In north India a helmet with an air filter matters most from late October to January. In 2026, Diwali on 8 November (Wikipedia) lands inside the forecast 1 to 10 November stubble-fire peak (India.com, 2026). A HEPA-grade filter removes the fine particles, but not the NO₂ and SO₂ from fireworks, so ride timing still matters.
Part of our Helmet air filter guide.
It is Sunday evening, 8 November. The first rockets go up over the flyover, and the traffic below has stopped moving.
Delhi has 5.93 million registered two-wheelers (Economic Survey of Delhi, 2026). On Diwali night in 2024, the layer of air trapping the smoke sat below 100 m, about the height of a 30-storey tower (Bartaria et al., 2026).
This week the headlines said El Niño will make Delhi's winter worse. The newest peer-reviewed study says that part is not settled. The part that is settled is already on the calendar.
Key takeaways
- On Diwali 2024, Delhi's PM2.5 rose 19% to 220.6 µg/m³, and the night-time boundary layer fell below 100 m, against about 1,240 m by day (Bartaria et al., 2026).
- Winter storms called western disturbances explain about 70% of the year-to-year swing in north India's winter PM2.5, but the El Niño-linked ocean signal tends to go with more of that winter rain, not less (Xie et al., 2026).
- With emissions held constant, Delhi's pollution runs 40 to 80% above its annual average from November to January, purely because the air stops moving (Guttikunda and Gurjar, 2012).
The research behind this
Peer-reviewed measurements this post is built on. Each card says what was measured, what was found, and what it cannot tell you.
Western disturbances control winter PM2.5
- What they measured
- About 500 government air monitors across north India (2015 to 2024), 440 winter storms tracked from 2000 to 2024, and how autumn sea temperatures predict winter rain.
- What they found
- These storms explain about 70% of the year-to-year swing in winter PM2.5. One strong storm in January cut PM2.5 by about 30%.
- What it can't tell you
- Only 10 years of PM2.5 records. It covers December to February, not the October to November fire season, and it makes no 2026 to 2027 forecast.
Diwali fireworks under a collapsed lid
- What they measured
- Air quality in 14 Indo-Gangetic cities around Diwali 2024, from ground monitors and satellites, plus the height of the boundary layer that traps pollution.
- What they found
- Delhi PM2.5 rose 19% to 220.6 µg/m³ on 1 November, then reached 233.8 µg/m³ after the festival. The night-time layer fell below 100 m, and SO₂ and NO₂ surged.
- What it can't tell you
- One festival year only. Fixed monitors, not what a person on the road breathed.
The weather alone makes winter worse
- What they measured
- A tracer model of Delhi's air in 2009 with emissions held the same every month, so any change came from the weather alone.
- What they found
- Concentrations ran 40 to 80% above the annual average from November to January, and 10 to 60% below it from May to July.
- What it can't tell you
- One year of weather, modelled, not measured on people. It does not split pollution by source.
Satellites link Delhi's haze to farm fires
- What they measured
- Satellite haze readings over Delhi, fire detections in Punjab and Haryana, and air-parcel tracks, every October and November from 2020 to 2024.
- What they found
- Daily fire counts correlated with Delhi's haze at r = 0.78, and 60% of air tracks from burning fields reached Delhi within 36 hours.
- What it can't tell you
- Satellite haze covers the whole air column, not the air at street level. Ground studies give fires a much smaller share.
Evening commuters breathe in the most
- What they measured
- Delhi air monitoring from 2019 to 2023, fed into a model of how much PM2.5 lands in the lungs by activity and time of day.
- What they found
- PM2.5 peaked at 750.5 µg/m³, and the evening commute dose ran up to 39% higher than the morning commute.
- What it can't tell you
- The dose is modelled from fixed monitors. It covers walking and commuting, not motorcycle riding.
What a Delhi main road adds in NO₂
- What they measured
- NO₂ exposure on 1,229 km of cycling across Delhi roads, and the dose breathed in per hour.
- What they found
- Main roads carried 47 µg/m³ more NO₂ than residential streets. One hour's dose was 403 µg on side streets and 482 µg on main roads.
- What it can't tell you
- Cyclists breathe harder than riders. NO₂ is a gas, which is why it matters here: a particle filter does not stop it.
What the studies actually measured
Six peer-reviewed studies carry this post. None of them put a monitor inside a rider's helmet. Together they cover the weather, the fires, the festival and the road.
The biggest is brand new. Xie et al. (2026), published on 9 September, used about 500 government monitors and tracked 440 winter storms over 24 years. It asked what makes one Delhi winter dirtier than the next.
The festival data comes from Bartaria et al. (2026). They followed 14 cities through Diwali 2024 and measured how high the trapping layer of air sat by day and by night.
The weather effect comes from an older model. Guttikunda and Gurjar (2012) held Delhi's emissions fixed for a whole year and let only the weather change.
The fire link comes from satellites (Kotrike and Sridhar, 2026). The commuter dose comes from five years of Delhi monitoring (Singh et al., 2025). The gas side comes from 1,229 km of cycling on Delhi roads (Apparicio et al., 2021).
Two of them disagree with each other on fires, and one disagrees with this week's news. Both disagreements are in the sections below.
Ride with filtered air
Filters haze PM2.5 on every ride
Easi Breezi mounts on your scooter or motorbike and feeds H11 HEPA-filtered air into your helmet. Bike-powered, no batteries, no charging.
The El Niño headline points the opposite way to the newest study
The news came from two outlets. ThePrint quoted Skymet's Mahesh Palawat saying El Niño weakens the western disturbances, the winter storms that rain Delhi's air clean (ThePrint, 2026).
Daily Jagran quoted SAFAR founder Gufran Beig calling this year's super El Niño a "unique event" and warning of slower winds and stagnant air. It also counted 47 stubble fires across five states between 15 and 23 September (Daily Jagran, 2026).
The first half of that argument holds. Xie et al. (2026) found the storms explain about 70% of the year-to-year swing in winter PM2.5. One strong storm in January cut PM2.5 by about 30%, moving most of Delhi's stations out of the worst band.
The second half does not. In the same paper, the ocean pattern that moves with El Niño correlates positively with winter-storm rain (r = 0.47), and with El Niño itself at r = 0.57 (Xie et al., 2026). The authors call that consistent with wetter Indian winters in El Niño years.
So the mechanism in the headlines runs the other way in the newest peer-reviewed model. The authors also say El Niño's influence on future storms remains uncertain, and they publish no forecast for this winter.
Treat any "worse winter" forecast for December to February as open until the autumn ocean data is in. Plan around November instead, because that part is already fixed.
November is the real risk, and it is on the calendar
Diwali moves every year. It fell on 31 October and 1 November in 2024, on 20 October in 2025, and falls on Sunday 8 November in 2026 (Diwali, Wikipedia).
Stubble fires are expected to rise through October and peak in the first 10 days of November (India.com, 2026). This year the festival lands inside that window.
Diwali against the forecast fire peak
Main Diwali night each year, 15 October to 15 November. Orange band: the 1 to 10 November fire peak forecast for 2026.
| Year | Main Diwali | Inside the forecast 1 to 10 November fire peak? |
|---|---|---|
| 2024 | 31 October to 1 November | 1 November is the first day of the window |
| 2025 | 20 October | No |
| 2026 | 8 November | Yes |
Diwali 2024 showed what the festival does on its own. Delhi's PM2.5 rose 19% to 220.6 µg/m³ on 1 November and reached 233.8 µg/m³ in the days after (Bartaria et al., 2026).
The same study found 80% of the 14 cities moved into severe or hazardous health categories. The authors say crop burning alone cannot explain the sharp night-time peaks (Bartaria et al., 2026).
Farm fires drive the worst days, but not the everyday baseline
Stubble burning has fallen hard. In 2025, Punjab logged 5,114 fires between 15 September and 30 November, down 93% on 2021 (ThePrint, 2026). Yet Delhi's annual PM2.5 still averaged 96 µg/m³ in 2025, and broke the daily standard on 212 days (CREA via Millennium Post, 2026).
The two big evidence lines on fires do not agree. From space, daily fire counts correlated with Delhi's haze at r = 0.78. Once fires were active, the chance of a very hazy day rose five times (Kotrike and Sridhar, 2026).
At street level, the share is small most of the time. In October and November 2025, farm fires made up about 4.2% of Delhi's PM2.5 on average, peaking briefly above 22% in mid-November (CSE, 2025).
The fair reading is that both are right about different things. Fires push Delhi into its worst episodes. Local traffic, industry and the weather set the high baseline the rest of the year.
For context, the WHO's 24-hour guideline for PM2.5 is 15 µg/m³ and its annual guideline is 5 (WHO, 2021). Delhi's ordinary year sits far above both, before a single rocket goes up.
Delhi's winter is a ventilation failure, and so is your helmet's
The engineering point is simple. When air stops moving, pollution stays where it was made. That is true for a city and for the space inside your helmet.
Start with the city. With emissions held identical every month, Delhi's modelled pollution ran 40 to 80% above the annual average from November to January (Guttikunda and Gurjar, 2012). Nothing extra was burned. The air simply stopped carrying it away.
At night the lid comes down. On Diwali 2024 the boundary layer over Delhi fell below 100 m at night, against about 1,240 m in the afternoon, when daytime mixing cut PM2.5 by 20 to 30% (Bartaria et al., 2026).
As a rough picture from those two heights (our arithmetic), the same smoke is packed into a layer about 12 times thinner.
The lid over Delhi, and the lid over your face
Left: by day the mixing layer is tall and smoke spreads out. Right: at night it collapses, and a stopped rider's vents move no air.
Now the helmet. Passive vents have no fan. They rely on ram air, and the push behind it rises with the square of road speed (NASA Glenn). Halve your speed and the push falls to a quarter. Stop, and it is zero.
So in a Diwali-night jam, the vents move almost no air. What little comes in is unfiltered street air, pulled from the layer the lid has just squashed down onto the road.
Timing adds to it. Evening commuters in Delhi took in up to 39% more PM2.5 than on the morning commute, and peak readings reached 750.5 µg/m³ (Singh et al., 2025). Fireworks are an evening event.
A HEPA filter stops the particles, not the firework gases
This is the limit every Indian buyer should hear before spending anything. A HEPA-grade filter is a particle trap. It catches PM2.5 well. It does not catch gases such as NO₂ and SO₂, which pass through fibre media.
Diwali makes both. Bartaria's team recorded SO₂ and NO₂ surging over the festival, on top of the particles (Bartaria et al., 2026).
Traffic makes NO₂ every day. On Delhi's main roads, cyclists met 47 µg/m³ more NO₂ than on residential streets. One hour's dose was 482 µg on main roads against 403 µg on side streets (Apparicio et al., 2021).
So a filter handles the part of Diwali air that is smoke and dust. For the gases, the only proven controls are distance and timing: side streets over main roads, and earlier rides over late ones.
Your ride's PM2.5 dose, with and without a filter
Pick the kind of day, then drag to your ride time. Illustrative particles-only estimate.
On an average 2025 day at 96 µg/m³, a 45-minute ride breathes in about 108 µg of PM2.5 unfiltered, or about 5.4 µg through a filter that stops 95%.
| Kind of day | PM2.5 (µg/m³) | Unfiltered dose | Filtered dose (95%) |
|---|---|---|---|
| Average 2025 day | 96 | 108 µg | 5.4 µg |
| Diwali 2024 | 220.6 | 248 µg | 12.4 µg |
| After Diwali 2024 | 233.8 | 263 µg | 13.2 µg |
By 2031, even a cleaner Delhi winter stays far above the WHO line
We projected a Delhi rider's winter to 2031. This is our synthesis, not a published forecast, so here is the working.
- Base: Delhi's 2025 annual PM2.5 of 96 µg/m³ (CREA, 2026).
- Winter factor: 1.4 to 1.8 times the annual level, from the weather-only model (Guttikunda and Gurjar, 2012).
- Flat case: no improvement gives a winter of 134 to 173 µg/m³.
- Optimistic case: if annual PM2.5 falls 3% every year from 2025, it reaches about 80 µg/m³ by 2031. Winter is then 112 to 144 µg/m³. The 3% is our assumption, not a government target.
Our projected 2031 Delhi winter PM2.5 even if the city's air improves 3% a year, against the WHO 24-hour guideline of 15 µg/m³.
Synthesised from CREA 2026, Guttikunda and Gurjar 2012 and WHO 2021The riders are growing too. Delhi had 5,927,775 two-wheelers in March 2026, and its whole vehicle fleet grew 7.93% in a year (Economic Survey of Delhi, 2026).
If two-wheelers keep that pace, Delhi reaches about 8.7 million of them by 2031. That growth rate is for all vehicles, so treat 8.7 million as a rough guide, not a count.
Put together: even with steady progress, a Delhi rider's winter commute in 2031 still runs at roughly 7 to 10 times the WHO daily guideline, for millions more people. We cover the everyday Delhi baseline in Delhi motorcycle air pollution.
What a clip-on filter changes on a stopped Delhi road
The problem in a jam is that the helmet's only airflow depends on speed and comes in unfiltered. The Easi Breezi unit runs off the bike's own power, with no battery, and pushes air through an H11 HEPA filter that stops 95% of PM2.5. It enters the helmet under the chin bar.
Because a fan moves the air, it keeps working at zero road speed, which is exactly when vents stop. It fits the helmet you already own. It filters particles, not gases, so the timing advice below still applies. For the Bengaluru side of the same question, see a helmet with an air purifier in India.
Frequently asked questions
Is a helmet with an air filter worth it in India?
For riders in north Indian cities, the case is strongest from late October to January. Delhi averaged 96 µg/m³ of PM2.5 in 2025 and broke the daily standard on 212 days (CREA, 2026). A filter cuts the particle part of that exposure.
Does a HEPA helmet filter stop Diwali firecracker smoke?
It stops the particles in the smoke. It does not stop gases such as SO₂ and NO₂, which surged over Diwali 2024 (Bartaria et al., 2026). For those, avoid main roads and late-evening rides.
When is Delhi's air worst for riders?
November to January, and especially evenings. The weather alone pushes winter pollution 40 to 80% above average (Guttikunda and Gurjar, 2012), and evening commutes carry up to 39% more dose than mornings (Singh et al., 2025).
Will El Niño make Delhi's winter worse?
Not proven. The press argument is that El Niño weakens winter storms, but the newest study finds the El Niño-linked ocean signal goes with more winter-storm rain (Xie et al., 2026). Treat December to February as an open question.
Can I use a clip-on filter with my existing helmet?
Yes. A clip-on unit feeds filtered air into the helmet you already wear through a tube under the chin bar. For the general questions, read our guide to a helmet with an air filter.
What to do between 1 and 11 November
- Move rides out of the evening on 7 to 9 November. Evening commutes carried up to 39% more PM2.5 dose than mornings in Delhi (Singh et al., 2025).
- Check the nearest government monitor before you leave. Look at the night-time readings, because that is when the lid is lowest.
- Take side streets where you can. Main roads carried 47 µg/m³ more NO₂, a gas no particle filter stops (Apparicio et al., 2021).
- Keep the visor shut in stopped traffic. An open visor lets unfiltered air straight in when the vents are doing nothing.
- Filter the air your helmet breathes. The Easi Breezi unit feeds H11 HEPA-filtered air at any speed, including at a standstill.
The El Niño part of this winter is still unknown. The Diwali part is fixed, so plan for it now.
Sources
- Economic Survey of Delhi 2025-26, via The Pioneer (2026). Delhi's vehicle count hits 87.6 lakh. 24 March 2026. dailypioneer.com
- Bartaria V, Kumar R, Kumar K, Jangid A (2026). Firecracker-induced air pollution and health burden during Diwali: ground and satellite assessment across 14 cities of the Indo-Gangetic Basin. Scientific Reports 16:28665. doi:10.1038/s41598-026-54531-y
- Xie Y, Hunt KMR, Gupta R, Goswami G, Zhou M, Dong W, Mauzerall DL (2026). Seasonal predictability of winter PM2.5 pollution severity in India through a strong pollution-extratropical storm connection. Science Advances 12:eaee4549. doi:10.1126/sciadv.aee4549
- Guttikunda SK, Gurjar BR (2012). Role of meteorology in seasonality of air pollution in megacity Delhi, India. Environmental Monitoring and Assessment 184:3199. doi:10.1007/s10661-011-2182-8
- Kotrike T, Sridhar V (2026). Tracing the haze: satellite-based assessment of stubble burning and air quality in Delhi. Air Quality, Atmosphere & Health 19:9. doi:10.1007/s11869-026-01885-x
- Singh AK, Pathak AK, Saini G (2025). Respiratory deposition of particulate matter in Delhi: a five-year assessment of exposure patterns and health risks. Scientific Reports 15:42601. doi:10.1038/s41598-025-26663-0
- Apparicio P, Gelb J, Jarry V, Lesage-Mann É (2021). Cycling in one of the most polluted cities in the world: exposure to noise and air pollution and potential adverse health impacts in Delhi. International Journal of Health Geographics 20:18. doi:10.1186/s12942-021-00272-2
- ThePrint (2026). El Niño and Delhi's winter pollution. Soumya Pillai, 25 September 2026. theprint.in
- Daily Jagran (2026). Delhi might see mild winter, worsened air quality this season. 24 September 2026. thedailyjagran.com
- Diwali. Wikipedia, festival dates. en.wikipedia.org
- India.com (2026). Winter warning: stubble burning expected to peak in the first 10 days of November. 24 September 2026. india.com
- Centre for Research on Energy and Clean Air (CREA), via Millennium Post (2026). Delhi tops PM10, second worst for PM2.5 in 2025. 10 January 2026. millenniumpost.in
- Centre for Science and Environment (2025). Beyond the burn: Delhi's winter smog intensifies even after stubble fires fade. 31 December 2025. cseindia.org
- World Health Organization (2021). WHO global air quality guidelines. who.int
- NASA Glenn Research Center. Dynamic pressure. grc.nasa.gov
Written by Ash, mechanical engineer and founder of Easi Breezi, building an active HEPA filtration system for motorcycle helmets (patent pending). Based between Hong Kong and Bali, riding daily in the traffic this blog writes about.