Multicontaminant air pollution in Chinese cities.

OBJECTIVE: To investigate multicontaminant air pollution in Chinese cities, to quantify the urban population affected and to explore the relationship between air pollution and urban population size.
METHODS: We obtained data for 155 cities with 276 million inhabitants for 2014 from China's air quality monitoring network on concentrations of fine particulate matter measuring under 2.5 μm (PM2.5), coarse particulate matter measuring 2.5 to 10 μm (PM10), nitrogen dioxide (NO2), sulfur dioxide (SO2) and ozone (O3). Concentrations were considered as high, if they exceeded World Health Organization (WHO) guideline limits.
FINDINGS: Overall, 51% (142 million) of the study population was exposed to mean annual multicontaminant concentrations above WHO limits - east China and the megacities were worst affected. High daily levels of four-contaminant mixtures of PM2.5, PM10, SO2 and O3 and PM2.5, PM10, SO2 and NO2 occurred on up to 110 days in 2014 in many cities, mainly in Shandong and Hebei Provinces. High daily levels of PM2.5, PM10 and SO2 occurred on over  146 days in 110 cities, mainly in east and central China. High daily levels of mixtures of PM2.5 and PM10, PM2.5 and SO2, and PM10 and SO2 occurred on over  146 days in 145 cities, mainly in east China. Surprisingly, multicontaminant air pollution was less frequent in cities with populations over 10 million than in smaller cities.
CONCLUSION: Multicontaminant air pollution was common in Chinese cities. A shift from single-contaminant to multicontaminant evaluations of the health effects of air pollution is needed. China should implement protective measures during future urbanization.

Introduction

Air pollution in cities is a major concern worldwide, irrespective of a country’s level of development. In high-income countries, air quality has improved substantially since the 1970s; however, the adverse health effects of exposure to relatively low-level pollution remains a public concern. In contrast, air quality in some middle- and low-income countries, such as China and India, has seriously deteriorated. Before the 1920s, the main cause of urban air pollution in high-income countries was the rapid spread of coal-fired industry during the second phase of the Industrial Revolution. The major contaminants produced by coal combustion are particulate matter and sulfur dioxide (SO2). After the1920s, a new source of air pollution emerged with the widespread use of the automobile, which emits particulate matter, nitrogen dioxide (NO2), lead and other contaminants. However in some middle- and low-income countries, e.g. China, the development of coal-fired industries and increased automobile use have overlapped, which has resulted in the emission of a complex mix of air contaminants.,

Most studies of the health effects of air pollution have focused on individual contaminants, such as particulate matter, NO2, SO2, ozone (O3) and carbon monoxide, with each considered to have an independent impact.– However, in reality the urban atmosphere is never confronted with a single contaminant but is actually exposed to a complex mix of different contaminants at varying times of the day and year. Consequently, people are more likely to be exposed to a mixture of contaminants than to a single substance, the resultant impact on human health can be highly varied. For instance, some contaminants (e.g. NO2 and O3) affect the respiratory system, some (e.g. particulate matter) affect the circulatory system and cause heart disease and others (e.g. SO2) affect the skin and mucous membranes. Although few epidemiological studies have looked at the combined effect of several air contaminants, it can be assumed that they will have an impact on different parts of human body. For example, the combination of NO2 and particulate matter pollution will affect both respiratory and cardiovascular systems., As it can lead to these complex conditions, exposure to multicontaminant air pollution is important and should be quantified, especially in rapidly urbanizing developing countries where mixtures of contaminants are common.,

Previous research has paid particular attention to understanding how specific contaminants affect public health in developing countries. Although important, this approach may underestimate the actual impact of urban air pollution on public health. In fact, there have been calls for a shift from a single-contaminant to a multicontaminant approach to countering the health effects of air pollution. The aims of this study were: (i) to document the mixture of air contaminants in Chinese cities both annually and diurnally; (ii) to determine the proportion of the urban population affected by multicontaminant air pollution; and (iii) to investigate the relationship between the size of the urban population and the frequency of occurrence of high levels of multicontaminant air pollution.

Methods

We obtained data on air quality for 155 cities (including all 31 provincial capitals and 124 major prefectural cities) from China's urban air quality monitoring network, which reports concentrations of air contaminants under the newly upgraded ambient air quality standard GB3095–2012. For this study, we used hourly concentrations of fine particulate matter less than or equal to 2.5 μm in diameter (PM2.5), coarse particulate matter with a diameter between 2.5 and 10 μm (PM10), NO2, SO2 and O3 for the whole of 2014. To assess pollution levels and their potential impact on public health, we used guideline values for annual and daily ambient air quality provided by the World Health Organization (WHO; Table 1). We averaged hourly concentrations to obtain annual means for all contaminants, 24-hour means for PM2.5, PM10 and SO2 and 8-hour means for O3. For the NO2 concentration, we retained the hourly values. Finally, we determined how frequently annual and daily multicontaminant air pollution due to various combinations of three, four and five contaminants (Table 2; available at: http://www.who.int/bulletin/volumes/96/4/17-195560) exceeded the values in Table 1 for individual substances. We obtained the size of the population in each of the 155 cities, as reported in the 2010 census, from the National Bureau of Statistics of China. In total, these cities accounted for 41.2% of China's urban population in 2010.

Table 1

WHO guideline values on ambient air quality, 2016

Contaminant	Annual limit	Daily limit
PM2.5	10 μg/m3 annual mean	25 μg/m3 24-hour mean
PM10	20 μg/m3 annual mean	50 μg/m3 24-hour mean
NO2	40 μg/m3 annual mean	200 μg/m3 1-hour mean
SO2	ND	20 μg/m3 24-hour mean
O3	ND	100 μg/m3 8-hour mean

ND: not determined; NO2: nitrogen dioxide; O3: ozone; PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm; SO2: sulfur dioxide; WHO: World Health Organization.

Table 2

Combinations of contaminants evaluated, air pollution study, China, 2014

No. of contaminants	Combinations of air contaminants
	Annual concentrations evaluated	Daily concentrations evaluated
Five	N/A	PM2.5, PM10, NO2, SO2 and O3
Four	N/A	(i) PM2.5, PM10, NO2 and O3; (ii) PM2.5, PM10, SO2 and O3; (iii) PM2.5, PM10, NO2 and SO2; (iv) PM2.5, O3, NO2 and SO2; and (v) PM10, O3, NO2 and SO2
Three	PM2.5, PM10 and NO2	(i) PM2.5, PM10 and O3; (ii) PM2.5, O3 and NO2; (iii) PM2.5, O3 and SO2; (iv) PM2.5, PM10 and NO2; (v) PM2.5, PM10 and SO2; (vi) PM10, SO2 and NO2; (vii) PM2.5, O3 and NO2; (viii) PM10, O3 and NO2; (ix) PM10, O3 and SO2; and (x) NO2, O3 and SO2
Two	(i) PM2.5 and PM10; (ii) PM2.5 and NO2; and (iii) PM10 and NO2	(i) PM2.5 and PM10; (ii) PM2.5 and O3; (iii) PM2.5 and NO2; (iv) PM2.5 and SO2; (v) PM10 and O3; (vi) PM10 and NO2; (vii) PM10 and SO2; (viii) O3 and NO2; (ix) O3 and SO2; and (x) NO2 and SO2

N/A: not applicable; NO2: nitrogen dioxide; O3: ozone; PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm; SO2: sulfur dioxide.

The main variable of interest in our study was exposure to a high level of multicontaminant air pollution, which was defined as occurring when the concentration of a contaminant exceeded the relevant WHO value in Table 1. Annual exposure to multicontaminant air pollution was assessed for combinations of two or three contaminants and daily exposure was assessed for combinations of two, three, four or five contaminants (Table 2). To investigate the impact of urbanization on air pollution, we determined whether there was a correlation between the size of the urban population and the proportion of days in 2014 during which the concentration of specific contaminants exceeded WHO guideline values. For this analysis, cities were divided into five groups by population size, according to China's new urban size standard: (i) less than 0.5 million; (ii) 0.5 to less than 1 million; (iii) 1 to less than 5 million; (iv) 5 to less than 10 million; and (v) 10 million or more. The correlation between the population size and the percentage of days in 2014 with a high level of multicontaminant air pollution was determined using nonlinear regression analysis.

Results

In total, 56 of the 155 cities analysed (36%) were exposed to mean annual concentrations of the contaminants PM2.5, PM10 and NO2 above WHO guideline values (Fig. 1). These cities had a combined population of 142 million out of a total study population of 276 million (i.e. 51%). In addition, all 155 cities were exposed to high annual concentrations of two-contaminant mixtures of PM2.5 and PM10 and 56 cities, with a total population of 142 million, were exposed to high annual concentrations of PM2.5 and NO2 and of PM10 and NO2. The cities with high annual multicontaminant exposure to either (i) PM2.5, PM10 and NO2; (ii) PM2.5 and NO2; or (iii) PM10 and NO2 were mainly located in east China, specifically in Hebei, Henan, Jiangsu, Shandong and Zhejiang Provinces and in the megacities of Beijing, Guangzhou, Shenzhen and Tianjin (Fig. 2).

Fig. 1

Cities with high mean annual air contaminant concentrations, by contaminant type, China, 2014

Fig. 2

Locations of cities with high mean annual air multicontaminant concentrations, China, 2014

NO2: nitrogen dioxide; PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm.

Notes: The mean annual air contaminant concentration in 2014 was classed as high if it exceeded the World Health Organization guideline value (Table 1).

The dark blue circles represent cities with high mean annual air contaminant concentrations of either: (i) fine particulate matter less than or equal to 2.5 μm in diameter (PM2.5), coarse particulate matter with a diameter between 2.5 and 10 μm (PM10) and nitrogen dioxide (NO2); (ii) PM2.5 and NO2; or (iii) PM10 and NO2. The mean annual air contaminant concentration in 2014 was classed as high if it exceeded the World Health Organization guideline value (Table 1).

Daily multicontaminant exposure

Only two cities, Dongying and Linyi in Shandong Province, had mean daily concentrations of all five contaminants (i.e. PM2.5, PM10, SO2, O3 and NO2) above WHO guideline values for 11–15 days (3–4%) in 2014 (Fig. 3). Weifang and Zibo in Shandong Province were exposed to high daily concentrations of the five contaminants for 8–11 days (2–3%) in the year. Jining in Shandong Province, Wuhan in Hubei Province and Jiayuguan and Jinchang in Gansu Province were exposed to high daily concentrations for 4–8 days (1–2%; Fig. 4). Other cities had less than 4 days (1%) with high concentrations of all five contaminants.

Fig. 3

Cities with high mean daily air concentrations of PM2.5, PM10, NO2, SO2 and O3, by annual frequency, China, 2014

Fig. 4

Locations of cities with high mean daily air concentrations of PM2.5, PM10, NO2, SO2 and O3, by annual frequency, China, 2014

NO2: nitrogen dioxide; O3: ozone; PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm; SO2: sulfur dioxide.

Notes: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1). The study included 155 cities with a combined population of 276 million.

NO2: nitrogen dioxide; O3: ozone; PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm; SO2: sulfur dioxide.

Note: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1).

Exposure to high mean daily concentrations of four contaminants was more common. In some locations, daily concentrations exceeded WHO guideline values for 73–110 days (20–30%) in 2014 for PM2.5, PM10, SO2 and O3 (Fig. 5) and for PM2.5, PM10, NO2 and SO2. The cities with the highest frequencies of exposure to high daily concentrations of the four contaminants PM2.5, PM10, SO2 and O3 were located in Shandong Province (Fig. 6; available at: http://www.who.int/bulletin/volumes/96/4/17-195560), whereas those with the highest frequencies of exposure to high daily concentrations of the four contaminants PM2.5, PM10, NO2 and SO2 were mainly located in Hebei and Shandong Provinces. High daily concentrations of other four-contaminant mixtures were rare: high daily concentrations of PM2.5, PM10, NO2 and O3 (Fig. 7 and Fig. 8; both available at: http://www.who.int/bulletin/volumes/96/4/17-195560), of PM2.5, O3, NO2 and SO2 and of PM10, O3, NO2 and SO2 were observed on less than 18 days (5%) in 2014 in most major Chinese cities.

Fig. 5

Cities with high mean daily air concentrations of PM2.5, PM10, SO2 and O3, by annual frequency, China, 2014

Fig. 6

Locations of cities with high mean daily air concentrations of PM2.5, PM10, SO2 and O3, by annual frequency, China, 2014

Fig. 7

Cities with high mean daily air concentrations of PM2.5, PM10, NO2 and O3, by annual frequency, China, 2014

Fig. 8

Locations of cities with high mean daily air concentrations of PM2.5, PM10, NO2 and O3, by annual frequency, China, 2014

O3: ozone; PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm; SO2: sulfur dioxide.

Notes: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1). The study included 155 cities with a combined population of 276 million.

O3: ozone; PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm; SO2: sulfur dioxide.

Note: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1).

NO2: nitrogen dioxide; O3: ozone; PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm.

Notes: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1). The study included 155 cities with a combined population of 276 million.

NO2: nitrogen dioxide; O3: ozone; PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm.

Note: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1).

Exposure to high mean daily concentrations of the three contaminants PM2.5, PM10 and SO2 was even more common: 110 cities with a total population of 173 million were exposed to this level of air pollution for more than 146 days (40%) in 2014 (Fig. 9). Those cities were mainly located in east and central China, particularly in Hebei, Henan, Shandong and Shanxi Provinces (Fig. 10). In addition, exposure to high daily concentrations of mixtures of the following three-contaminant combinations were observed on 18–146 days (5–40%) in many cities: (i) PM2.5, PM10 and O3; (ii) PM2.5, O3 and SO2; (iii) PM2.5, PM10 and NO2; (iv) PM2.5, SO2 and NO2; (v) PM10, O3 and NO2; and (vi) PM10, O3 and SO2 (Table 3). However, high daily concentrations of the three-contaminant mixtures of (i) PM2.5, O3 and NO2, (ii) PM10, SO2 and NO2, and (iii) NO2, O3 and SO2 were observed on less than 18 days (5%) in 2014 in major Chinese cities.

Fig. 9

Cities with high mean daily air concentrations of PM2.5, PM10 and SO2, by annual frequency, China, 2014

Fig. 10

Locations of cities with high mean daily air concentrations of PM2.5, PM10 and SO2, by annual frequency, China, 2014

Table 3

Frequency of high mean daily concentrations of air contaminants in 155 cities, by number of contaminants, China, 2014

No. of contaminants	Contaminant combinations with high mean daily concentrationsa
	High frequency (> 40% of days in 2014)	Medium frequency (5–40% of days in 2014)	Low frequency (< 5% of days in 2014)
Four	No cities	PM2.5, PM10, SO2 and O3 (46 cities)	PM2.5, PM10, NO2 and O3 (56 cities)
		PM2.5, PM10, NO2 and SO2 (25 cities)	PM2.5, O3, NO2 and SO2 (53 cities)
			PM10, O3, NO2 and SO2 (54 cities)
Three	PM2.5, PM10 and SO2 (147 cities)	PM2.5, PM10 and O3 (73 cities)	(PM2.5, O3 and NO2 (56 cities)
		PM2.5, O3 and SO2 (46 cities)	PM10, O3 and NO2 (57 cities)
		PM2.5, PM10 and NO2 (31 cities)	NO2, O3 and SO2 (55 cities)
		PM2.5, SO2 and NO2 (26 cities
		PM10, SO2 and NO2 (27 cities)
		PM10, O3 and SO2 (47 cities)
Two	PM2.5 and PM10 (155 cities)	PM2.5 and O3 (76 cities)	O3 and NO2 (55 cities)
	PM2.5 and SO2 (147 cities)	PM2.5 and NO2 (33 cities)
	PM10 and SO2 (147 cities)	PM10 and O3 (74 cities)
		PM10 and NO2 (32 cities)
		O3 and SO2 (47 cities)
		NO2 and SO2 (28 cities)

NA: not applicable; NO2: nitrogen dioxide; O3: ozone; PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm; SO2: sulfur dioxide.

a The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1).

PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm; SO2: sulfur dioxide.

Notes: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1). The study included 155 cities with a combined population of 276 million.

PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm; SO2: sulfur dioxide.

Note: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1).

Exposure to high daily concentrations of two contaminants was extremely common: 145 cities with a total population of 269 million were exposed to mean daily concentrations of PM2.5 and PM10 above WHO guideline values for more than 146 days (40%) in 2014 (Fig. 11). High concentrations of the two contaminants PM2.5 and SO2 were also observed on more than 146 days (40%) in 116 cities with a total population of 184 million (Fig. 12; available at: http://www.who.int/bulletin/volumes/96/4/17-195560) and high concentrations of PM10 and SO2 were equally frequently observed in 111 cities with a total population of 175 million (Fig. 13; available at: http://www.who.int/bulletin/volumes/96/4/17-195560). The affected cities were mainly located in provinces in the east of China: Hebei, Henan, Shandong and Shanxi Provinces (Fig. 14, Fig. 15 and Fig. 16; all available at: http://www.who.int/bulletin/volumes/96/4/17-195560).

Fig. 11

Cities with high mean daily air concentrations of PM2.5 and PM10, by annual frequency, China, 2014

Fig. 12

Cities with high mean daily air concentrations of PM2.5 and SO2, by annual frequency, China, 2014

Fig. 13

Cities with high mean daily air concentrations of PM10 and SO2, by annual frequency, China, 2014

Fig. 14

Locations of cities with high mean daily air concentrations of PM2.5 and PM10, by annual frequency, China, 2014

Fig. 15

Locations of cities with high mean daily air concentrations of PM2.5 and SO2, by annual frequency, China, 2014

Fig. 16

Locations of cities with high mean daily air concentrations of PM10 and SO2, by annual frequency, China, 2014

PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm.

Notes: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1). The study included 155 cities with a combined population of 276 million.

PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; SO2: sulfur dioxide.

Notes: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1). The study included 155 cities with a combined population of 276 million.

PM10: coarse particulate matter with a diameter between 2.5 and 10 μm; SO2: sulfur dioxide.

Notes: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1). The study included 155 cities with a combined population of 276 million.

PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; PM10: coarse particulate matter with a diameter between 2.5 and 10 μm.

Note: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1).

PM2.5: fine particulate matter less than or equal to 2.5 μm in diameter; SO2: sulfur dioxide.

Note: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1).

PM10: coarse particulate matter with a diameter between 2.5 and 10 μm; SO2: sulfur dioxide.

Note: The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1).

Population size

In general, daily multicontaminant air pollution was less frequent in cities with a population greater than 10 million than in smaller cities in our study. For example, the proportion of days in 2014 during which the mean daily concentrations of all five contaminants exceeded WHO guideline values was not significantly affected by population size in cities with fewer than 10 million inhabitants but the portion was substantially lower in cities with a population greater than 10 million (Fig. 17). Similarly, the frequency of exposure to high mean daily concentrations of four contaminants was comparable among cities with populations ranging from 0.5 to 10 million but was lower in cities with a population less than 0.5 million or greater than 10 million (Fig. 18). This variation was also observed for high mean daily concentrations of three contaminants: the frequency was similar in cities with populations ranging from 0.5 to 10 million but lower in those with a population less than 0.5 million or greater than 10 million (Fig. 19). For exposure to high daily concentrations of two contaminants, there was no substantial variation in frequency among cities with a population less than 10 million, whereas the frequency was markedly lower in cities with a population greater than 10 million (Fig. 20). There was a significant inverse U-shaped relationship between the size of the urban population and the observed frequency of high mean daily concentrations of four contaminants (Fig. 18). In addition, there were inverse U-shaped relationships between population size and the frequency of high mean daily concentrations of three and two contaminants but the relationships were weaker (Fig. 19 and Fig. 20).

Fig. 17

Cities with high mean daily air concentrations of five contaminants, by city population, China, 2014

Fig. 18

Cities with high mean daily air concentrations of four contaminants, by city population, China, 2014

Fig. 19

Cities with high mean daily air concentrations of three contaminants, by city population, China, 2014

Fig. 20

Cities with high mean daily air concentrations of two contaminants, by city population, China, 2014

Notes: The five contaminants were: (i) fine particulate matter less than or equal to 2.5 μm in diameter; (ii) coarse particulate matter with a diameter between 2.5 and 10 μm; (ii) nitrogen dioxide; (iv) sulfur dioxide; and (v) ozone. The mean daily air contaminant concentration was classed as high if it exceeded the World Health Organization guideline value (Table 1).

Notes: The mean daily air contaminant concentrations of any four study contaminants were classed as high if they exceeded World Health Organization guideline values (Table 1). The five study contaminants were: (i) fine particulate matter less than or equal to 2.5 μm in diameter; (ii) coarse particulate matter with a diameter between 2.5 and 10 μm; (ii) nitrogen dioxide; (iv) sulfur dioxide; and (v) ozone. The equation for the regression line is y = –0.0042 x + 0.0218 x + 0.0047.

Notes: The mean daily air contaminant concentrations of any three study contaminants were classed as high if they exceeded World Health Organization guideline values (Table 1). The five study contaminants were: (i) fine particulate matter less than or equal to 2.5 μm in diameter; (ii) coarse particulate matter with a diameter between 2.5 and 10 μm; (ii) nitrogen dioxide; (iv) sulfur dioxide; and (v) ozone. The equation for the regression line is y = –0.0184 x + 0.1219 x – 0.0644.

Notes: The mean daily air contaminant concentrations of any two study contaminants were classed as high if they exceeded World Health Organization guideline values (Table 1). The five study contaminants were: (i) fine particulate matter less than or equal to 2.5 μm in diameter; (ii) coarse particulate matter with a diameter between 2.5 and 10 μm; (ii) nitrogen dioxide; (iv) sulfur dioxide; and (v) ozone. The equation for the regression line is y = –0.0216 x + 0.1086 x + 0.1266.

Discussion

Although our study was based on data for only one year, it provides a snapshot of air pollution in major Chinese cities and demonstrates that multicontaminant air pollution was very common in 2014. These findings underscore the need to assess multiple air contaminant concentrations at the same time to obtain a more realistic picture of urban air quality and its potential impact on public health. Consequently, a change in air quality guidelines is required, with the establishment of guidelines on multicontaminant mixtures. The globally recognized, ambient air quality guidelines produced by WHO were designed to help reduce the health effects of air pollution in 1987. They were based on a review of the scientific evidence and its implications. The guidelines, which were updated in 1997 and 2005, now specify daily and annual limits for five major ambient air contaminants. In addition, some regions and countries have established their own air quality standards. For instance, the European Union, Japan and the United States of America were quick to update their air quality guidelines, whereas some middle- and low-income countries, e.g. China, established their own standards in response to high levels of pollution. However, all these guidelines and standards treat each contaminant in isolation or choose a single major contaminant as an indicator of air quality. For example, China uses an air quality index based on the maximum value of each individual contaminant's concentration to indicate air quality.

Multicontaminant ambient air pollution is also important for public health research at both the urban and regional level. In the past, very little attention has been paid to multicontaminant exposure and research efforts have primarily focused on the health effects of individual contaminants. Initially, the reason for this focus was the difficulty of evaluating the medical effects of exposure to several contaminants. In addition, there was little understanding that multicontaminant ambient air pollution is common. However, without detailed research into the medical consequences of multicontaminant exposure, the disease burden will be underestimated. The influential Global Burden of Disease Study 2013 considered both ambient and household air pollution. Still, the only ambient air contaminants included were particulate matter and ozone, no consideration was given to other contaminants. We recommend that research into air pollution and its health effects should pay more attention to multicontaminant ambient air pollution, especially in middle- and low-income counties where current pollution levels are often higher than in high-income countries. In particular, by devoting attention to multicontaminant mixtures, researchers could raise public awareness of the complex nature of ambient air quality and stimulate greater interest in air pollution prevention.

As a result of rapid urbanization during the last century, more than half of the world's population now lives in cities. This rise in the urban population and the associated intensification of social and economic activity have had a substantial impact on urban air quality. Thus, urbanization and its effect on air quality are among the most important issues for achieving sustainable urban and regional development. Researchers have studied the relationship between urbanization and typical air contaminants in both developed and developing countries., For example, the concentration of the traditional air contaminant NO2 has been observed to increase exponentially with population size, though the value of the exponent varies between locations. In contrast, for PM2.5, the relationship between its concentration and urban population size is much more variable across continents and countries. In our study, we found an inverse U-shaped relationship between urban population size and the frequency of high daily concentrations of three contaminants, whereas other researchers have demonstrated no clear relationship. Furthermore, we discovered that a high level of multicontaminant air pollution was less common in cities with a population of more than 10 million than in smaller cities, which is contrary to general expectations that larger cities would be more polluted. The likely explanation is that large cities have implemented extensive environmental protection measures and that many polluting industries have been relocated to smaller cities. This observation casts new light on multicontaminant air pollution and its relationship to urbanization. We suggest that future research should pay more attention to the process of urbanization and its impact on multicontaminant ambient air pollution, particularly in middle- and low-income countries. Our findings highlight the varied pattern of multicontaminant air pollution in Chinese cities and confirm the view that pollution in developing countries should be expected to vary greatly across both time and space. Consequently, the results of this research should be relevant not only to China but also to other middle- and low-income countries facing similar challenges with multicontaminant air pollution.