Charts published by Tehran Air Quality Control Company’s website, Airnow.tehran.ir, show that air quality was healthy on only one day in the month, as the air quality index was 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).
TAQCC data show that during the month, “moderate” status was the most prevailing air condition, as the index hovered between 51 and 100 in May’s remaining 28 days.
Sensitive groups in the capital were warned to limit their outdoor activities due to air pollution in two consecutive days, as the index entered the threshold of unhealthy for the sensitive group (101-150) on May 22 and 23.
Children, the elderly, pregnant women and those with respiratory and cardiovascular conditions fall in the above group.
Analyses illustrate that the pollutant responsible for the toxic index recorded in the two days was ground level ozone.
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.
TAQCC data indicate that air quality condition was slightly better in the same period of last year.
In May 2019, AQI had two “good” and 29 “moderate” days.
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 is rather a relief from smog, ozone and dust particles are to be blamed for the high levels of pollution.
Key Factors
Following the government’s temporary cancellation of the restrictive traffic rules in Tehran, the so-called “Traffic Scheme” and “Air Pollution Control Scheme” are not being enforced. This is in line with efforts to help people observe social distancing criteria to prevent the Covid-19 disease from spreading further.
The Traffic Scheme is usually enforced in a 3,000-hectare area in the center of the city, where cars, except public transportation vehicles, are barred from entering the area between 7 a.m. and 6 p.m. Entering the zone costs up to 517,000 rials ($3.2) for private vehicles.
APCS defines an 88.5-square-kilometer area in central Tehran, which could be entered for free over a maximum of 20 days each season, or 80 days a year.
In addition to the fact that the suspension of the schemes has motivated citizens to use their private cars without being worried about getting charged or fined, the government also advised citizens to avoid the hazardous public transportation system as far as possible to flatten the curve of the novel coronavirus infection.
The measures have led to more clogged roads and heavier traffic jam in the city and consequently more polluted air, the officials believe.
However, although AQI charts show fewer days with “good” air quality, its difference with the index in May 2019 is not significant. Therefore, slightly more polluted air this year cannot be solely blamed on private car emissions.
According to a study conducted last year by TAQCC, mobile sources, including private cars, taxis, motorcycles, minibuses, buses, heavy-duty vehicles and airplanes, are blamed for 76% of the total PM2.5 in Tehran’s air quality.
Based on these findings, the highest level of emission at 31% is spewed by passenger buses, even more than all the stationary sources of pollution in the city. 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%.
The data illustrate that the least polluting groups are private vehicles and taxis with a respective contribution of 1.6% and 0.4%.
This is while not only have the capital’s urban managers always censured passenger vehicles for their detrimental effects on air pollution, but also set more stringent traffic regulations, schemes and fines to curb the use of private cars.
Ineffective efforts to relocate 300 industrial units in Tehran have been to no avail. Over the past many years, the city’s mayor and councilors have failed to address the issue effectively, blaming others for the worsening air pollution.
Smog in Tehran takes the lives of over 3,500 people every year, Iran’s Health Ministry reported.