Protect Home From Winter Smog India

Why Winter Air Is Different

Every year from around October, air quality across the Indo-Gangetic plain deteriorates
sharply, and every year the same advice circulates: stay indoors. That advice is incomplete in a
way that matters, because indoors is not automatically safe.

Three things change in winter, and they compound:

• The air stops mixing. In summer, warm air rises and carries pollution
  upward. In winter, cold air settles near the ground and traps everything in it — the same
  emissions produce far higher concentrations.
• Sources add up. Crop residue burning, vehicle exhaust, construction dust,
  industry and biomass burning for warmth all land in the same stagnant layer.
• Homes close up. Windows stay shut, which helps — but only where the
  building is actually sealed, which most Indian homes are not.

What Happens Inside Your Home

Outdoor particulate does not stop at the wall. It enters continuously through gaps around
window frames, under doors, through unused exhaust vents and service ducts, and through the
ordinary opening and closing of doors. A closed window in an unsealed frame is a filter with
holes in it.

Meanwhile your home generates its own load. Cooking is a significant source — high-heat oil
techniques central to Indian cooking aerosolise fine particulate directly into the kitchen.
Incense and candles are combustion. So is smoking indoors.

The practical implication is that on a bad day, indoor levels in an unprepared home usually
sit somewhere below outdoor levels but nowhere near clean — and after cooking, the kitchen can
briefly be the worst air in the building.

Step 1 — Seal, Before You Buy Anything

This is the least glamorous intervention and reliably the highest return per rupee. If
outdoor infiltration is your dominant source — and in most urban Indian homes it is — then
reducing the inflow does more than filtering after the fact.

• Weather-strip window frames, starting with the ones facing a road. Foam
  or rubber adhesive strip, available at any hardware shop.
• Fit a brush or rubber seal to door bottoms, especially the main door. The
  gap under a door is often the single largest opening in a home.
• Block unused exhaust vents. Old kitchen or bathroom vents that no longer
  have a fan are open pipes to outside.
• Check the service duct. In apartments, the gap around plumbing risers is
  a common and completely unnoticed pathway.

Step 2 — Ventilate at the Right Hours

Sealing a home completely creates a different problem: carbon dioxide, humidity and indoor
pollutants accumulate with nowhere to go. Filtration does not fix that — an air purifier does
not add oxygen or remove CO2. You still need fresh air.

The refinement is when. Pollution concentrates near the ground when air is cool and
still — worst overnight, in the early morning, and again after sunset when the ground cools.
The air usually mixes upward and improves through the middle of the day.

So ventilate in the afternoon, briefly and with purpose: open windows on
opposite sides for cross-flow, ten to fifteen minutes, then close and re-seal. On a severe day
there may be no good hour, and the trade shifts toward keeping it shut.

Step 3 — Filter the Rooms You Actually Use

Filtration is third on this list, not because it does not work, but because it works on a
smaller scale than people expect. An air purifier cleans one closed room. It
does not treat a flat, and it cannot outrun an open window.

Two rules decide whether it does anything:

• Size it for the room. Manufacturers quote a coverage area calculated at a
  low air-change rate. A practical adjustment: treat the printed area as roughly double the room
  it genuinely handles in a polluted winter.
• Put it where you spend time. The bedroom first — that is eight hours of
  continuous exposure and the easiest room in the house to keep closed.

What to Do on a Severe Day

When AQI runs past 400, the ordinary routine changes:

• Keep everything shut. Skip the afternoon ventilation window.
• No outdoor exercise. Exertion multiplies how much air you draw in;
  a morning run on a severe day is the worst possible combination.
• Add no indoor sources. No incense, no candles, no indoor smoking. Use the
  kitchen extractor for every meal, switched on before the oil goes in.
• Run filtration continuously in the occupied rooms, doors closed.
• If you must go out, wear a fitted N95. Cloth and surgical masks do very
  little for particles this size, and fit matters as much as the rating.

The September Checklist

Everything here is easier and cheaper before the season starts than during it.

Do this in September	Why now
Weather-strip windows and doors	Highest return, lowest cost. Hardware shops sell out in November
Service the kitchen chimney	A grease-clogged filter barely extracts. Cooking is your indoor source
Replace filters in any purifier you own	A saturated filter entering peak season is furniture
Run the window-sill test	Tells you whether to spend on sealing or filtration
Buy the N95s you’ll need	Availability and price both worsen once the season is reported

Frequently Asked Questions

How do I protect my home from Delhi’s winter pollution?

In order of return: seal the gaps around windows and doors so outdoor air stops leaking in continuously; time your ventilation for the afternoon rather than early morning or evening; and run correctly sized HEPA filtration in the one or two rooms you actually occupy, with those rooms closed. Sealing costs the least and does the most, which is why it comes first.

What should I do when AQI is 400 or above?

Treat it as a stay-indoors day. Keep windows and doors shut, skip outdoor exercise entirely, run filtration in the rooms you occupy, and avoid adding indoor sources — no incense, no candles, and use the kitchen extractor for every meal. If you must go out, a properly fitted N95 does more than a cloth or surgical mask, which do very little for PM2.5.

Does keeping windows closed actually help with smog?

Yes, substantially — but only if the gaps are sealed too. A closed window with an unsealed frame still admits outdoor air around the edges continuously. Weather-stripping the frames and fitting a brush seal at door bottoms is usually the single highest-return intervention in an Indian home, and it costs a few hundred rupees.

When is the best time to open windows in winter in Delhi?

Generally early to mid afternoon. Pollution concentrates near ground level when air is cool and still, which is worst overnight and in the early morning, and again after sunset. The afternoon, when the air has warmed and mixed upward, is usually the cleanest window of the day — though on a severe day there may not be a good hour at all.

Is indoor air cleaner than outdoor air in winter?

Not automatically. Outdoor particulate infiltrates continuously through gaps, and homes add their own sources — cooking, incense, candles, smoking. Without sealing and filtration, indoor levels in a Delhi winter tend to track outdoor levels at a somewhat lower value, not at a safe one.