The release of the US-based Health Effects Institute’s (HEI) State of Global Air report has generated the latest alarm over air pollution in India. The report includes an analysis of 25 years of Global Burden of Disease (GBD) data assessing the impact of air pollution on mortality.

According to 2015 GBD data, PM2.5 (particulate matter with a diameter of 2.5 micron) contributed to 4.2 million deaths globally, 52% of which occurred in China and India. In 2015, ground-level ozone caused 254,000 deaths worldwide, with India accounting for 42% of these deaths.

The report, and media attention, has been focused on particulate matter pollution which has the most immediate harmful impact. However, other pollutants such as sulphur dioxide (SO2) and nitrogen dioxide (NO2) also pose significant health hazards. SO2 is emitted by the burning of fossil fuels, especially coal. The gas can get converted to particulate matter through chemical reactions in the air. It can also result in acid rain that can hurt the environment, in particular, crops. NO2 is also generated from burning of fuels, and its chief source of emissions in cities is vehicles. High levels of NO2 can worsen the symptoms of those with respiratory diseases, and also cause acid rain. NO2 can also form particulate matter and ozone, both of which are harmful to health.

A previous Plainfacts column looked at the evolution of measurement of pollutants in the particulate matter category in India. This piece looks at the state of SO2 and NO2 levels and their measurement in India.

According to data from India’s Central Pollution Control Board (CPCB), SO2 levels in Indian cities have declined since the 1990s. The decline in Indian cities, as shown by the CPCB data, is due to a fuel shift from biomass-based fuels to LPG, and from coal to kerosene. For instance, SO2 levels averaged 18 µg/m3 (microgram per meter cube) over 1990-98 in Delhi, but by 2015, it had come down to 5 µg/m3. This is well within the upper limit of 50 µg/m3 for SO2 as mandated by the CPCB’s National Ambient Air Quality Standards (NAAQS). An analysis of SO2 levels for most cities shows a continuously declining trend in the presence of SO2 in the air.

However, satellite data from Nasa paints a different picture. Data from the Ozone Monitoring Instrument (OMI) of Nasa’s Aura satellite shows that total SO2 emissions over India have nearly doubled between 2005 and 2014.

Nasa’s global database takes into account 491 continuously emitting point sources that release more than 30 kilotonnes of SO2 per year. In India’s case, Nasa took into account 47 large point sources that emitted SO2: only one of these was of volcanic origin, and almost all the rest of the sources were large coal-based power plants.

What explains the discrepancy between the CPCB and Nasa data? Tarun Gupta, professor at the department of civil engineering, IIT Kanpur, points out that both have shortcomings. He points out that CPCB measures ground-level data, which is relevant for measuring health impact.

However, it has few monitors and these are primarily located in urban areas, away from sources of emissions such as coal plants. He argues that CPCB also does not readily collaborate with academics and researchers. Satellite data, on the other hand, comes with a high degree of uncertainty. It is difficult to use satellite data to pinpoint the health impact of SO2 emissions from tall smoke stacks in power plants as these emissions could travel vast distances depending on meteorological conditions.

In a research paper titled, A global catalogue of large SO2 sources and emissions derived from the Ozone Monitoring Instrument, Vitali E. Fioletov (from the Air Quality Research Division, Environment Canada) and his coauthors acknowledge the total degree of uncertainty for the data at 55% to 67%.

When it comes to NO2, CPCB data shows a mixed picture. Fuel emission norms such as the Bharat stage emission standards have been effective in containing pollution. Despite the growth in vehicles in cities, most cities remain under or close to the safety limit of 40 µg/m3 annual average.

However, this does not mean that NOx (oxides of nitrogen, including NO2) levels have not increased. In a 2012 paper published in the Environment Science and Technology Journal, titled Increase in NOx emissions from Indian thermal power plants during 1996-2010: Unit-based inventories and multi-satellite observations, Zifeng Lu and David G. Streets, from the Argonne National Laboratory in the US, analysed NOx emissions from Indian thermal power plants over 1996 to 2010, and found that emissions increased at least 70% over this period.

Opinion is divided on estimates of damage by NO2 and ozone. Gupta cautions that the HEI’s findings on ozone-related mortality are problematic because ozone monitoring is very recent in India, and most of the ozone data used by GBD comes from satellite observations. Moreover, high particulate matter in India counteracts ozone. Sagnik Dey, assistant professor at the Centre for Atmospheric Sciences in IIT Delhi, agrees that the GBD’s methodology in estimating ozone-related deaths has a large uncertainty range. Nonetheless, he argues that the GBD provides a qualitative scale to compare countries with respect to the health impact of pollution.

Pollution measurement in India is currently too focused on a few cities. Data from few monitoring stations or satellites, offer an incomplete picture. India can come up with effective measures to fight air pollution only if the CPCB improves coverage and encourages research on the data it generates.

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