This article focuses on Tehran megacity, one the largest population centers

and most polluted cities in western Asia.

Air Pollution Challenges in the Megacity of

Tehran, Iran

Air Pollution Challenges in Tehran, Iran by Farhad Azarmi and Mohammad Arhami

em • The Magazine for Environmental Managers • A&WMA • April 2017

Air Pollution Challenges in Tehran, Iran by Farhad Azarmi and Mohammad Arhami

em • The Magazine for Environmental Managers • A&WMA • April 2017

Air pollution has been placed in the top 10 health risksencountered by human beings worldwide.1 Tehran megacity,capital of Iran, is the nation’s largest metropolitan area with aland area of 657 km2.2 Tehran has a population of approxi-mately 10 million, which continues to grow and suffers fromair pollution as consequence of rapid urbanization during the last several decades.3 Deterioration of air quality has alsobeen significant in several other fast growing cities in Iran,such as Mashhad, Arak, and Ahvaz,4 but the focus here is Tehran.

Air Pollution Situation in TehranTehran is rated as one of the world’s most polluted cities andis frequently immersed by smog, making breathing difficult.5

The smog contains a toxic blend of pollutants that have seriousenvironmental consequences (e.g., climate change, visibilitydegradation) and adverse health effects (e.g., lung cancer,respiratory disorders, mortality, and morbidity), and comprisesa variety of toxic metallic and organic compounds.6,7 In 2007,3,600 people died in a single month as a result of pollution,8

and Tehran’s air pollution has been proven to be responsiblefor several billions of dollars in relate costs each year.9 Exposureto sulfur dioxide (SO2), nitrogen dioxide (NO2), and ozone(O3) has resulted in approximately 1,460, 1,050, and 820

excess cases of mortality, respectively, per year in the city of Tehran.10

Each year, frequent episodes, whereby pollution exceeds air quality standards, are reported in Tehran. Figure 1 showsthe number of polluted days from 2004 to 2014 in Tehranowing to different pollutants, including fine and coarse partic-ulate matter (PM2.5 and PM10), NO2, O3, and carbonmonoxide (CO). A total of 120–230 days each year were reported polluted based on the recorded Air Quality Index(AQI) values. An AQI value of 100 corresponds to the national air quality standard for the pollutant; AQI valuesbelow 100 are normally assumed satisfactory (healthy day),while values above 100 are considered to be unsatisfactory(unhealthy day). The main pollutant responsible for polluteddays is PM2.5. Figure 1 clearly shows a considerable increasein the number of polluted days since measurements of PM2.5began in 2010. In the years up to 2007, CO levels were frequently above standard levels. However, the CO problemhas diminished since 2007 due to various counter measures,such as the phasing out of a fraction of old, highly pollutingvehicles and forcing industries to relocate outside the city perimeter.

Figure 1. The number of polluted days in Tehran due to the increments in concentrations of PM10, PM2.5, NO2, CO,and O3 (2004–2014). The pie chart represents fractions of contribution of each pollutant to the total number of thepolluted days.Source: Tehran Air Quality Company’s report in 2015 (available at air.tehran.ir).

Air Pollution Challenges in Tehran, Iran by Farhad Azarmi and Mohammad Arhami

em • The Magazine for Environmental Managers • A&WMA • April 2017

self-cleaning capabilities of the megacity.7 Tehran’s geographicalsituation, in combination with meteorological conditions suchas wind speed, wind direction, temperature, and frequentdust storms, affect the pollutants transport and concentrationsduring all four seasons of the year. In winter months, for example, Tehran experiences temperature inversions duringwhich motionless polluted air remains close to the surface,raising the pollutant concentrations to high levels.13 The massconcentrations of pollutants are often considerably higher inwinter compared with other seasons in Tehran.13 In additionto the prevailing urban background concentrations of particles,the city is also subject to numerous dust storm episodes eachyear. During these episodes, which mainly occur during thewarmer seasons (spring and summer), dust particles blown to the city substantially increase the PM levels.14 Dust stormscan carry pollutants hundreds of miles and substantially limitvisibility and affect population health.

Current and Future Air Pollution StudiesIn recent years, studies have focused on identifying the majorair pollutant characteristics, performing source apportionmentsof pollution, determining temporal variation in concentrationsof pollutants in various meteorological conditions, and fore-casting pollution concentrations by implementing variousmodels. Other studies have assessed how Tehran’s growingurban population, energy consumption, culture, and vehiclescombine to raise concentrations of the main air pollutants.2-14

However, due to the lack of high-quality emissions data, manyuncertainties remain about the physicochemical features of

Major Causes of Pollution in TehranA mixture of sources emits pollutants to the atmosphere ofthe city (see Figure 2). Like many other large cities, vehiculartraffic is one of the main sectors responsible for air pollution.Along with rapid population growth, the city vehicular fleethas also grown exponentially. The city has approximately 3 million cars in the streets daily, and some of them are morethan 20 years old.5,10 Traffic is often heavy, pumping highamounts of exhaust emissions and pollutants into the atmo-sphere.11 On-road vehicles are known as the major sourcesof nitrogen oxides (NOx), PM10, PM2.5, and CO.2,12 Somestudies have shown that approximately 80 percent of thesepollutants are produced by mobile sources.10

The combustion of fossil fuels (mainly natural gas and diesel)in the industrial sector also releases pollutants into the air, including CO, SO2, NOx, and PM10.5 These stationarysources also generate a notable fraction of pollutants in thecity, since 20 percent of the total energy demand of Iran isconsumed in Tehran.10 Secondary pollutants (e.g., aerosolsand ozone) are also formed in the atmosphere through photochemical reaction of precursor organic vapours and inorganic gases emitted from abovementioned sources.

Unfavorable geographic and weather conditions also affectTehran’s air quality.12 Mountains surround the city to the northand east (Alborz Mountain Ranges), trapping the pollutantswhen the wind speed is not strong enough to blow themaway.2 Densely packed high-rise buildings also restrict the

Figure 2. A number of the main sources of air pollution in Tehran.

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em • The Magazine for Environmental Managers • A&WMA • April 2017

effectively. Hence, extensive studies to suggest the best management practices to control air pollution and assess alltheir effects are required. The findings of dedicated futurestudies will enable local authorities to develop national regulations, mitigation strategies, and risk assessment and toreduce emissions from the major sources (e.g., urban trans-port and energy production) and improve the air quality inTehran. In summary, the extent of the air pollution in Tehranhighlights the need for urgent actions to reduce pollutionthrough regulations and mitigation strategies. em

Air Pollution Challenges in Tehran, Iran by Farhad Azarmi and Mohammad Arhami

pollutants and particles released from different sources, theiremission characteristics, exposure levels, and suitable exposurelimits. Further research is required to monitor the emissionlevels of pollutants arising from various sources and theirphysicochemical properties. Moreover, emission inventoriesspecifying the contributions of multiple sources (e.g., industrialand vehicular sources) are needed.

The current national regulations and available guidelines forcontrolling the release of pollutants into the atmosphere havenot been successful in managing the air pollution problem

The extent of the air pollution in

Tehran highlights the need for urgent

actions to reduce pollution through

regulations and mitigation strategies.

Farhad Azarmi is a post-doctorate researcher, and Mohammad Arhami, Ph.D., P.E., is an associate professor, both in the civilengineering department at Sharif University of Technology, Tehran, Iran. E-mail: arhami@sharif.edu.

AcknowledgmentThe authors thank the Air Quality Control Company in Tehran for providing the recorded air pollutants data.

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8. Keivani, R. A review of the main challenges to urban sustainability; Int. J. Urban Sust. Dev. 2010, 1 (1-2), 5-16.9. Abbaspour, M.; Soltaninejad, A. Design of an environmental assessment model on the effect of vehicle emission in greater Tehran on air pollution with economic

sensitivity; Int. J. Environ. Sci. Technol. 2004, 1 (1), 27-38.10. Naddafi, K.; Hassanvand, M.S.; Yunesian, M.; Momeniha, F.; Nabizadeh, R.; Faridi, S.; Gholampour, A. Health impact assessment of air pollution in megacity of

Tehran, Iran; Iranian J. Environ. Health Sci. & Eng. 2012, 9 (1), 1-7.11. Halek, F.; Kianpour-Rad, M.; Kavousirahim, A. Seasonal variation in ambient PM mass and number concentrations (case study: Tehran, Iran); Environ. Monit. Assess. 2010, 169 (1-4), 501-507.

12. Hosseinpoor, A.R.; Forouzanfar, M.H.; Yunesian, M.; Asghari, F.; Naieni, K.H.; Farhood, D. Air pollution and hospitalization due to angina pectoris in Tehran,Iran: A time-series study. Environ. Res. 2005, 99 (1), 126-131.

13. Arhami, M.; Hosseini, V.; Shahne, M.Z.; Bigdeli, M.; Lai, A.; Schauer, J.J. Seasonal trends, chemical speciation, and source apportionment of fine PM in Tehran;Atmos. Environ. 2017, 153, 70-82; http://dx.doi.org/10.1016/j.atmosenv.2016.12.046.

14. Arhami, M.; Farokhzad, H.; Kamali, N. Air Pollution in Tehran: Key issues and forecasting; EM April 2014, 22-26.