One January evening a couple of Delhi winters ago, I was at my doctor’s. During the routine examination, he discovered that my blood pressure was rather high: 160/100 to be precise. I asked him what I should do; he said, “walk regularly, reduce salt intake and come back next week”. So I did. The reading remained the same, so I asked him again what I should do; again he said, “walk regularly, reduce salt intake and come back next week”. This went on for a few more weeks. Finally, after five or six weeks of consistently high readings, my doctor prescribed a medicine and added, “walk regularly, reduce salt intake and come back next week”.

Popping pills after an isolated high blood pressure reading is something no doctor worth his/her, er, salt would recommend. Instantaneous readings can vary wildly depending on a wide range of reasons – cold weather, a full bladder or the white coat. It is true of blood pressure; it is also true of air pollution. Its sources are many – from power plants to industries, from open burning of dried leaves to dust from construction sites, from vehicular emissions to road dust.

Then there are atmospheric conditions. Broadly speaking, weak or no winds result in higher concentrations of particulate matter; moderate winds disperse PM thereby reducing their level, while higher wind speeds increase levels of PM10 (mostly by bringing in dust from construction sites and open fires in neighbouring areas) but not PM2.5. Of course, a lot also depends on whether the sun actually shines – a quick glance at AQI (Air Quality Index) readings over a 24-hour period reveals the relationship between a clear sky and low levels of PM2.5, PM10 and NO2 (but not O3), be it Dwarka or the US Embassy.

Over the festive season, the sun shone rather brightly on Delhi – in fact, New Year’s Eve was the warmest in five years and five degrees above normal. Sometime during the final weekend of the holidays, Delhi debuted its ambitious odd-even rule. Even as the first couple of days passed off smoothly, critics cried themselves hoarse reminding everyone how January 4th – a Monday and the first true working day after the implementation of the scheme – would be the first real test.

Well, Monday the 4th of January came and brought with it a blanket of smog. That, a distinct lack of wind and an increase in the number of vehicles on the streets ferrying working professionals back to their offices after a break resulted in the inevitable – pollution levels rose through the day. The smog persisted through the working week – if anything, it intensified, resulting in the cancellation of many trains and flights.

So how much water does this comparison hold? Apart from the fact that January 1-7 saw more smog, lesser sunlight and lesser wind than December 25-31, there is also another factor at play: the number of cars on the streets in the last week of 2015 vis a vis the first week of 2016. Excluding the New Year weekend, the latter comprised four working days; excluding the Christmas weekend and New Year’s Eve, the former comprised just three working days, on which many working professionals – the ones who drive to work – would have been on leave.

Then there is the question of how many vehicles are actually off the streets on any given day. The rough math is easy. Data from the 2011 census shows that 10.79% Delhiites drive to work while 14.05% ride. Since two-wheelers are exempt, one is left with only about 40% private vehicles. Since either odd- or even-numbered vehicles are plying on any given day, one is down to 20%. And since there are another 24 categories under which vehicles are exempt – women, ambulances, embassy, defence, government, police, VVIP, CNG, hybrid, battery-operated etc – it is safe to say that only about 10% of all private vehicles are required to be off the streets on any given day.

Including the number of public transport vehicles (buses, cabs, etc.), the odd-even rule applies to only about 6% of Delhi’s vehicles. Then there are those who feign ignorance or are willing to cough up the Rs 2000 fine. Taking these figures as well as weather conditions into consideration, it is actually impressive that pollution levels have dipped, if only marginally. If taking one of 20 vehicles off the streets has had this impact, imagine what removing more can do. Bear in mind that apart from emissions, vehicles are also responsible for pollution in indirect ways, such as kicking up road dust, a huge slice of the pollution pie.

Another thing: Which polluted air are we breathing? Minimum stack height requirements ensure that power plants and other polluting industries release their noxious fumes high up in the sky; what we essentially inhale are vehicular emissions and road dust kicked up by vehicles, both of which are substantially greater in traffic jams. While taking just one in 20 vehicles off the streets, the odd-even rule has significantly reduced idling time and overall transit time.

Finally, the blood pressure analogy: Readings should not be analysed over a limited time-span. To find out if this really works, the odd-even scheme has to be repeated for extended durations of at least a fortnight at different times of the year and under different weather conditions.

Simultaneously, other steps need to be taken. Improving public transport by increasing the number of buses, making the Metro ply from 4am to 1am and at peak frequency from 6am to 11pm and reviving the Ring Railway as well as encouraging cycling and walking by constructing dedicated paths for the same are to combating pollution what, well, regular walks are to hypertension; deterring people from using private conveyance by raising parking fees and making monthly car-free days mandatory, the equivalent of a sodium-regulated diet.

[Kaushik Chatterji is an independent journalist based in New Delhi; he was formerly with Hindustan Times]