Delhi NCR air quality: what PM2.5 does, and how to get clean air at home

Every year from about the second week of October, the air across Delhi NCR becomes the thing everyone talks about and almost nobody acts on usefully. Part of the reason is that the public conversation is about outdoor air, which no household can change, and the useful conversation is about indoor air, which a household can change substantially and fairly cheaply.

This article is the second one. We rent air purifiers, so we have an interest here and you should read it knowing that. We have tried to keep it to what is measurable, and the longest section is about the things that do not work, including two that get sold as air purifiers.

What PM2.5 is, and why it is the only number worth watching

PM2.5 means particulate matter smaller than 2.5 micrometres across, roughly a thirtieth the width of a human hair. Size is the whole story. Larger particles get caught in the nose and upper airway and are cleared. PM2.5 is small enough to reach the deepest part of the lung, and a fraction of it crosses into the bloodstream from there. That is why the health literature links it not just to asthma and bronchitis but to heart attacks, strokes and adverse pregnancy outcomes — it is a cardiovascular exposure, not only a respiratory irritant.

The World Health Organization tightened its air quality guideline in 2021 to 5 µg/m³ for PM2.5 as an annual mean, and 15 µg/m³ as a 24-hour mean. India's National Ambient Air Quality Standard is considerably looser at 40 µg/m³ annually. Delhi NCR does not reliably meet either. Through the winter peak, daily readings in the hundreds are ordinary rather than exceptional, and the AQI figure quoted on the news is usually driven by the PM2.5 sub-index.

Why it happens on a schedule

The seasonality confuses people into thinking Delhi's air is fine for eight months. It is not; it is merely survivable. There is a baseline of local emissions running all year — vehicle exhaust, construction and road dust, industrial output, waste burning, generator sets — and then two seasonal factors stack on top of it.

1. 1Stubble burning. From early-to-mid October, post-harvest crop residue is burned across Punjab and Haryana, and the prevailing north-westerlies carry the smoke into the NCR basin. This is what makes October worse than September, almost overnight.
2. 2The winter inversion. As nights get colder, a layer of cold air settles at ground level under warmer air above it. The mixing height — the vertical space pollution can disperse into — collapses from a couple of kilometres to a few hundred metres. The same quantity of emissions now occupies a fraction of the volume.

The inversion is the bigger factor and the less discussed one. It is also why November is consistently worse than October even though stubble burning tapers: the smoke stops arriving, but everything Delhi emits itself stops dispersing. And it is why the air is at its worst around dawn, when the inversion is strongest, rather than at rush hour.

What actually works indoors

You cannot filter a city. You can filter a room, and the results in a sealed room are genuinely dramatic — this is one of the few interventions in public health where a household can move its own exposure by an order of magnitude in half an hour. Three things do the work, in this order.

1. Seal the room. This is free and it is half the result

A purifier fighting an open window is a fan with a filter in it. Close the windows and the door of the room you are treating. Deal with the gaps: the strip under the door, unsealed window frames, the unused exhaust fan opening, the gap around a window air-conditioner. Draught excluders and tape cost a few hundred rupees and improve the result more than a larger machine would. If you do nothing else this winter, do this in your bedroom.

2. An H13 True HEPA filter, sized to the room

H13 is a grade rather than a marketing adjective: it means at least 99.95% capture at the most-penetrating particle size, around 0.3 microns, which brackets PM2.5. Behind it, activated carbon handles smoke odour and volatile organics — that is the difference between air that tests clean and air that smells clean during a stubble-burning week. A pre-filter in front catches coarse dust so the expensive stage is not wasted on it.

One unit handles a bedroom or study up to roughly 350 square feet with the door shut. It will not clean a flat. Run it continuously on a low setting rather than on maximum for two hours: steady removal beats bursts, and low is quiet enough to sleep through. We go into sizing, placement and what a season costs in renting an air purifier for the Delhi NCR smog season.

3. Stop adding particles indoors

This is the step everyone skips and it can dominate everything else. Indian cooking generates serious PM2.5 — tadka, deep frying and tawa cooking can push a kitchen reading past a bad outdoor day, and in most NCR flats that air reaches the bedrooms within minutes. Use the chimney or extractor every single time, not just for the obviously smoky dishes, and keep the kitchen door shut while cooking. Then: no incense or dhoop in a sealed room, no candles, no smoking indoors, and no mosquito coils, which are among the worst indoor sources there are.

Four things that do not work, including two sold as purifiers

We get asked about all four of these regularly, and telling people the truth about them costs us nothing, because none of them is a product we sell.

• Houseplants. The famous NASA study was conducted in sealed laboratory chambers and concerned volatile organic compounds, not PM2.5. Reproducing that air-exchange rate in a real room would take something on the order of a plant per square foot. Keep the plants because you like them.
• Ionisers and "plasma" units. These charge particles so they stick to surfaces rather than removing them, so your readings can improve while the particles are simply on your walls and floor, available to be resuspended. Many also generate ozone, itself a respiratory irritant. Avoid, particularly in a bedroom.
• Water-based "air washers" and bowls of water. Water captures large dust well and PM2.5 essentially not at all. Humidity is a separate question from particulates and treating them as one thing is how this product sells.
• A cloth or surgical mask indoors. Neither seals to the face nor filters at PM2.5 grade. A well-fitted N95 works outdoors, on a bad day, for the time you are exposed. Indoors, filtering the room is the better answer.

Who should act earlier than everyone else

The harm from PM2.5 is not distributed evenly, and some households should treat mid-October as a deadline rather than a suggestion: homes with infants and young children, whose lungs are still developing and who breathe faster relative to body mass; anyone with asthma, COPD or another chronic respiratory condition; people with existing cardiovascular disease, given the bloodstream pathway; pregnant women; and older adults. Ground-floor and roadside flats also start from a worse baseline than higher floors set back from traffic.

If someone in the house is in one of those groups, the bedroom purifier stops being a comfort purchase. None of this is medical advice and we are not qualified to give any — if there is a diagnosed respiratory or cardiac condition in the house, the person to plan the winter with is their doctor.

When to open the windows, because you still should

A permanently sealed room accumulates carbon dioxide, moisture and indoor VOCs, and gets stuffy in a way that is its own problem. The answer is not to stop ventilating but to time it. Avoid dawn and late evening, when the inversion is strongest and readings peak. Early afternoon is usually the least-bad window, when the sun has lifted the mixing height. Open up for ten to fifteen minutes, then close and let the purifier recover the room — on a sized unit that takes twenty minutes or so.

Check a live reading for your own locality rather than a city-wide AQI before deciding. The spread across NCR on a single afternoon can be a factor of two, and a monitor ten kilometres away is not describing your street.

One related point, while you are sealing the house

The other Delhi NCR exposure worth measuring is the water, and it is easier to fix. Groundwater across much of the region tests between 800 and 2000 TDS against an acceptable limit of 500, and unlike the air it is a solved problem at household scale. We cover the numbers in what TDS your drinking water should be, and we will test your tap free whether or not you rent anything from us.