The transition from rainy to winter season which occurs at around October-November. This is the time when the water accumulated in trenches & shallow topography zones, start getting stagnated. Unlike the high temperature of summer heat, the low temperature of cold winter does not evaporate these stagnant water bodies easily. Hence, they end up becoming breeding ground for insects, especially mosquitoes, which then leads to spread of communicable diseases.
Indians, who were already experts in the Sciences of Yoga and Ayurveda, thousands of years ago itself, had all the relevant knowledge back then, to come up with a solution, which could actually be carried out as a fun-filled tradition so that it could be sustained &passed on across generations.
The major constituent of firework is Gunpowder which in turn comprises of Saltpeter (Potassium Nitrate), Sulfur and Charcoal. While the Sulfur and Charcoal act as fuel to prolong the combustion, it is actually the Saltpeter which gives the characteristic purple flame due to which Fireworks are used not only for military but aesthetic (celebratory) purposes as well.
Historians around the world have confirmed that the knowledge of gunpowder existed in ancient India. With the knowledge of saltpeter, one of the simplest ways of using it to destroy such insects, was to use them in tiny explosives or sparklers, which could release Potassium-Nitrate, which in turn could destroy germs and hence curb dangerous insects. In fact, even today, saltpeter is used as basic ingredient in home-remedy-kits to prepare mosquito-repellents.
Thus it can be deduced that, the Indians, thousands of years ago, came up with such simple yet effective solution of destroying harmful disease-borne-insects and germs, by simply exploding firecrackers (or lighting up as sparklers), in which saltpeter was the basic ingredient, thereby keeping water-borne & communicable diseases at bay.
As there is a “If taken in excess divine nectar also will become poison”, firework in Deepavali has its own limitation to avoid air and noise pollution. Have a happy and safe Deepavali.