It is that time of the year when all Hindus are shamed for celebrating one of their most important festivals - Diwali. Apart from lights, the festival is celebrated with fireworks and crackers. The common refrain is that the festival causes pollution, and serious harm to health of kids and adults alike. Besides which, we are also reminded of animals not liking loud noises, and the pollution killing off birds - all for one day.





While smoke is an issue on Diwali day, we will take stock of the facts about pollution - and Diwali's contribution to it. There are few threads in this problem: a) the overall pollution levels and the source thereof, b) the persistence of firecracker / Diwali pollution, and c) the economics of firework / crackers in the recent years.





A Greenpeace report [ here ] reports that all around the year, Indian metropolii are uninhabitable with respect to air quality. Particularly, Delhi has the distinction of being the most polluted city in the world. Average year round air pollutant levels [both PM2.5 and PM10] in Delhi were >5x the safe levels as specified by India, and ~10x of that specified by WHO. The GP report identified construction and vehicular traffic to be the two largest contributors of pollution. The third largest contributor was practice of burning paddy husk in nearby states. While such a detailed analysis has not been done for other cities like Bangalore, anecdotal evidence suggests that construction dust may be the biggest culprit, when it comes to air pollution. Besides construction dust and vehicular pollution, bad infrastructure [as in bad roads] creates suspended particular matter that are effective respiratory pollutants. This is particularly true of metros where civil amenities are more of a wish than a reality. From the above points, it is clear that majority of pollution in our cities do not happen due to Diwali, and more importantly, unlike smoke during Diwali - these are year round persistent issues.





There was a research paper published by IIT-Kanpur in 2016, which studies the issue of pollution and its causes in great detail; in and around the NCR [Delhi]. One particular figure in this report [Figure 2.5] is educative. The figure has been reproduced below:









The time series graph shows the pollution levels in Delhi during the month of November (year) 2013. The first data point is Diwali day - and one does see that the pollutant levels were much higher than the mean [red and blue dotted lines]. What is interesting however, are the pollution level spikes on the 5th of November, on the 9th and 11th, and on 21 through 23rd. In fact, the pollution levels on the 23rd of November are far higher than on Diwali day. One may argue that the Diwali pollution persists for three weeks, but the data falsifies such a hypothesis. In fact, we find that the pollutant level on Diwali day falls well below the mean levels by the next day. Furthermore, the median pollutant levels across the month also seem to be well below mean levels. These two data together suggests that the pollution caused by Diwali is extremely short-lived [<24Hrs], and is no worse than the usual sporadic spikes of pollutants that are observed in Delhi.





Another aspect of Diwali that gets condemned is the noise pollution. While firecrackers used to be noisy [upto 140dB] in the past, continuous revisions in acceptable noise levels have brought down the overall noise levels to less than 100dB - and that was the maximum noise by a firecracker. In fact, the overall noise levels across several cities were measured to be between 60dB and 90dB [far lesser than the 100dB of the noisiest cracker]. Compare this with a typical noise level in a city junction: honking creates noise in excess of 100dB





A look at the differential noise between a normal day and Deepavali noises shows that in many places, the differences are marginal. In fact, in some cities, the differences are so marginal that one may consider it to be well within expected variation. This figure is from a Central Pollution Control Board study on pollution during Diwali across India [2014]. Sadly, this report doesn't provide confidence intervals for the noise levels; nor does it provide any P-values for the differences in noise between normal and Deepavali day. What is clear, is that Deepavali likely to be as noisy as any normal working day in Indian cities.







A statistical test for Bangalore for Normal and Diwali days [screen shot below] shows that the mean difference in noise between a normal and festival day in the city is barely 2dB. In other words, the difference is negligible. Diwali doesn't create any extra-ordinary noise pollution than an normal day.









Last but not least, we come to the question of economics. Several reports such as this one point out that fire cracker sales have been falling drastically, on an annual basis. This particular report cites a 20% reduction [year on year] in sales for five consecutive years. Such drastic reduction in fire cracker sales would mean at least a halving of total sales [taking a base line of 100 INR, -20% CAGR for 5 years leaves us with 32 INR - or a 68% reduction]. In one particular year, 2015-2016, the fall was 25%. Yet, some newspaper reports claimed a 40% increase in pollution on Diwali due to firecrackers. This doesn't compute: how can one cut down firecracker purchase by almost 70%, and still end up with 40% increased smoke? Does it mean that firecrackers are creating 3 times as much smoke as before? While it is "possible", it is not probable. And the jury will be out on this one until specific studies are done. On the other hand, given the first graph we saw - one could argue that majority of the pollution is due to other causes, and Diwali smoke only marginally adds to it. This seems to be the more likely scenario - at least for Delhi.





In summary, pollution during Diwali - of both the air, and noise, are likely marginal, and more importantly, extremely temporary in nature. Vilification of both the festival and those who celebrate it on environmental grounds, seems like vested hatred than anything else.





I shall sign off now - gotta go buy my fire crackers. Happy Diwali.