Diwali and the Air: What Fireworks Really Release
A cracker lasts a second, but what it throws into the air stays much longer. Here is the chemistry of the bang, the colours and the smoke, and what measurements in Delhi show after the festival.
By Saurabh Dhakad, PhD scholar, Atmospheric Chemistry and Climate Change Lab, School of Environmental Sciences, Jawaharlal Nehru University · · 4 min read
Diwali is two days away, on Sunday 8 November. The diyas are bought and the air over Delhi has already turned ‘very poor’. The early warning system for Delhi expects it to slip into ‘severe’ from 6 to 8 November. Stubble smoke, traffic and still winter air are all part of this. But for a few hours on Diwali night, fireworks become the biggest single source. So what exactly goes up when a cracker bursts?
What is inside a cracker
Most crackers start with black powder, an old mix of about 75% potassium nitrate, 15% charcoal and 10% sulphur. The nitrate is the oxidiser. It supplies oxygen, so the charcoal and sulphur can burn very fast even inside a closed paper tube. A simple textbook form of the reaction is:
2KNO3 + S + 3C → K2S + N2 + 3CO2
In words, a small solid turns into a lot of hot gas in a fraction of a second, and that sudden push is the bang. Real crackers are never this clean. Some of the sulphur leaves as SO2, and the potassium leaves as fine salt particles. Many crackers also use potassium perchlorate (KClO4) as a stronger oxidiser.
The colours come from metal salts. Each metal glows with its own colour when it is heated:
- Strontium gives red.
- Barium gives green.
- Copper gives blue.
- Aluminium, magnesium and titanium give white sparks, and iron gives gold.
What the measurements show
These metals are rare in normal city air, so they work like fingerprints. Researchers in Delhi used an automatic metal monitor to follow PM2.5 over three Diwalis, from 2019 to 2021. Potassium, aluminium, strontium, barium and sulphur showed sharp peaks on Diwali night. Together, the measured elements reached 181 µg/m³ as a 12-hour average in 2019, 117 in 2020 and 151 in 2021. More than 85% of this elemental mass came from firecrackers. An older Delhi study had found barium and potassium in PM10 about 264 times higher than on normal days.
The total particle load jumps too. An analysis of CPCB data by Climate Trends found that Delhi’s average PM2.5 went from 156.6 µg/m³ before Diwali 2025 to about 488 µg/m³ after it, roughly three times higher. The worst hours were between 10 PM and 1 AM.
Why metals and perchlorate matter
Most of these particles are small enough to reach deep into our lungs, and barium or strontium salts are not things our bodies need. Perchlorate is a quieter worry. The US health agency ATSDR notes that it affects the thyroid gland’s ability to take up iodine, which the body needs to make thyroid hormones. Perchlorate also dissolves easily and travels in water. In Ahmedabad, a 2020 study found 65 µg/L of perchlorate in the Sabarmati river one day before Diwali, and pointed mainly to runoff from firework making, not only bursting.
Cold, calm November nights make it all worse. In the Delhi study, the air’s ability to spread smoke on Diwali night was lowest in 2021, so the same smoke stayed packed close to the ground.
What this means
None of this asks us to give up the joy of the festival. It asks us to choose how we celebrate. CSIR-NEERI’s green crackers leave out barium nitrate and are reported to cut emissions by about 30%. That helps, but less smoke is still smoke. Fewer crackers, one shared colony display instead of every lane bursting its own, and more diyas than bombs will all lower the night-time peak. Children, older people and anyone with asthma can stay indoors with windows shut between 10 PM and 1 AM. And for those of us who study the air, measuring metals and perchlorate in air, rain and lakes after every Diwali is the best way to know whether these choices are working.
References and further readings
- Faisal, M., Ali, U., Kumar, A., Hazarika, N., Singh, V. and Kumar, M. (2024). Festive fireworks in Delhi: A major source of elemental aerosols established through dispersion normalized PMF in a multiyear study. Atmospheric Environment, 323, 120394.
- Kumar, M. (2020). Runoff from firework manufacturing as major perchlorate source in the surface waters around Diwali in Ahmedabad, India. Journal of Environmental Management, 273, 111091.
- ATSDR (US Agency for Toxic Substances and Disease Registry). ToxFAQs: Perchlorates.
- Smith, P. E. (2021). How do fireworks work? A pyrotechnics chemist explains the science behind the brilliant colors and sounds. The Conversation.
- Carbon Copy (28 Oct 2025). Firecrackers caused pollution to spike three-fold this Diwali: report (Climate Trends analysis of CPCB data).
- Business Standard/PTI (5 Nov 2026). Delhi AQI may turn severe on Diwali, worsen further after festival: experts. See also The Hitavada (31 Oct 2019), CSIR-NEERI develops firecrackers that reduce air pollution by 30%.