Delhi’s battle against air pollution has been the talking point in media and news outlets over the past two months. On November 8th, the city’s air quality monitoring systems registered an AQI (Air Quality Index) of 999, a reading several times over the safe limit of 150.
A thick haze had blanketed the national capital region, forcing residents indoors and causing schools to be shut down for the week. Poor visibility stopped traffic in many areas in the city.
Persistent levels of pollution in the city caused hospitals to fill up with patients afflicted by the smog. In December, the haze disrupted a Test match between India and Sri Lanka, when Sri Lankan cricketers were taken ill on the field.
Till December 6th, Delhi’s air quality remained unhealthy and was mostly classified as ‘very poor’, with the Air Quality Index fluctuating between 215–380 on a scale of 500. Increased wind speed provided temporary relief last week when the AQI dropped to 194, putting Delhi’s air quality in the ‘moderate’ category.
However, the System of Air Quality and Weather Forecasting and Research (SAFAR) recorded increasing AQI at its Delhi monitoring stations (average of 300) on December 9th, with conditions expected to worsen over the next few days.
So, what’s causing the persistent problems with air quality in Delhi?
The city is no stranger to air pollution. In fact, for many years now, residents have learned to expect the smog that shrouds the city every winter. NASA Visible Earth shows images of a thick haze spanning the breadth of the Himalayan foothills, from as far back as 2003.
The seasonal smog is a mixture of smoke from crop burning in neighboring states Punjab and Haryana, mixed with fog and pre-existing pollutants in the air. Due to cold temperatures and low winds, the particulate pollution remains suspended in the air for several weeks. The haze affects north India, and regions from Pakistan and Bangladesh.
However, though the spike in Delhi’s particle pollution is due to smoke from seasonal crop burning, air quality remains poor in the national capital region (NCR) through the year. Since 2007, studies on Delhi’s air quality have examined the underlying cause of air pollution, and emphasized the rising trends of particulate matter (PM) concentrations in the air. The reports mainly focused on diesel-fueled vehicles as the primary contributor to Delhi’s pollution.
With India’s launch of the Air Quality Index (AQI) in 2014, a quantitative assessment of air pollutants was made available to the Indian public. AQI measures levels of different pollutants in the air (PM10, PM2.5, SO2, NO2 and CO). PM10 and PM2.5 refer to minute particles, of diameter 10 and 2.5 micrometers. To put this in context, the diameter of fine beach sand is on average 90 micrometers.
Through 2015 and 2016, statistical models and data collection suggested that Delhi’s air pollution could not be solely due to increasing numbers of private vehicles on the road. A 2016 study from IIT Kanpur examined the sources of Delhi’s air pollution, and comprehensively studied PM distribution in the air.
Highlights from the study underlined the fact that Delhi’s particulate pollution shows a seasonal variation. During winter, the majority of the PM (PM10 – PM2.5) is from biomass burning (17 – 26 %), secondary particles (25 – 30 %) and vehicles (20 – 25 %). Whereas in summer, dust from construction sites and dirt roads are the major contributors of PM (26 – 27 %).
Other significant sources of pollution include coal and fly ash, as well as burning of municipal solid waste (MSW). Secondary particles are emissions from power factories, cooking oil, industries and incinerators.
Health hazards due to rising levels of PM
According to India’s National Ambient Air Quality Standards, PM values of 100 micrograms per cubic metre for PM10, and 60 micrograms per cubic metre for PM2.5 are considered safe. During the Delhi smog, PM2.5 reached 710 micrograms, which is 11 times the recommended exposure limit. PM10 reached 412 micrograms.
High PM2.5 is considered the more damaging form of air pollution, since it can lodge deep into our lungs and enter the bloodstream. This causes respiratory disorders, and prolonged exposure to particulate matter increases the risk of heart disease, stroke, lung cancer and respiratory infections.
A UNICEF analysis in December warned that high levels of air pollution affect brain development in young children. Children exposed to ultrafine PM2.5 particles, and polycyclic aromatic hydrocarbons (PAHs), are at high risk of developing structural damages to the brain. PAHs are commonly found in areas with dense automobile traffic. Prolonged PAH exposure also increases the risk of cancer.
During winter, PAH levels in Delhi rise to very high concentrations (average of 80 ng/m3) compared to summer (average of 15 ng/m3), increasing the risk of PAH exposure in the city during winter. No safe levels are recommended for PAH since it’s carcinogenic.
Air quality in urban India
The deteriorating conditions over the years have affected urban life in the NCR, so much so that doctors in Delhi have advised at-risk residents to move out of the city to safer zones.
However, though Delhi has been under fire for worsening air quality, it may not even be the most polluted city in India. According to WHO Urban Air Pollution database from 2016, Indian cities Gwalior, Allahabad and Patna registered an annual mean PM2.5 of 176, 170 and 149 respectively, putting Gwalior as the second most polluted city in the world that year.
Delhi recorded an annual mean of 122, and ranked as the 11th most polluted city in 2016.
More recently, Indian city Mumbai was engulfed in a haze on December 9th, with PM2.5 recorded as high as 305 in some places, though average AQI in the city was 176.
AQI data from monitoring stations across India on December 10th showed that air quality predominantly varies between moderate to severe, with north Indian cities such as Patna, Gaya and Lucknow classified as ‘severe’ (AQI 318 – 420), Delhi, Varanasi and Kanpur as ‘very poor’ (AQI 212 – 290), and Agra as ‘poor’ (AQI 195).
In central India, air quality in Nagpur, Vishakhapatnam, Hyderabad and Mumbai was ‘poor’ (AQI 150 – 180), while further south the AQI appears to improve with Bengaluru classified as ‘satisfactory’ (AQI 41), and Chennai as ‘moderate’ (AQI 140).
Apart from regional human activities, it is important to consider that geographical location, as well as seasonal variations in the region affect air quality trends across the country.
It is also important to note that AQI data is available only for urban cities. No data is currently available for rural regions. Rural areas, villages and towns have a subset of emission sources such as factory outlets, higher exposure to indoor air pollution, coal and fly ash, unpaved road dust, seasonal crop residue burning or biomass burning, and high use of pesticides on farms.
Biomass burning may be the major seasonal contributor to air pollution in rural areas; however, this is speculative since no air monitoring data is available.
Biomass burning: other potential hazards
Seasonal crop burning is practiced in Punjab and Haryana during the winter. A possible hazard from agricultural waste burning is the potential release of a class of pollutants called POPs (Persistent Organic Pollutants), due to the burning of rice or wheat field residues.
POPs are non-degradable chemical compounds that remain in the environment and are biomagnified, building to highly toxic concentrations in the fat tissues of animals on the higher end of the food chain. These pollutants can migrate over long distances, causing both regional and global contamination.
The burning of agricultural waste has been reported to cause a significant increase in dioxin and furan emissions (PCDD/F) in China. Both compounds are persistent organic pollutants. In addition, due to past and current use of organic chlorinated pesticides in India, soils in many regions are rich in chemical organic pollutants. The presence of these pesticides could further increase dioxin emissions when crop residues are burnt.
Long-term exposure to dioxins can cause developmental problems, compromise the immune system and cause cancer.
Currently, there is no data available regarding air pollution by POPs, linked to crop burning in India. Due to the simultaneous increase of PM levels during biomass burning, further research is required to assess the chemical stability of any particle-bound POPs in the air.
As can be inferred from Delhi’s current situation, pollution in urban cities is a result of emissions from multiple sources, and is found throughout the country. Though Delhi has taken the heat for poor air quality control, other regions in the country regularly contend with air pollution as well. Urban areas appear to be most affected, but this is not certain as no data is available for India’s many rural regions.
However, with rapid urbanization and migration of people to megacities, there is a need for strategic urban planning that reduces particulate pollution. Several pollutant prevention and emission control measures are therefore required, in addition to further studies that examine rural emission sources, and monitor other classes of air pollutants at these sources.
We also need to remember that each city may have its own class of gaseous pollutants to contend against, depending on varied human activities. For instance, in a conference report from 2016, Chennai was found to have the most contaminated soil among Indian cities for high concentrations of PCB (polychlorinated biphenyls). The second highest contamination was reported in a village near Bengaluru. PCB is also a persistent organic pollutant.
Urban cities Mumbai and Delhi showed high levels of PCB in the air, primarily due to informal electronic-waste recycling, and burning of MSW.
Since air-borne particles have long-range spatial dispersal patterns, the problem requires a comprehensive and long-term systemic solution in order to have a significant impact on improving air quality in India.