Wet-Cleaning for Settled Dust, Closing Windows During Haze, Air Purifier CADR Sizing
5 min read
For a Singapore flat during unhealthy haze, the one cleaning method the National Environment Agency names for particles is wet-cleaning — mopping or wiping — to remove settled dust. Ventilation and filtration carry the rest of the load: close doors and windows while the outdoor air is unacceptable, re-open them when it improves, and shut the outdoor intake on a window-unit air-conditioner. [1]
This is a live decision. The haze portal's 5 October update reported that some air quality stations in Singapore were reading Unhealthy to Hazardous levels, with dry weather expected to continue over most parts of the southern ASEAN region and the hotspot and smoke haze situation expected to remain escalated, with a continued risk of transboundary haze. [2]
Which reading you are acting on
NEA's haze portal separates two numbers that answer different questions. The 1-hour PM2.5 concentration indicates current air quality, and the public is advised to refer to the latest 1-hour readings when planning activities within the next few hours. The 24-hour PSI is the planning number for tomorrow: NEA notes that scientific studies on the health effects of particulate matter are based on a 24-hour exposure, and advises using the 24-hour PSI forecast and the corresponding health advisory to plan ahead, such as work or school tomorrow. That forecast is only available during the haze season. [3]
On the PSI scale, 0-50 is Good, 51-100 Moderate, 101-200 Unhealthy, 201-300 Very Unhealthy and above 300 Hazardous. If the question is what to do with the flat tonight, the current 1-hour PM2.5 figure is the relevant one, because the sealing decision depends on the air outside right now rather than on tomorrow's forecast. Checking it before you start tidying takes a moment. [2] [3]
Wet-cleaning for settled dust, and what plants are not
NEA's practical tips name wet-cleaning methods, such as mopping or wiping, as a way to remove settled dust. If your evening routine is up for review, that is the method to use for dust that has already settled. The document carrying this advice is dated 2014, so read it as long-standing guidance rather than a fresh alert. It does not discuss dry-dusting, and so offers no verdict on that habit either way. [1]
The other idea needs care with its evidence. In a 2 October Straits Times column, National Parks Board-certified horticulturist Wilson Wong traced the belief that plants double as air purifiers to the 1989 NASA Clean Air Study, which looked at sealed environments such as space stations. He noted that many such studies ran in sealed enclosures, unlike a home, where door gaps and the opening and closing of doors let outdoor pollutants in. His conclusion: houseplants are simply not effective at filtering the air in a room during the haze, and dedicated HEPA-filter purifiers are the better reliance. Wong still gave plants a task indoors — wiping or washing leaves with a damp cloth to clear accumulated grime and unblock their pores — which is plant care, not particle control. [4]
Closing windows during haze, and the window unit
NEA states both sides of the sealing trade-off. When the outdoor air quality becomes unacceptable, closing doors and windows helps reduce the rate at which haze particles enter the home. But an enclosed indoor environment may lead to an accumulation of heat, carbon dioxide or other pollutants, especially in highly occupied spaces. That is why the same guidance says to re-open windows and doors when the outdoor air quality improves, and to do so periodically rather than leaving a sealed room shut indefinitely. [1]
One instruction in the same list concerns the air-conditioner: fans or air-conditioners may be used for air circulation and cooling, but if the unit draws unfiltered air from outside, as window units do, close the outdoor air intake openings. A window unit left drawing in outdoor air works against the sealing, so treat its intake as part of the same check as the windows themselves. [1]
Air purifier CADR sizing for a sealed room
Purifiers work on the assumption that the room is shut. NEA warns that they may not work at their specified performance ratings if the doors and windows of the room are not closed, and that the number and size of units also depend on the effectiveness of the purifiers used, airflow obstructions and the leakiness of the room. Leave the unit's air flow unobstructed, and check that it preferably has an effective filter, such as a HEPA filter, to remove fine particles. [1]
Size is arithmetic. NEA's rule is that the smoke CADR should be at least three times the room volume in cubic metres, or one-twelfth of the volume in cubic feet. In NEA's own worked example, a 40 square metre room with a 3 metre height and a volume of 120 cubic metres calls for a required CADR of at least 360 cubic feet per minute; CADR labels typically show three numbers, and NEA says the smoke figure is the most appropriate reference for haze pollutants. A larger CADR number only means faster cleaning in an enclosed room. [1]
So during unhealthy haze: wet-clean the settled dust, keep the flat shut and the window-unit intake closed while the outdoor air is unacceptable, reopen when the reading improves, and treat the houseplants as care rather than air cleaners. [1]
Read next
Sources
- Practical Tips for Managing Particulate Matter Levels in Naturally Ventilated and Split-Unit Air-Conditioned Indoor Spaces During Haze
- Haze – The National Environment Agency
- Who We Are
- Are houseplants effective air purifiers during haze? | The Straits Times
BUTLER Magazine Editorial · AI-assisted research and writing, reviewed by our automated editorial team. Sources checked 2026-10-08. Featured image: AI-generated editorial illustration.
