The National Capital and the areas around it struggled for breath in October and early in November as pollution levels reached hazardous levels .

Contributing to the filthy air was smoke from crop stubble burnt in the nearby agricultural states of Punjab and Haryana, vehicular and industrial emissions in the capital as well as dust from construction work. Low temperatures during the period strengthened the smog blanket as the colder air made it more difficult for the pollutants to disperse.

As pollution levels rose, Delhi’s Graded Response Action Plan went into effect. Construction activity was halted and schools were ordered shut.

Significantly, the odd-even scheme went into effect. The scheme, which allows vehicles with odd-numbered licence plates on the road on dates with odd numbers and those with even-numbered plates on others, was implemented for the first time in 2016 in two phases. It ran from 8 am till 8 pm on all days except Sundays. Two-wheelers and vehicles driven by women were exempt from the scheme.

The third edition was enforced from November 4 to 15, but was suspended on November 11 and November 12 on the occasion of Guru Nanak Jayanti.

Compared to the second phase in April 2016, the fines levied by the police on motorists violating the scheme this year fell by 50%. In April 2016, fine had been imposed on 9,576 motorists, reported the Hindustan Times. This year, action was taken against 4,885 motorists .

But did reducing the number of vehicles on the road result in lower pollution levels? Experts are wary of drawing conclusions from the data, adding that an analysis of pollution levels must consider multiple factors, including weather conditions.

What data shows

For this analysis, picked four air-quality monitoring stations spread around Delhi – RK Puram in South Delhi, Anand Vihar in East Delhi, Jawaharlal Nehru Stadium in Central Delhi and Punjabi Bagh in West Delhi.

All four stations are monitored by the Delhi Pollution Control Committee, an autonomous body under the Delhi government. Daily averages were used from the hourly data recorded from 9 am till 9 pm between November 4 and November 15. On some days, data was not recorded by the monitoring agency for a few hours.

In the chart below, it can be seen that PM 2.5 levels started to increase after October 27.

Particulate matter smaller than 2.5 microns (or about a ten-thousandth of an inch) is particularly dangerous to human health. Such particles are small enough to travel deep into the respiratory system, potentially hurting lung function. The National Ambient Air Quality Standards require PM 2.5 concentration be less than 60 micrograms per cubic metre of air, in any given 24 hour period.

On November 3, levels peaked as Delhi experienced its worst air quality during the season.

On November 4, on the first day of odd-even, these levels suddenly dropped and the city had cleaner air to breathe. On the next day, the air quality improved further as the wind speed picked. This trend continued until November 10 after which PM 2.5 levels started to increase steadily.

Similar trends were also noticed in PM 10 levels or particulate matter the size of 10 micrometers. also analysed the 24-hour average PM 2.5 concentrations in the regions adjoining Delhi, where the scheme was not implemented.

In Ghaziabad, Uttar Pradesh and Gurugram, Haryana, a similar drop in particulate matter in the air was observed between November 4 to 10. Both places, because of their proximity to Delhi, could be expected to experience similar wind patterns as the capital. The data indicates that Delhi’s PM 2.5 levels were marginally lower than its adjoining regions, but this was also true on some of the days before the odd-even scheme was introduced.

Outcome in 2016

On November 15, the Supreme Court expressed concern about the effectiveness of the odd-even policy. Studies about the operation of the scheme in 2016 showed that there were marginal improvements in Delhi’s air quality as a result of several factors.

When the policy was first implemented between January 1 and 15, 2016, particulate matter 2.5 levels fell by 4%-6%, found a study by researchers from the Indian Institute of Technology-Kanpur, Indian Institute of Technology-Delhi, The Energy Research Institute and National Physical Laboratory in Delhi, and Indian Institute of Tropical Meteorology in Pune.

The study noted that this was not a significant consequence of the policy, considering that it was taken as an emergency measure. “The failure is attributed to stable meteorological conditions (winds are not strong enough to disperse PM 2.5 away) during the period and there was no control over PM 2.5 outside the periphery of the city,” the study states.

Another study concluded that the first phase of odd-even in 2016 led to a reduction of 13% in particulate matter concentrations. The study was conducted by researchers from the University of Chicago and Harvard University. The study noted that the reduction in particulate matter was highest in the mornings from 11 am till 2 pm, but no such improvement was noticed at night.

The study also found that compared to levels in other states, Delhi did not show any significant improvement in the second phase of the policy in April 2016.

Interestingly, the study noted that similar car rationing schemes implemented in China and Mexico did not yield satisfactory results because of non-compliance.

The Central Pollution Control Board also carried out a study of the two phases in 2016. It concluded that a single factor could not result in substantial reductions in pollution and that an “integrated approach” was needed.

“With no clear trend and wide fluctuations observed in the concentrations, it is evident that the meteorology and emissions from other polluting sources have been major factors impacting air quality of Delhi during the period,” the study noted.