The residents of Iranian capital inhaled less polluted air in May compared with the same period of last year, Tehran Air Quality Control Company’s data show.
However, charts published by TAQCC website, Airnow.tehran.ir, show that the air quality index did not fall under 50.
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).
The company’s charts show that “moderate” status was the dominating air condition, as the index hovered between 51 and 100 throughout the month.
However, on May 29, AQI levels reached its highest of 99 due to the increasing density of pollutants.
Things were a bit different in May 2020 as sensitive groups in the capital were advised to limit their outdoor activities due to air pollution on two consecutive days, as the index entered the threshold of unhealthy for sensitive groups (101-150).
Children, the elderly, pregnant women and those with respiratory and cardiovascular conditions fall in the above group.
The index remained in the “moderate” range in the rest of the month, the data indicate.
Analyses show that pollutants responsible for the toxic index recorded on the two days were mostly ground level ozone and PM2.5 (tiny particles smaller than 2.5 micrometers).
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.
The pollutant’s density is projected to rise in the coming months, as the season becomes warmer.
Bad Ozone
Besides ozone that occurs naturally in the Earth's troposphere 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 three years ago.
Main Culprits
Apart from 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 several times now.
“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 catalytic converters,” Shahsavani said, adding that TM is responsible for cracking 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.
