg·m⁻³ which was 2.08 times as high as that of the second level criterion in China (75 μg·m⁻³) during the observation period. Compared with the non-heating period, all heavy metals concentrations increased during heating period. The growth rates of Pb and Ni were the highest and the lowest, which were 88.03 and 5.11 percent, respectively. Second, the results of enrichment factor indicated that the EF values of all heavy metals were higher during the heating period in comparison with during the non-heating period, but the degree of enrichment of all heavy metals remained unchanged. Not only those, Cr and Ni were minimally enriched and were affected by both human and natural factors, Pb, Cu and Zn were significantly enriched and were mainly affected by human factors, the enrichment of Cd was much higher than that of the other heavy metals, exhibiting extremely high enrichment, mainly due to human factors during the whole sampling period. The results of the pollution load index indicated that the proportions of the number of highly and very highly polluted PM_2.5-bound metals were the highest during the heating period, while the proportion of moderately polluted PM_2.5-bound metals was the highest during the non-heating period. The combined pollution degree of heavy metals was more serious during the heating period. Third, according to the health risk assessment model, we concluded that the non-carcinogenic and carcinogenic risks caused by inhalation exposure were the highest and by dermal exposure were the lowest for all kinds of people. The overall non-carcinogenic risk of heavy metals via inhalation and subsequent ingestion exposure caused significant harm to children during the non-heating and the heating periods, and the risk values were 2.64, 4.47, 1.20 and 1.47, respectively. Pb and Cr exhibited the biggest contributions to the non-carcinogenic risk. All the above non-carcinogenic risks exceeded the standard limits suggested by EPA (HI or HQ < 1). The carcinogenic risk via inhalation exposure to children, adult men and women were 2.10 × 10⁻⁴, 1.80 × 10⁻⁴, and 1.03 × 10⁻⁴ during the non-heating period, respectively, and 2.52 × 10⁻⁴, 2.16 × 10⁻⁴ and 1.23 × 10⁻⁴ during the heating period, respectively. All the above carcinogenic risks exceeded the threshold ranges (10⁻⁶~10⁻⁴), and Cr posed a carcinogenic risk to all people.

Beijing, which is the capital of China, suffers from severe Fine Particles (PM_2.5) pollution during the heating season. In order to take measures to control the PM_2.5 pollution and improve the atmospheric environmental quality, daily PM_2.5 samples were collected at an urban site from 15 November to 31 December 2016, characteristics of PM_2.5 chemical compositions and their effect on atmospheric visibility were analyzed. It was found that the daily average mass concentrations of PM_2.5 ranged from 7.64 to 383.00 μg m⁻³, with an average concentration of 114.17 μg m⁻³. On average, the Organic Carbon (OC) and Elemental Carbon (EC) contributed 21.39% and 5.21% to PM_2.5, respectively. Secondary inorganic ions (SNA: SO₄²⁻ + NO₃⁻ + NH₄⁺) dominated the Water-Soluble Inorganic Ions (WSIIs) and they accounted for 47.09% of PM_2.5. The mass concentrations of NH₄⁺, NO₃⁻ and SO₄²⁻ during the highly polluted period were 8.08, 8.88 and 6.85 times greater, respectively, than during the clean period, which contributed most to the serious PM_2.5 pollution through the secondary transformation of NO₂, SO₂ and NH₃. During the highly polluted period, NH₄NO₃ contributed most to the reconstruction extinction coefficient (b′_ext), accounting for 35.7%, followed by (NH₄)₂SO₄ (34.44%) and Organic Matter (OM: 15.24%). The acidity of PM_2.5 in Beijing was weakly acid. Acidity of PM_2.5 and relatively high humidity could aggravate PM_2.5 pollution and visibility impairment by promoting the generation of secondary aerosol. Local motor vehicles contributed the most to NO₃⁻, OC, and visibility impairment in urban Beijing. Other sources of pollution in the area surrounding urban Beijing, including coal burning, agricultural sources, and industrial sources in the Hebei, Shandong, and Henan provinces, released large amounts of SO₂, NH₃, and NO₂. These, which were transformed into SO₄²⁻, NH₄⁺, and NO₃⁻ during the transmission process, respectively, and had a great impact on atmospheric visibility impairment.