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.
Detailed charts released by Tehran Air Quality Control Company indicate that ozone pollution has hit a record in Tehran's history of poor air conditions.
TAQCC reports that since the beginning of the current Iranian year (March 21), ground-level ozone has been responsible for 15 days of air pollution, according to Financial Tribune.
The concentration of the pollutant reached its highest on July 1, pushing up the Air Quality Index to 160, showing "unhealthy" status for all groups.
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).
Detailed data analysis of ozone pollution shows that things have recently become worse.
During the same period of last year (started March 2018), ozone pollution was recorded on 11 days in the capital and AQI did not pass the threshold of 150, which shows unhealthy conditions for sensitive groups.
Children, the elderly, pregnant women and those with respiratory and cardiovascular conditions fall in the above group.
In the year before (started March 21, 2017), bad ozone was recorded on only two days, increasing the AQI to "unhealthy for sensitive groups" status.
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 is a result of chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC), created in high concentration in the presence of sunlight.
Hossein Shahbazi, director of modeling and forecast at TAQCC, earlier told ISNA that "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.
Experts say 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 is rather a relief from smog, ozone and dust particles are to be blamed for the high levels of pollution.
Mohammad Rastegari, the Department of Environment’s deputy for environmental monitoring, believes that although the contaminant has not turned into a serious issue in Iran yet, this year might be the first time Tehran experiences the highest levels of ground-level ozone.
Ahad Vazifeh, an official with Iran Meteorological Organization, told ISNA that the high ozone pollution is expected to persist in the capital for the next several days.
Tehran Municipality Blamed
Besides the hot weather—the main reason behind ozone formation in the already polluted metropolis, some officials also blame Tehran Municipality.
During Wednesday's meeting of the workgroup for air pollution reduction, Abbas Shahsavani, the head of health group at the Health Ministry, criticized the inaction of Tehran Municipality in identifying sources of the secondary pollution and alleviating the problem.
"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," he 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.