According to charts published by Tehran Air Quality Control Company’s website, Airnow.tehran.ir, Tehran’s residents experienced mostly “moderate” air quality during the month, meaning that the Air Quality Index stood between 51 and 100 on 26 days.
The index categorizes conditions dictated by a measure of polluting matters into good (0-50), moderate (51-100), unhealthy for sensitive groups (101-150), unhealthy (151-200), very unhealthy (201-300) and hazardous (301-500).
TAQCC data show that the remaining four days of August were unhealthy for the sensitive group, as AQI hovered between 101 and 150.
During these four days, the sensitive group, which includes children, the elderly, pregnant women and those with respiratory and cardiovascular conditions, were advised to limit their outdoor activities.
Analyses indicate that pollutants responsible for the toxic index recorded in the polluted days were mostly ground level ozone, of which the highest concentration reached 114.
Consecutive analyses indicate that in July, Tehran residents breathed in one unhealthy air quality with the AQI reaching a high of 162.
The sensitive group was strongly advised to stay home on 13 days as the air quality index stood in the range of unhealthy for the group.
According to the TAQCC charts, the remaining 17 days showed moderate air quality. Comparative data indicate that air quality records were less satisfactory during the year-ago month.
In August 2020, “moderate” status was recorded on 21 days and the remaining 10 days were announced dangerous for the sensitive group.
Bad ozone and PM2.5 pollutants had the highest concentration during the months under study.
Scientific studies have shown that ground-level ozone is a secondary pollutant, formed when pollutants emitted by cars, power plants, industrial boilers, refineries, chemical plants and other sources react chemically in the presence of sunlight.
Bad Ozone Pollution to Persist
Besides ozone that occurs naturally in the Earth's stratosphere and forms a protective layer that shields the earth from the sun's harmful ultraviolet rays, there is "bad" ozone in the lower atmosphere, near ground level, which is considered a harmful air pollutant.
The ground-level ozone results from chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC), created in high concentration in the presence of sunlight.
Experts say emissions from industrial facilities, electric utilities and motor vehicle exhausts, gasoline vapors and chemical solvents are some of the major sources of NOx and VOC.
Ozone is a colorless gas, 1.5 times denser than oxygen.
Breathing ozone can trigger a variety of health problems, particularly for the children, the elderly and people of all ages who have lung disorders such as asthma. Ground-level ozone can also have harmful effects on sensitive vegetation and ecosystems.
Recommended remedial measures to alleviate this kind of pollution are similar to those regularly suggested for other types of pollution, such as detecting and removing mobile or stationary sources of nitrogen oxides and hydrocarbon emissions.
Vehicles burning diesel fuel and carburetor-equipped motorcycles are the most common producers of toxic emissions that lead to the emergence of ground-level ozone.
The capital's pollution levels often reach unhealthy levels in the cold season when the phenomenon of inversion occurs, but PM2.5 is chiefly responsible for poor air quality in this period.
In the warm season, which offers relief from smog, the high levels of pollution are blamed on ozone and dust particles.
TAQCC charts indicated that ozone pollution hit a record in Tehran's history of poor air conditions last year.
Ahad Vazifeh, an official with Iran Meteorological Organization, had told the local media that the high ozone pollution is expected to persist in the capital for the next several days.
Main Culprits
Besides the hot weather, which is the main reason behind ozone formation in the already polluted metropolis, some officials also blame Tehran Municipality.
The Health Ministry has criticized the inaction of TM in identifying the sources of pollution and alleviating the problem on several occasions.
“Catalytic converters installed on vehicles are not in conformity with the required standards, if at all vehicle owners bother to equip their cars with the device," Abbas Shahsavani, the head of health group at the ministry, said.
A catalytic converter is an exhaust emission control device that reduces toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction.
“Many commercial vehicles and public transport buses, which generously add to air pollution in the city, do not use catalyst converters,” Shahsavani said, adding that it is TM's responsibility to monitor and crack down on smog-inducing two- and four-wheelers plying the streets of Tehran.
The capital’s urban managers have always censured passenger vehicles for their detrimental effects on air pollution and set several traffic regulations, schemes and fines to curb the use of private cars.
This is while a recent study by TAQCC on the nature and sources of pollutants shows that the highest level of emission is spewed by passenger buses, equal to 31%.
Heavy-duty vehicles are the next most polluting source with 23.7%, followed by motorcycles with 10%, airplanes with 5% and minibuses with 4.3%.
All these mobile sources account for a total of 76% of PM2.5 particles released into the air and the remaining 24% are emitted by industrial units.