Delhi's pollution year, month by month
Why winter is worst, when stubble smoke and Diwali hit, and a month-by-month checklist so you're ready before the season starts.
Delhi’s air is polluted all year, but how bad it gets depends heavily on the weather and the season.
Why winter is the worst
- Temperature inversions: on cold, still nights a layer of warm air sits above cooler air at the ground and acts like a lid. Pollution from traffic, industry and burning gets trapped close to where people breathe.
- Low wind and a shallow mixing layer mean pollution isn’t blown away or diluted upwards.
- Extra burning: crop stubble in neighbouring states (mainly October–November), firecrackers around Diwali, and biomass burned for heating.
- Fog and humidity help particles grow and linger.
The year at a glance
| Months | Typical air | Main drivers | What to do |
|---|---|---|---|
| July–September | Cleanest of the year | Monsoon rain washes out particles | Best time to buy and test purifiers, service ACs, fix seals |
| Late September | Starting to worsen | Monsoon withdraws, winds drop | Replace purifier filters and car cabin filter; stock respirators; see the doctor for action plans |
| October | Poor, often Very Poor by the end | Stubble burning begins, cooler nights | Clean room ready; purifiers running continuously from mid-month |
| Diwali (Oct/Nov) | Sharp spikes, often Severe | Firecrackers on top of everything else | Windows shut, purifiers on high, sensitive people indoors that night and the next day |
| November | Very Poor to Severe; often the worst month | Stubble smoke + inversions | Full protection; GRAP stages likely in force |
| December–January | Very Poor, Severe episodes | Strong inversions, fog, heating | Full protection; watch for winter infections in older adults |
| February | Improving, still Poor at times | Warmer days, more wind | Keep the clean room going on bad days |
| March–June | Moderate to Poor, dusty | Dust storms (more PM10), summer ozone in afternoons | Purifiers on dusty days; avoid exercising in hot afternoons |
These are typical patterns. The worst month varies by year: November in some winters (stubble smoke, Diwali and the first inversions together), December or January in others (deepest inversions and heating). Wheat-residue burning in April–May has a smaller effect because spring weather disperses smoke better.
Daily pattern in winter
Particle levels are often highest late evening through early morning: on winter nights the mixing layer can drop below 100 metres. Levels are lower in the afternoon, when the ground warms and air mixes higher. India’s health ministry advises ventilating only between 12 noon and 4 pm on bad days, and avoiding outdoor exercise early morning and late evening.
Set a reminder
Put a note in your calendar for mid-September: check filters, order spares and respirators, and book doctor visits for vulnerable family members before the season starts.