Haze outside and a stuffy room inside: when should you open the windows?

First, check what your equipment actually does

A portable particle-filtering purifier recirculates air through a filter. A standing fan moves air. Neither arrangement, by itself, establishes an outdoor-air supply. A split aircon may cool a room without providing fresh air; check whether your particular system has a separate ventilation provision.

NEA's updated building-ventilation guidance distinguishes split-unit spaces without fresh-air supply and says air cleaning does not replace ventilation. Most of that document is for non-residential premises, so its detailed building-system prescriptions should not become a DIY instruction for your flat.

Next, look up the purifier's display in its manual. Does the number mean PM2.5, a manufacturer's air-quality score, or something else? A PM-only sensor does not measure CO2. Its green or blue light cannot certify the whole room's air quality. The US EPA's consumer-monitor guidance explains that sensors detect only the pollutants they are designed for, and colour alerts depend on the manufacturer.

Check the outdoor reading without mixing up the numbers

On NEA's site or myENV, check your reporting region and the reading's time.

• The one-hour PM2.5 concentration reflects recent outdoor fine-particle conditions.
• The 24-hour PSI is an index based on a longer averaging period and has an accompanying health advisory.
• Your purifier's indoor reading comes from another location and instrument. Its colour band is not NEA's band.

NEA explains the measures and their uses. Its one-hour activity guide is for immediate outdoor decisions; it does not supply a universal numerical window-opening threshold. Smell and visibility alone are also unreliable substitutes for the official readings.

Three situations at home

These are illustrative decisions, not measurements from a tested flat.

The purifier looks fine, but the occupied room feels stuffy

Check the display's meaning before accepting its colour as reassurance. If it measures particles, it has not answered the ventilation question. Feeling stuffy does not establish a CO2 level either.

Review outdoor conditions and the room's ventilation arrangements. Turning up a particle purifier is not a substitute for resolving a lack of air exchange. If the problem keeps recurring, discuss the actual setup with your landlord, building manager or a qualified ventilation professional.

One-hour PM2.5 has fallen, but the daily PSI is still elevated

The two readings can move differently because they cover different periods. Keep the health advisory in view, but don't wait for both displays to show an identical number or colour.

NEA's household approach is to ventilate when outdoor air improves. Reassess as conditions change, and close openings again if outdoor air worsens. The guidance does not establish a guaranteed-safe number of minutes or a rule that works for every flat.

There is no aircon, and closing the windows makes the room too hot

A low particle reading does not resolve the heat problem. NEA allows fans or aircon for circulation and cooling, but that is not a promise that a fan will make every closed room tolerable.

Avoid treating “keep everything shut” as an instruction to remain in an overheating space. Consider whether another suitable indoor space is available, check current official advice, and seek help with a persistent ventilation or cooling problem. There is no universal window schedule that resolves this trade-off for everyone.

Keep the home setup within its safety requirements

Follow appliance instructions and preserve ventilation required by the building or fuel-burning appliances; do not tape over permanent vents to chase a lower purifier number. If you don't know what an opening serves, ask before blocking it.

Reduce avoidable indoor particle sources, including smoking and candles, as NEA's household tips advise. For recurring problems, get the ventilation arrangements assessed rather than improvising changes to air intakes or exhausts.

The useful check is simple: what is happening outdoors, what does the indoor device actually measure, and how is the occupied room being ventilated? Keep all three in view. For personal health precautions, follow the current official advisory and advice from your clinician.

About this guide

Sources checked on 8 October 2026. This is a source-based explanation for Singapore households, not an indoor-air assessment, medical recommendation or purifier test. Everyday Sift has not measured the example homes or tested equipment for this guide.