Silent killers SO2, NO2 turned Delhi into a gas chamber in November

Since pollution control agencies were busy with the exceptionally high levels of particulate matter — PM2.5 and PM10, the most dominant pollutants in Delhi’s air, nitrogen dioxide and carbon monoxide escaped scrutiny.
A child wears a face mask for protection from air pollution in Delhi.(Reuters)
A child wears a face mask for protection from air pollution in Delhi.(Reuters)
Updated on Dec 04, 2017 09:21 AM IST
Copy Link
Hindustan Times, New Delhi | By

The concentration of poisonous gases such as nitrogen dioxide and carbon monoxide rose up to seven times their safe limit in November, when Delhi was covered by a choking haze for several days.

Since pollution control agencies were busy with the exceptionally high levels of particulate matter — PM2.5 and PM10, the most dominant pollutants in Delhi’s air, these gases escaped scrutiny.

The Supreme Court-appointed Environment Pollution (Prevention and Control) Authority considers the part matter levels to enforce the Graded Response Action Plan, which was for the first time this year. The EPCA is planning to take into account other pollutants from next year.

Read: Delhi air pollution interrupts play in India vs Sri Lanka Test, visitors wear masks on field

Levels of ozone and ammonia also fluctuated drastically.

While SO2 and carbon monoxide can linger in the air from 10 days to a month, prolonged exposure to high levels of these gases could trigger a range of respiratory and cardiac diseases.

SO2, NO2 and volatile organic carbon can also trigger the formation of secondary pollutants such PM2.5 – ultra fine particles that can go deep inside the lungs.

Nitrogen Dioxide

High level of NO2 in the air has become a concern for experts and scientists. Data from air monitoring stations show that NO2 levels shot up by more than seven times above the safe standards in areas such as Punjabi Bagh over the last four November days. It had shot up to 588ug/m3 on November 28.

“NO2 is primarily the result of burning diesel. It can’t be said for certain why it increased to such levels in Punjabi Bagh without a detailed analysis, but it could be because of the high density of vehicular population,” said Anumita Roychowdhury executive director at Centre for Science and Environment.

Sulphur Dioxide

The gas, one of the key elements of smog, has dropped well below the safe standards in Delhi primarily because of use of cleaner fuels. Though it did not breach the safe standards for a single day, in NCR cities such as Ghaziabad and Bhiwadi, it increased beyond the safe standard.

“Monitoring SO2 is important because it triggers the formation of secondary pollutants. SO2 often condense and convert into other pollutants, reflecting a reduction in its own level,” said SN Tripthi head of the Centre for Environmental Science and Engineering at IIT Kanpur.


Ozone is a threat to asthma patients and can cause premature death, if it is high even for a short duration during the day. It forms primarily when nitrogen oxides, carbon monoxide (CO) and volatile organic compounds (VOCs), react in the atmosphere in the presence of sunlight.

The level of O3 shot up above the safe standards on some days at places such as Anand Vihar, Delhi University North campus and Lodhi Road. The maximum level was recorded at DU where it had shot up to 143 above the permissible limit of 100.

Carbon Monoxide

On Lodhi Road, it had shot up at least three times above the safe standards on certain days. Once in the air, this gas can linger for more than a month and its presence affects concentrations of other greenhouse gases, including methane, ozone and carbon dioxide.

“Carbon monoxide is usually emitted during burning of garbage or incomplete combustion,” said Dilip Ganguly assistant professor at Centre for Atmospheric Sciences at IIT Delhi.

Other gases: Other pollutants such as Benzene and Ammonia have also spiked sometimes in Delhis air in certain places. But on most of the days they found to be under safe limits.

Benzene, which is usually found in emissions, also escapes into the atmosphere when petroleum products are transported or handled. A fuel station (petrol pump) in the vicinity of an air monitoring station could push up levels of this gas. Its level often shot up in places such as Shadipur, Punjabi Bagh and Anand Vihar.

Levels of Ammonia also shot up in Delhi on some days but have never crossed the safe standards.

“Some pollutants may spike momentarily at some stations for various reasons but their overall levels are under control. This time, we experienced a far better November as far as pollution levels are concerned. Efforts like GRAP has started to show its effects and despite unfavourable weather conditions, pollution is still in very poor category,” said D Saha, head of the air quality laboratory at CPCB.


    Joydeep Thakur is a Special Correspondent based in Kolkata. He focuses on science, environment, wildlife, agriculture and other related issues.

Close Story

Less time to read?

Try Quickreads

  • Official removing the fallen tree from the railway track at Parsik tunnel. (HT Photo)

    Loco pilot averts major mishap after tree falls on railway track near Thane

    The presence of mind of a locomotive pilot on Monday night averted a major mishap after a tree from the Parsik tunnel fell and blocked track number 6 of the railway network between Thane and Kalyan station. The incident occurred around 8:45 pm when a Gulmohar tree fell on the track just moments before the Lokmanya Tilak Terminus-Bhagalpur express was about to arrive at the tunnel.

  • Moist east wind raised humidity levels in the Capital on Monday. (HT PHOTO)

    Delhi’s air quality in moderate category, mercury expected to settle around 41°C

    Delhi's air quality was in the moderate category on Friday morning as the mercury was expected to settle around 41 degree Celsius on Tuesday. Moist east wind raised humidity levels in the Capital on Monday. The mercury touched 40.2C at 2.30 pm on Monday in Safdarjung, the base weather station, when humidity was at 45%, taking the heat index, or real feel, to 52C, the highest so far this year.

  • A slum in Laxmi Nagar area of east Delhi. (PTI File Photo)

    DUSIB approaches college-goers for survey to fix issues in slums, night shelters

    In a bid to identify issues faced in 1,000 community toilets, the Delhi Urban Shelter Improvement Board, the Delhi government's slums management agency, is seeking the help of college students to carry out a survey at toilets run by the agency in the city, and also identify problems faced in nearly 800 night shelters. With the survey, the agency aims to identify beneficiaries under the slum and JJ cluster rehabilitation policy of the Delhi government.

  • Shiv Sena leader Sanjay Raut. (HT PHOTO)

    ‘Uneducated ones are like walking corpses’: Sanjay Raut's veiled dig at rebels?

    Shiv Sena MP Sanjay Raut – under fire from his party's dissident faction for calling them 'living corpses' – on Tuesday, tweeted out a one-liner that contained the phrase 'chalti firti lashain' (moving dead bodies), in what seemed a dig at the rebel group. 'Jahalat', an Urdu word, roughly translates to being unknowledgeable/stupid. The full translation, therefore, reads: “Stupidity is like death, and fools are walking corpses.”

  • The Agnipath scheme provides for the recruitment of youths in the age bracket of 17.5 to 21 for four years. (HT photo)

    Bihar: Trainers, coaching institutes wary of Agnipath impact

    The Centre's Agnipath scheme for recruitment in the armed forces has evoked mixed responses ever since it was announced by the government. There is now disappointment among group of trainers and coaching institutes in Bihar engaged in mentoring students for their career paths. Rahul Kumar, who is from Ujiarpur (Samastipur), is a trainer for Army job aspirants. An Army job aspirant Ajit Kumar said the passion for the job was the main driving force.

Story Saved
Saved Articles
My Reads
Sign out
New Delhi 0C
Tuesday, June 28, 2022