Idle at a red light in central Singapore and the concrete around you is heating the air, and that rising warmth is why the smoke does not thin out where you are breathing it. NUS meteorologist Koh Tieh Yong told the Straits Times that urban heat off concrete drives turbulence that holds haze particles close to the ground, which is why the central region reads worst. On 19 September its hourly PM2.5 hit 149 µg/m³ at noon, one point under NEA's “high” band, while the rest of the island's 24-hour PSI stayed moderate all day (Straits Times via Yahoo). NEA publishes two numbers for this haze, and neither is measured at your visor. Five peer-reviewed studies show what is and is not known about the air at rider height, and this post walks through each.
Part of our Haze and air quality by city guide.
Key takeaways
- Central Singapore's 1-hour PM2.5 reached 149 µg/m³ at noon on 19 September, one point below NEA's “high” band (151 to 250), while the other regions' 24-hour PSI stayed in the moderate range all day (Straits Times via Yahoo News Singapore; NEA bands).
- The two NEA numbers can point opposite ways: on 14 September central 1-hour PM2.5 was 113 µg/m³ at 10am while the 24-hour PSI still read 78 to 99 (moderate), and on 20 September at 1pm every region's hourly PM2.5 was normal while central PSI read 107 (Mothership; Straits Times via Yahoo).
- NUS's Koh Tieh Yong attributes the central region's worse readings to urban heat from concrete, which drives rising air and turbulence that keeps PM2.5 close to the ground; NTU's Steve Yim adds that in dry conditions the particles come from both haze and local emissions (Straits Times via Yahoo). Near a signalised intersection in Korea, black carbon fell from a median 3.1 µg/m³ at ground level to 2.2 µg/m³ at 100 m (Lee & Kwak, 2020).
- On 15 September all five regions were unhealthy at once for the first time since 2019, with central PSI at 154 at daybreak (Straits Times via Yahoo).
The research behind this
Five peer-reviewed studies this post leans on. Each card says what was measured, what was found, and what it cannot tell you. None of them is a Singapore reading at rider height.
Urban heat islands and urban pollution islands: a 30-year review
- What they measured
- A systematic review of research since 1990 on how urban heat and urban pollution interact: 40 papers analysed in depth, from 16 countries and 11 climate zones.
- What they found
- The heat and pollution link is an established research field, sorted into five topics, including how heat changes pollutant chemistry and how the effect differs between day and night.
- What it can't tell you
- It maps the literature. It is not a measurement in Singapore or of a rider, and it does not say how much of central Singapore's excess is heat-driven.
Near-road air pollution at a signalised intersection: drone monitoring and modelling
- What they measured
- A drone hovering at eight heights from 0 to 100 m beside a signalised intersection in Chuncheon, Korea, from 8 to 9am on six mornings in March and April 2019, plus a fixed monitor at 30 m and a dispersion model.
- What they found
- Median black carbon was 3.1 µg/m³ on the ground and 2.2 µg/m³ at 100 m, and measured black carbon and modelled NOx roughly halved by about 20 m.
- What it can't tell you
- An inland Korean city in spring, not Singapore and not a motorcycle. Heights come in 10 m steps, so it cannot separate handlebar from face height, and PM2.5 fell off more gently than black carbon.
Route choice and commuters' exposure to air pollution in Taipei
- What they measured
- Three commuting routes in Taipei modelled from Taiwan EPA station data for motorcycling, cycling and walking, checked with a real-time particle monitor carried along a motorcycle route.
- What they found
- The shortest route gave motorcyclists a much higher cumulative dose (3,340.8 µg/m³ PM2.5) than the cleanest route (912.5 µg/m³), and the monitor showed high concentrations with short-duration peaks.
- What it can't tell you
- The cumulative figure is built from interpolated station data and is not a health outcome. It describes Taipei, not Singapore.
Inhaled dose of pollutants in different micro-environments
- What they measured
- A systematic review of 46 papers that estimated the dose people inhale in different micro-environments, including transport.
- What they found
- Exposure concentration and time spent are among the most important parameters, especially in transport environments, and a higher breathing rate during active commuting raises the dose.
- What it can't tell you
- Mostly particle studies, and not specific to motorcycles. The breathing-rate effect applies to cyclists and walkers more than to seated riders (our reading).
PM10 during episodic haze in Malaysia: domestic or solely transboundary?
- What they measured
- Hourly PM10, gas and weather records from Malaysia's Department of Environment at four stations (Kelang, Melaka, Pasir Gudang, Petaling Jaya) across historic haze years.
- What they found
- Air masses came from Sumatra during the episodes, PM10 tracked CO strongly to moderately, and its correlation with NOx was weak in all four areas, which the authors link to a small domestic contribution.
- What it can't tell you
- It measures PM10 rather than PM2.5, at Malaysian stations rather than Singapore, and a correlation is not a source breakdown.
What the studies actually measured
No study has put a sensor at a Singapore rider's face during this haze, so the honest picture is five studies that each measured one link in the chain. Read them in order and note where each one stops.
Does urban heat hold pollution near the ground? Ulpiani (2021) reviewed the research since 1990 on the link between the urban heat island and what the field calls the urban pollution island. The review analysed 40 papers in depth, from 16 countries and 11 climate zones, and sorted them into five topics, among them how heat changes pollutant chemistry and how the effect differs between day and night (Ulpiani, 2021). That tells us NUS's Koh Tieh Yong is pointing at an established research question, not making a stray remark. It does not tell us how much of central Singapore's excess this haze comes from heat, because it is a map of the literature and not a measurement of one city.
How does concentration change with height beside a road? Lee and Kwak (2020) hovered a drone at eight heights between the ground and 100 m next to a signalised intersection in Chuncheon, Korea, from 8 to 9am on six mornings in spring 2019. Median black carbon, a marker for vehicle exhaust, was 3.1 µg/m³ on the ground and 2.2 µg/m³ at 100 m, and measured black carbon and modelled NOx both roughly halved by about 20 m (Lee & Kwak, 2020). PM2.5 fell off more gently than black carbon, because black carbon is a large share of what vehicles emit, and the gradients faded out above the 50 m building height. Read this as support for one idea only: traffic pollution is richest closest to the road. Ten-metre steps cannot separate a handlebar from a helmet, so the “rider height” reading in Singapore remains an inference.
What does a motorcyclist actually inhale? Li et al. (2017) modelled three commuting routes in Taipei from Taiwan EPA station data, then carried a real-time particle monitor along a motorcycle route. The shortest route gave motorcyclists a much higher cumulative dose (3,340.8 µg/m³ PM2.5) than the cleanest route (912.5 µg/m³), and the monitor showed “high-concentration exposure and short-duration peaks” (Li et al., 2017). The lesson for a reader comparing NEA's two numbers is that short peaks are what a 24-hour average is built to smooth away. The limit is that the cumulative figure is interpolated from stations, and Taipei is not Singapore.
Does time in the queue matter? Borghi et al. (2021) reviewed 46 papers on the dose people inhale in different micro-environments. Their conclusion was that exposure concentration and time spent are among the most important parameters, especially in transport environments, and that breathing rate matters too (Borghi et al., 2021). Dose is concentration multiplied by time, so a rider who crawls through a dense layer pays on both terms. The breathing-rate effect is mainly about cyclists and walkers, so we do not lean on it for riders.
Is the smoke the haze, or is it us? Addiena et al. (2023) analysed hourly records from four Malaysian stations across historic haze years. Air masses came from Sumatra during the episodes, PM10 tracked CO strongly to moderately, and its correlation with NOx was weak in all four areas, which the authors link to a small domestic contribution (Addiena et al., 2023). That leans the other way from NTU's Steve Yim, who told the Straits Times that in dry conditions downtown Singapore's particles come from both haze and local emissions. The two need not conflict, since one is Malaysian PM10 in earlier episodes and the other is downtown Singapore in dry weather, but neither confirms the other, and we say so rather than pick a side.
Put together, the studies support three claims: heat and pollution are linked in cities, traffic pollution is richest near the road, and short peaks and time spent drive what a rider breathes. They do not support a measured figure for the air at a Singapore rider's face, and this post does not offer one.
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What has the Singapore haze done since 4 September?
It has come in waves, not as one event. Central Singapore's 24-hour PSI first dipped into NEA's unhealthy band (101 to 200) on 4 September, and the 20 September reports call it the first haze-driven unhealthy reading since October 2023 (Straits Times via Yahoo). On 15 September all five regions were unhealthy together, the first time since 2019, with central at 154 at daybreak and the other regions between 104 and 128 (Straits Times via Yahoo).
Then it cleared and came back. Central crossed into unhealthy again from 1pm on 19 September (Straits Times via Yahoo). By 9am on 20 September central read 122 and the east 101 (AsiaOne). By 1pm central was 107, the west 97 and the east 87, so the east and west had slipped back to moderate while the centre stayed unhealthy (Straits Times via Yahoo).
Counting those reports, central Singapore has had at least three separate unhealthy stretches in 16 days: 4 September, 15 September, and 19 to 20 September. That count is mine, read off the published readings, not an NEA statistic.
Keep the scale honest. NEA's 20 September outlook was for the 24-hour PSI to stay “in the high-moderate to mid-unhealthy range” (AsiaOne): unhealthy, not hazardous. And the authorities' stated haze-risk window runs June to October, driven by warmer, drier conditions from El Niño and the Indian Ocean Dipole (Straits Times via Yahoo), so this may not be over. We wrote the rider's contingency plan for a Singapore haze in August, before any of it arrived. This post is about what the first three weeks showed.
One boundary: Easi Breezi has not measured Singapore's air. Every Singapore number here is NEA's, as reported by the press.
Singapore Haze, Central Region: Why the Same Smoke Reads Higher Downtown
Because the central region is the most built-up part of the island, and heat off concrete keeps smoke near the ground. Koh Tieh Yong, an adjunct associate professor of meteorology at NUS, told the Straits Times that urban heat from concrete structures warms the air above the ground, producing hot, rising air parcels that enhance turbulence, and that turbulence holds PM2.5 close to the ground (Straits Times via Yahoo).
Steve Yim of NTU's Centre for Climate Change and Environmental Health adds a second ingredient. In Singapore's dry conditions, the particles can come from both the haze and local emissions in the downtown area. Traffic is one of those local sources, and a rider at a stop line is inside it.
Why central Singapore's smoke stays at street level
Side view of a downtown street on a hazy, dry day
Read that as physics, not as measurement. What the published reporting supports is that the central region reads worse and that the experts explain it with heat and turbulence at ground level. What it does not give is a vertical profile. Nobody has published the difference between a weather-station inlet and a rider's face in a stopped queue, so anything about “handlebar height” is an engineering inference. It is a reasonable one, because the mechanism acts on exactly the layer a rider is in, but it is still an inference.
Which number should a rider plan on: the 24-hour PSI or the hourly PM2.5?
Plan the ride on the hourly PM2.5 and treat the 24-hour PSI as the day's background. NEA's own advice, as the Straits Times reported it, is to use the hourly PM2.5 reading for immediate activities and the 24-hour PSI to plan the day ahead (Straits Times via Yahoo). The 1-hour PM2.5 reading is the most current indication of air quality, as Mothership put it (Mothership), while the PSI is a 24-hour figure that carries yesterday's smoke along with today's. That is why they disagree. The health stake is real at either timescale, since fine particulate matter can penetrate the lungs and enter the bloodstream (WHO). We have already covered what PSI, AQI and API each measure; this section is only about which one to look at before you start the bike.
NEA bands the hourly reading as normal at 55 µg/m³ or below, elevated at 56 to 150, high at 151 to 250 and very high at 251 or more (NEA). The three days below show the two numbers pulling apart.
Same day, two numbers: which one told the truth?
Pick a day. Central Singapore, NEA readings as reported by the press.
On 14 September the hour was already elevated at 113 µg/m³ while the 24-hour PSI still read moderate; on 19 September the hourly peak of 149 µg/m³ came at noon and the PSI crossed into unhealthy at 1pm; on 20 September the hour had cleared to normal while central PSI still read 107 (Mothership; Straits Times via Yahoo). Bands from NEA; the unhealthy PSI band is 101 to 200.
| Date | 1-hour PM2.5 | 24-hour PSI |
|---|---|---|
| 14 Sept | 113 µg/m³ at 10am, elevated band | 78 to 99 at 11am, moderate |
| 19 Sept | 149 µg/m³ at noon, elevated band | Central unhealthy from 1pm, other regions moderate all day |
| 20 Sept | Normal band in all regions at 1pm | Central 107 at 1pm (120 at 10am), unhealthy |
The 149 reading is worth a sense of scale. It is about 2.7 times the ceiling of NEA's normal band (149 against 55; NEA). It is also the kind of short peak that Li et al. (2017) saw on the Taipei motorcycle route, and that a 24-hour figure dilutes. It came at noon, and the PSI did not cross into unhealthy until 1pm.
Why can't helmet vents fix a layer of smoke at the ground?
Because a vent moves air only when the bike moves, and the worst air is where the bike is stopped. Passive helmet ventilation depends on forward speed to build a pressure difference across the shell. At a stop line in central traffic that difference collapses, and PM2.5 is small enough to pass straight through vent mesh and comfort foam regardless. We set out the vent physics in full during the August haze run, so the short version is enough here: the stop-start central corridor is exactly the case vents handle worst.
Three unhealthy spells in 16 days: what does 2031 look like?
Plan for spells, not seasons, and expect at least one more before the decade turns. The verified events give two intervals, and they disagree. The gap between the October 2023 haze-driven unhealthy reading and September 2026 is about three years (Straits Times via Yahoo). The gap between all five regions being unhealthy in 2019 and again in 2026 is seven (Straits Times via Yahoo).
That is EB's own arithmetic from two data points, and it is a synthesis, not a forecast. Two intervals are not a cadence. If the shorter gap repeats, the next unhealthy reading lands around 2029. If the longer one does, it is 2033, outside the window. So “one more before 2031” is plausible, not scheduled. We looked at the wider direction of travel in is haze getting worse every year.
The more useful lesson for a rider is within-season clustering. Central Singapore had at least three unhealthy stretches in 16 days, so the planning unit is the spell: a few days of unhealthy central air, repeatedly, with better days between. Riders who wait for a “haze season” to start preparing miss the first spell.
Filtering at the face, not at the weather station
A weather-station reading and a 24-hour average cannot follow you through a queue downtown. Easi Breezi does not depend on either. A vehicle-powered fan draws air through an H11 HEPA element rated at ≥95% on PM2.5 and into the helmet, so the air at your face is filtered whether or not the vents are moving, and whatever the PSI reads. It is patent pending.
Frequently Asked Questions
Why is the haze worst in the central region of Singapore?
NUS's Koh Tieh Yong says the central region is highly urbanised, and heat from concrete drives rising air and turbulence that keeps PM2.5 near the ground. NTU's Steve Yim adds that in dry conditions the particles include local downtown emissions as well as haze (Straits Times via Yahoo).
Should I use the 24-hour PSI or the 1-hour PM2.5 to decide whether to ride?
For the ride you are about to take, use the 1-hour PM2.5, which is the more current number. The 24-hour PSI carries the previous day's smoke, so it can read moderate while the hour is already elevated (14 September, per Mothership) or read unhealthy after the hour has cleared (20 September, per Straits Times via Yahoo). NEA's own advice is the hourly reading for immediate activities and the 24-hour PSI for planning the day ahead. Use the PSI as the day's background.
What are NEA's PM2.5 bands?
Normal is 55 µg/m³ or below, elevated is 56 to 150, high is 151 to 250, and very high is 251 or above (NEA).
Does a normal hourly PM2.5 reading mean the haze has gone?
No. At 1pm on 20 September the hourly PM2.5 was normal in every region while central PSI still read 107, and NEA's outlook that day was still “high-moderate to mid-unhealthy” (AsiaOne). A clear hour can sit inside an unhealthy day.
Is this year's Singapore haze worse than earlier years?
The reports call it the first haze-driven unhealthy reading since October 2023 (Straits Times via Yahoo) and the first time all five regions were unhealthy together since 2019 (Straits Times via Yahoo). The highest central reading verified for this post is a PSI of 154 on 15 September, inside the unhealthy band of 101 to 200 (same source). This post does not compare it with earlier episodes' peaks.
Ready to breathe cleaner on every ride?
Easi Breezi is $199, with worldwide shipping at a flat $15, duties and import taxes included. See the Easi Breezi unit, and for the wider picture on what riders breathe, start with our guide to motorcycle air pollution.
Sources
- Ulpiani, G. (2021). On the linkage between urban heat island and urban pollution island: three-decade literature review towards a conceptual framework. Science of the Total Environment, 751, 141727. doi.org/10.1016/j.scitotenv.2020.141727
- Lee, S.-H., & Kwak, K.-H. (2020). Assessing 3-D spatial extent of near-road air pollution around a signalized intersection using drone monitoring and WRF-CFD modeling. International Journal of Environmental Research and Public Health, 17(18), 6915. doi.org/10.3390/ijerph17186915
- Li, H.-C., Chiueh, P.-T., Liu, S.-P., & Huang, Y.-Y. (2017). Assessment of different route choice on commuters' exposure to air pollution in Taipei, Taiwan. Environmental Science and Pollution Research, 24(3), 3163-3171. doi.org/10.1007/s11356-016-8000-7
- Borghi, F., Spinazzè, A., Mandaglio, S., et al. (2021). Estimation of the inhaled dose of pollutants in different micro-environments: a systematic review of the literature. Toxics, 9(6), 140. doi.org/10.3390/toxics9060140
- Addiena, A. R. N. A., Noor, N. M., Mohd Jafri, I. A., et al. (2023). Variability of PM10 level with gaseous pollutants and meteorological parameters during episodic haze event in Malaysia: domestic or solely transboundary factor? Heliyon, 9(6), e17472. doi.org/10.1016/j.heliyon.2023.e17472
- National Environment Agency, Singapore. 1-hr PM2.5 readings and band descriptors. haze.gov.sg/resources/1-hr-pm2.5-readings
- World Health Organization. What are the WHO air quality guidelines? who.int
- The Straits Times via Yahoo News Singapore (Sept 2026). Why is Singapore's central region registering the worst air quality during this year's haze? sg.news.yahoo.com
- The Straits Times via Yahoo News Singapore (20 Sept 2026). Haze in S'pore: PSI reading in central region remains unhealthy, east and west dip back to moderate. sg.news.yahoo.com
- Mothership (14 Sept 2026). 1-hour PM2.5 in central Singapore hits 113. mothership.sg
- AsiaOne (20 Sept 2026). Haze watch: Singapore wakes up to unhealthy air quality in central, eastern and western areas. asiaone.com
Written by Ash — mechanical engineer and founder of Easi Breezi, building an active HEPA filtration system for motorcycle helmets (patent pending). Based between Hong Kong and Bali, riding daily in the traffic this blog writes about.