Introduction

Air pollution occurs when the air contains gases, dust, fumes or odor in harmful amounts, that is, amounts that could be harmful to the health or comfort of humans and animals or which could cause damage to plants and materials (Gurjar et al. 2012).

Major sources of air pollution in urban areas are stationary sources such as industrial plants and mobile sources mainly in the transportation sector, which uses fossil fuels as the source of energy (World Health Organization 2010). High smokestacks from industrial sources, such as refineries and coal-fired electrical power generating plants, can carry pollutants such as acidic aerosols and sulfur oxides over hundreds of kilometers by air that moves freely across international borders (Environmental Protection Agency 2009).

Urban air quality is a major concern throughout the world (World Health Organization 2014). This, in fact, is an immense issue with diverse effects on earth environments in different parts of the world or even within a specific region.

Air pollution is a major worldwide concern in an increasing number of megacities around the world (Prinn et al. 2005). Generally speaking, all mitigation plans by governments are aiming toward execution of programs which can ultimately result in the reduction in pollutant’s concentrations. In order to deal with the complex nature of air pollution, especially with respect to health, has been a motive to define the so-called indicators which simplify the collected monitoring data and to make them suitable for reporting to the public and decision making by authorities (Gurjar et al. 2012).

The Pollutant Standards Index (PSI) was introduced in the mid-1970s to provide a uniform system of measuring pollution levels for the major air pollutants. The PSI was used in a number of countries, including the USA and Singapore; however, in early 1980s, the US EPA replaced the PSI with a new index, so-called Air Quality Index (AQI), which pays more attention to air quality. AQI measures and reports on six key urban air pollutants: ozone (O3); particulate matter (PM10, PM2.5); carbon monoxide (CO); nitrogen dioxide (NO2); and sulfur dioxide (SO2) (Ontario Ministry of the Environment 2005; Ahadi et al. 2013).

These pollutants have long been associated with human adverse health and environmental effects at elevated levels. Hourly readings for each pollutant at various stations are assigned an AQI value from zero upward using a common scale (Ontario Ministry of the Environment 2006). EPA calculates the AQI for major air pollutants regulated by the Clean Air Act (Environmental Protection Agency 2009). Each category has a descriptive name reflecting levels of health concern (ranging from good through very hazardous), an associated color, and an advisory statement (Committee on Environmental Health 2004). The higher the AQI value, the greater the level of air pollution, and the greater the health concern. AQI values at or below 100 are generally thought of as satisfactory, relations and features of AQI are presented in US EPA, Kansas Department of Health and Environment Bureau (Environmental Protection Agency 2009; U.S. Environmental Protection Agency Of Standards 2006; Kansas Department of Health and Environment Bureau 2005).

To focus more on the health factor in air pollution indices, a new index called Air Quality Health Index (AQHI) was introduced in early 2000 to describe the connection between the general “health situation” and ambient air pollution in urban areas (Development Status Report of the Commissioner of the Environment 2009).

The AQHI was applied in 10 urban areas of Canada. It measured on a color-coded scale from 1 to 10+ (Chen and Copes 2013). The higher the number, the greater the health risk associated with local air quality. The AQHI numeric ratings (from 1 to 10+) are grouped into risk categories (low, moderate, high, very high) that are designed to identify the level of health risk in a “quick and easy” manner. The AQHI is a tool that could be used by people to reduce their short-term exposure to air pollution and plan, on a daily basis, to modify their behavior and reduce their personal health risk. Therefore, the AQHI responds to the acute, or short-term, changing levels of health risk associated with air pollution. Each category is associated with specific health advice for those at risk (children, seniors, people with heart or lung conditions and diabetics), and the general population. The relations and features of AQHI are presented in Hasselback, Health Canada and Environment, and Hindy and Pennell (Hasselback 2010; Health Canada and Environment 2009; Hindy and Pennell 2010; Health Protection Agency for the Committee on the Medical Effects of Air Pollutants 2011).

During the recent years, some other indices such as Aggregate Risk Index (ARI) (Sicard et al. 2012) have been introduced. However, none of these indices has reflected safety factors and diverse economic impacts due to the problem of air pollution. Therefore, an integrated AQI, called, Air Quality Risk Index (AQRI) has been introduced which not only considers environmental conditions and health issues but also brings economic losses involved into account. Moreover, a case study has been carried out with the available data of the year 2010 which was used to evaluate the state of Tehran megacity AQRI.

Materials and methods

Over the past four decades, as stated, different indices have been introduced for air quality, but none of them have paid attention to the three factors of health (R H), safety (R S) and environment (R E) simultaneously.

The diverse consequences of air pollution are not limited only to the health and environment, but air pollution also imposes direct and indirect expenses on a community. For example, it can damage building’s facade, increase accidents due to reduced visibility, cause the closure of the government offices, educational institutions and increase the hospital admissions due to respiratory diseases, which impose economic losses and external costs on metropolitan economy. This was a reason to bring the safety (damage and loss) (R S) item into the definition of AQI. Therefore, AQRI can be used as an index to control air quality, health conditions and economic losses due to air pollution. As a result, AQRI can be used as an urban air quality management tool for policy makers.

The National Health Service (NHS) of the UK published a report in 2007/2008 stating that the hospital admissions triggered by air pollution have imposed additional costs to the country. For example, in 2007/2008, there were over 74,000 emergency admissions to hospitals because of asthma, and the NHS’s non-elective spell tariff was £612 million for that year. The same report has indicated that air pollution has put considerable expenses on the shoulders of communities. These expenses have cost the tax payers £8.5–£20.2 billion in the year 2005. The Air Quality Management Resource Centre of the UK has noted that the health impacts of air quality in the UK are almost twice that of physical inactivity, estimated to be £10.7 billion per annum (House of Commons Environmental Audit Committee 2010). Air pollution has adverse short-term effects on health (Katsouyanni et al. 1996) and bears major impact on public health especially on cardiovascular problems (Kassomenos et al. 2008; Sunyera et al. 2003). Clear links between asthma and other respiratory diseases and air quality indicate that health risks (R H) are also another important factor to be considered in AQRI (House of Commons Environmental Audit Committee 2010).

Air pollution has a wide range of environmental impacts, including loss of biodiversity, reduction in crop yields and contribution to climate change. At the present time, NOx, SOx, CO, PM2.5 and PM10 have the most significant impacts on the environment. For example, excess nitrogen from emissions of ammonia and NOx leads to excessive plant growth and decay (eutrophication) that disturbs the biodiversity of both land-based and water-based ecosystems. It also contributes to the acidification of ecosystems, which has adverse effects on natural and semi-natural habitats.

Ozone has a direct effect on plants, damaging leaf structure and reducing growth, and as a result, damaging their defense mechanisms. Exceeded ozone exposures to critical loads can affect crops, forests and semi-natural vegetation over substantial areas. Ground-level ozone also contributes to global warming indirectly by reducing carbon absorbed by vegetation (U.S. Environmental Protection Agency 1997).

As previously described, critical loads of exposure to one or more air pollutants will increase the risk of damage to certain sensitive elements of the environment that are essential for survival of many species on Earth. Thus, paying attention to environmental risks (R E) is not only important for biodiversity, but is also important for quality of life and for human activities.

Acknowledging these factors, that is, R H, R S and R E, AQRI can be defined as follows.

Table 1 presents an outline of the sources, impacts, prevention and control methods for the most important air pollution factors (U.S. Environmental Protection Agency 1997, 2006).

Table 1 Outline of the sources, impacts, prevention and control methods for the most important air pollutants
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This outline indicates that AQI as environmental risk index, R E, safety (damages and losses), as related economic damages, R S, and AQHI as health-related risk, R H, indices can be integrated in order to define and measure AQRI. It should be noted that in calculation of AQI, the six pollutant items are used as indicated in Eq. (1) (Environmental Protection Agency 2009), while for the calculation of AQHI, only three pollutants are used as shown in Eq. (2):

$$I_{\text{p}} = \frac{{I_{\text{Hi}} - I_{\text{Lo}} }}{{BP_{\text{Hi}} - BP_{\text{Lo}} }}\left( {C_{\text{p}} - BP_{\text{Lo}} } \right) + I_{\text{Lo}}$$
(1)

I p, Air Quality Index (AQI); C p, the rounded concentration of pollutant; BP Hi, the concentration breakpoint that is ≥ to C p; BP Lo, the concentration breakpoint that is ≤ to C p; I Hi, the AQI value corresponding BP Hi; I Lo, the AQI value corresponding BP Lo

$${\text{AQHI}}:\frac{10}{{10.4\left( {100\left( {{\text{e}}^{{\left( {0.000871 \times {\text{NO}}_{2} } \right)}} - 1 + {\text{e}}^{{\left( {0.000537 \times {\text{O}}_{3} } \right)}} - 1 + {\text{e}}^{{\left( {0.000487 \times {\text{PM}}_{2.5} } \right)}} - 1} \right)} \right)}}$$
(2)

All pollutant concentrations in this algorithm are 3-h average and in terms of \(\frac{{\upmu{\text{g}}}}{{{\text{m}}^{ 3} }}\) (Chen and Copes 2013).

In order to evaluate the damages and losses (safety factor), R S, the amount of damages per capita due to different consequences of poor urban air quality should be evaluated. For example, for the case of a resilient city with acceptable air quality, there are no amount of damage and therefore the R S is zero, while if the amount of damage exceeds 10 million dollars for a city with a population of 1 million, the R S is 10 (dollars per capita/per day), which indicates a high value for R S.

Calculation of AQRI

Air Quality Risk Index (AQRI) can be evaluated using three issues of health AQHI (R H), safety (damages and losses) (R S) and environment AQI (R E), which the amount of R H, R S, R E can be evaluated from Eq. (3).

$$R_{i} = {\text{SS}}_{i} \times {\text{LO}}_{i}$$
(3)

R risk; i H, S, E; L likelihood of occurrence; SS subjective severity.

Subjective severity (SS), which indicates the severity of damages considering each of the three elements, that is, AQHI, safety (damages and losses), and AQI can be evaluated from Table 2.

Table 2 The subjective severity of Air Quality Risk Index
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Likelihood of occurrence (LO) is the probability of occurrence of a possible event in a certain time period in four levels: 1—low, 2—medium, 3—high and 4—very high.

Risk can be calculated from Table 3, by multiplying topical severity of each case to its likelihood of occurrence, Eq. (3).

Table 3 Air Quality Risk Index matrix
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The AQRI can be calculated from Eq. (4):

$${\text{AQRI}} = \frac{{R_{\text{H}} + R_{\text{S}} + R_{\text{E}} }}{3}$$
(4)

AQRI, Air Quality Risk Index; R H, health risk; R S, safety risk; R E, environment risk.

The evaluated amount of AQRI, as shown in Table 4, will provide a guideline which indicates the level of activities in order to safeguard the environment, health standards and safety measures to reduce the consequences risk (Abbaspour and Ahmadi 2014).

Table 4 Air Quality Risk Index guideline
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It is also possible to give weighting coefficient to each element. This depends on their importance as presented in Eq. (5):

$${\text{AQRI}}_{\text{W}} = \frac{{R_{\text{H}} W_{\text{H}} + R_{\text{S}} W_{\text{S}} + R_{\text{E}} W_{\text{E}} }}{{W_{\text{H}} + W_{\text{S}} + W_{\text{E}} }}$$
(5)

R H, health risk; R S, safety risk; R E, environment risk; W, weighted factors 0 < w ≤ 1.

Case study

Tehran, Iran’s capital and largest city, has the highest rate of urbanization with a population of 13.7 million in 2010 (Statistic Center of Iran 2011; The world bank 2013). This megacity has a large, densely populated area of 750 km2 (Sodoudi et al. 2014). It is located in valleys and is surrounded to the north, northwest, east and southeast by mountains with heights between 1000 and 3800 m (Madanipour 2006).

Similar to other megacities of the world, Tehran faces serious air quality problems which are not only caused by its in habitants, but also caused by its geographical location (Naddafi et al. 2012).

The levels of air pollutants in Tehran throughout different years are shown in Fig. 1, and the AQI of Tehran for several years is shown in Fig. 2 (Ahadi et al. 2013).

Fig. 1
figure 1

Levels of air pollutants in Tehran (in different years) (Ahadi et al. 2013)

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Fig. 2
figure 2

The AQI for Tehran (in different years) (Ahadi et al. 2013)

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In order to have a reliable estimation, the 2010 data were used to evaluate the state of AQRI. According to Air Quality Control Company (AQCC) report, levels of air pollutants in Fig. 1 are used.

Therefore, the AQHI (SSH) obtained from Eq. (1) is in the range of 1–3, which means “low” see Table 2.

Economic losses (SSS) due to air pollution according to the World Bank report in 2010 reached 10 milliard dollars, which, in terms of dollars per capita/per day, lies in “very high” zone level in Table 2.

Also AQI (SSE) has been reported by AQCC in the range of 51–100 for most of 2010 days which lies in “medium” range in Table 2.

As Fig. 2 illustrates, the likelihood of occurrence (LO) of unhealthy days is 104, that is 30 % of the year 2010, and stands as “medium” see Table 3.

Thus, by multiplying each subjective severity (SS) with its likelihood of occurrence (LO), the R i number will be specified and, by placing the R i s in AQRI formula, the AQRI therefore can be determined by the calculation of mean value of these three risk values.

The likelihood of occurrence for each subject can be determined according to the weighting coefficient and frequency of occurrence as defined. From Fig. 2, it can be illustrated that there are more than 104 days as unhealthy and, in 2014, only 20 days has been reported as healthy and clean, and from expert opinions, the related LOs, R H,S,E are, respectively, 2, 3, 2:

$$\begin{aligned} R_{\text{H}} & : 2 \times 2 = 4 \\ R_{\text{S}} & :4 \times 3 = 12 \\ R_{\text{E}} & : 2 \times 2 = 4 \\ \end{aligned}$$
(6)
$${\text{AQRI}} \to \frac{4 + 12 + 4}{3} = \frac{20}{3} = 6.6$$

Using Table 4, this indicates that the median range measure is required for this special case.

Even though AQI of the year 2010 for Tehran stands at clean zone, nevertheless, AQRI indicates that there is a need to change the common activities, establish safety measures and set new policies to reduce the amount of pollutants produced.

Results and discussion

The present study indicates the need for introducing a new AQI, which can cover all three factors of health, safety and environment with their related risks in an integrated manner.

AQRI is a steppingstone for decreasing financial losses and minimizing the environmental and related risks involved in achieving the sustainability and resiliency in cities.

The problem of AQI is that usually, the highest Ip (AQI of each pollutant) cannot reflect the actual situation of air quality.

The advantage of AQHI over AQI is that AQHI considers the integrated value of the three pollutants (NO2, PM2.5 and O3), but its defect is that it does not include CO concentration.

As is clearly shown, AQI and AQHI cannot reflect the consequences of air pollution for megacities like Tehran, while AQRI can provide enough information for decision makers to take preventive or recovery measures and set required legislations.

Conclusion

This paper provides a new procedure to quantify the new index called AQRI. This index can be used as a tool for integrated urban air quality monitoring and can also be applied as a guideline for residential, commercial and industrial activities.

Since the risks of damage due to poor quality of air is evaluated in this index, one can provide a proper tool to take required measures which can help the resiliency of megacities such as Tehran.

The authors are currently working to improve this index by introducing some new parameters, such as weighting factors according to experts’ opinions on each of the three factors of health, safety and environment.