On the morning Airscan ran its monthly indoor air quality assessment at Merdeka 118 in September, our outdoor sensors across Kuala Lumpur were reading outdoor PM2.5 at 70 µg/m³ to 105 µg/m³, around 7 times the World Health Organisation’s recommended 24-hour guideline of 15 µg/m³. The damaging transboundary haze from Indonesia has been choking Malaysia (the entire region, for that matter)for the better part of a month, and KL was doing what it always does in the haze season: closing windows, checking air quality indexes, and hoping that staying indoors offers some protection against the toxic smog raging outside.
For most office workers here, that trust is misplaced. Airscan tests and monitors indoor air quality across the region, lots of corporate offices included. During a pollution episode the pattern is depressingly consistent. Indoor PM2.5 tends to rise and fall in lockstep with the air outside, the peaks a touch lower but with much of the same curves. In one multinational’s modern, well-sealed office, we’ve seen PM2.5 inside a closed conference room – far from the lobby, the entrance and lifts – track the street reading movements almost exactly.
Video: Example PM2.5 hourly overlay of outdoor and indoor data. Taken over 4-day period from outdoor sensor clusters (in KL city center) and indoor sensor clusters (on the border of KL, approximately 7-10km away from the outdoor cluster!).
The culprit is the ventilation system itself. Never mind ageing buildings with old systems, commercial HVAC in most Malaysian offices tends to be designed for thermal comfort and basic air exchange: it draws in outdoor air, cools and dries it, and pushes it all throughout the building, haze and all. The filtration stage usually runs filters rated MERV 8 or lower, which deal with dust and coarse matter but were never engineered to catch the fine particles that haze is made of. PM2.5, as the name says, is particles 2.5 micrometers and smaller – small enough to slip past standard commercial filters, reach deep into the lungs, and in its finest fraction, cross into the bloodstream, where epidemiological studies tie it firmly to heart attacks and stroke. Whether you are at a desk by the window, eating in the pantry, or in a presentation in a meeting room, the air in a typical Malaysian office with typical centralised ventilation during a haze is closer to the air outside than you would realise.
Merdeka 118 tells a different story.
The air quality in Merdeka 118
Airscan served as WELL Performance Testing Agent (WELL PTA) for Merdeka 118’s WELL certification, work that helped make it the tallest building in the world to achieve WELL Core Platinum and WELL Platinum for their PNB’s office fit-out. Since then, Airscan has continued with monthly indoor air quality testing for PNB, covering both the parameters set out in Malaysia’s Industry Code of Practice on Indoor Air Quality 2010 and the annual performance testing required to maintain WELL certification. The September round happened to fall on one of the worst outdoor air quality days in years – certainly the worst Merdeka 118 has ever experienced since opening in 2024.
Of the 32 sampling points this month, 31 recorded PM2.5 at or below 15 µg/m³, the threshold set by WELL Building Standard v2 and aligned with the WHO’s 24-hour guideline. One point touched 18 µg/m³, just slightly above and was still 74% cleaner than what was outside. Worth being precise here, because the standards measure different things: ICOP 2010 sets no PM2.5 limit at all, but governs respirable particulates at 150 µg/m³ and carbon dioxide at a 1,000 ppm ceiling, and on both counts the building was comfortably compliant. The meaningful PM2.5 benchmark is WELL’s, and the areas we tested met it while outdoor PM2.5 ranged from 70 – 105 µg/m³.
What made the result striking was the ventilation data behind it. The building’s fresh air dampers were open at high percentages, pulling in large volumes of outdoor air rather than recirculating it, which is what most buildings would be doing at this level of air pollution. Carbon dioxide across every point held between 590 and 860 ppm, well below the 1,000 ppm ICOP and WHO ceiling, but also confirming that the building was ventilating at full capacity. In most buildings those damper positions during a haze would mean disaster for occupants.
PNB’s occupants were safe because of what sits upstream from them. Merdeka 118’s incoming air passes through two filtration stages:
- MERV 7/8 primary filters, which take out the coarse particles and protect the next stage
- And MERV 15 secondary filters, near-HEPA grade and the gold standard in air-handling units, which capture the fine particles haze consists of.
The system is designed to hold indoor air quality even when outdoor intake is high – which is exactly what the September results demonstrated. Ventilation running wide open at full capacity, pulling in heavily polluted outdoor air, and cleaning it to levels that met the strictest available threshold.
The false comfort of indoor environments in Malaysia
The default assumption across commercial real estate is that the building envelope offers real protection during an outdoor air pollution event. Even the standard public advice during haze is to stay indoors. Yes, the envelope holds the line against the everyday pollutant load – construction dust, road grime, the ambient background of a city. But haze is another matter. Its particles are finer, its concentrations higher, and its episodes last days or weeks (or in our case… months). Our continuous indoor monitoring across KL offices with centralised ventilation show indoor PM2.5 during these events tracking the outdoor data. The sealed meeting room and the executive floor offer no more separation from the lobby.
PNB’s investment in Merdeka 118’s mechanical systems reflects a view of indoor environmental quality as an engineered outcome, maintained by equipment that performs under the highest stress. The October haze was the stress test, and the building passed even with its dampers wide open.
Haze-proofing the building you have
For everyone whose building doesn’t come with Merdeka 118’s state of the art systems, the practical question is what can be done now, and the answer differs depending on whether you control the air handling.
If you do not, like most office workers, the realistic step is to invest in high quality air purifiers with a true HEPA filter. You need multiple units to purify air for a 1,500 sq ft open space. Go bug your office managers now. And if possible, try to work from sealed meeting rooms temporarily and place the air purifiers in those rooms. A desk-side unit will not rescue a whole open-plan floor, but it can turn a small closed room into something far better than the corridor outside.
If you do have a say in the building’s system, there is a defined process. Begin with a baseline: measure what your indoor air quality does during a haze. Check out Binnenluchtkwaliteitmeting En Continuous Monitoring. And have an engineer check whether your air handlers can tolerate a higher-grade filter, because a denser filter raises resistance and a blower that cannot push against it will simply move less air. Where the system allows, upgrade the primary filtration to the highest grade it will carry – MERV 13 is the floor for smoke, and higher is better. As Merdeka 118 shows, a dedicated high-MERV secondary stage is what makes a genuine difference. Where the fans cannot take it, deep-pleat filters spread the media over more surface and breathe more easily, and supplemental filtration or a designated cleaner-air space can hold part of the building to a better standard. None of it works blind, which is why continuous air quality monitoring and a written plan for when readings climb matter as much as the hardware.
The haze returns every year. For most buildings in Kuala Lumpur, the question that the September data poses is a simple one: what is your filtration actually rated for, and what happens to the air you breathe on the day filtration is needed most? The answer decides whether we’re protected from the haze or merely unaware that we’re not.
