Your motorcycle helmet is too hot because its vents run on road speed. At 10 km/h a vent gets about 1/36 of the push it gets at 60 km/h, and at a red light none (NASA Glenn). Cracking the visor lets heat out, but in haze season it lets smoke in unfiltered.
Part of our Helmet heat and cooling guide.
The light is red, the tarmac is shimmering, and sweat is running into your eyebrows. You flick the visor up two clicks. The air that comes in smells of burning.
That small move is the whole problem. This year the heat and the haze come from the same El Niño, and climate models say extreme El Niño years come twice as often from here on (Cai et al., 2014).
This post explains why the helmet traps you, and what the next five years look like.
Key takeaways
- Helmet vents run on road speed: the pressure driving air into a vent at 10 km/h is about 1/36 of the pressure at 60 km/h, and zero at a standstill (NASA Glenn).
- In California, deaths rose 21.0% on days with both extreme heat and extreme PM2.5, against 6.1% for heat alone and 5.0% for PM2.5 alone (Rahman et al., 2022).
- Climate models project extreme El Niño events rising from about one every 20 years to one every 10 (Cai et al., 2014). At that rate, the chance of at least one such season in 2027 to 2031 is about 41%, up from about 23%.
The research behind this
Peer-reviewed measurements this post is built on. Each card says what was measured, what was found, and what it cannot tell you.
Heat and PM2.5 on the same day, California
- What they measured
- Deaths in California from 2014 to 2019, matched against daily heat and PM2.5 at each person's home census tract, in a case-crossover design.
- What they found
- Deaths rose 21.0% on days with both extremes, against 6.1% on heat-only days and 5.0% on PM2.5-only days.
- What it can't tell you
- A US state, not the tropics. Exposure is at home, not on a bike. The combined estimate has a wide range (6.6 to 37.3%).
Heat and air pollution in 620 cities
- What they measured
- 22,630,598 deaths in 620 cities across 36 countries, 1995 to 2020, in each city's six warmest months, against temperature and four pollutants.
- What they found
- Each extra 10 µg/m³ of PM10 raised deaths 0.54% on the coolest days and 1.21% on the hottest days.
- What it can't tell you
- City-wide averages from fixed monitors, not a rider's breathing zone. PM2.5 results were weaker than PM10.
Fire smoke and extreme heat, Northern Thailand
- What they measured
- Deaths in Upper Northern Thailand against PM2.5 from vegetation fires and days above the 90th percentile of daily maximum temperature.
- What they found
- Deaths rose 0.9% per 10 µg/m³ of fire smoke and 12.8% on extreme heat days, but the two did not multiply each other.
- What it can't tell you
- All-cause deaths only. It disagrees with the California study on whether heat and smoke combine.
Extreme El Niño events double
- What they measured
- 20 climate models, comparing extreme El Niño counts in 1891 to 1990 against 1991 to 2090 under high greenhouse emissions.
- What they found
- Extreme events rose from 101 to 212 per 2,000 model years, from about one every 20 years to one every 10.
- What it can't tell you
- A model projection on a high-emission path. "Extreme" is defined by Pacific rainfall, not by haze.
Heat strain in motorcycle gear
- What they measured
- 12 men pedalling lightly for 90 minutes in motorcycle protective clothing at 25, 30 and 35 °C, with 40% humidity.
- What they found
- At 35 °C, core temperature rose 0.02 °C a minute, with moderate hyperthermia (above 38.5 °C) predicted within 105 minutes.
- What it can't tell you
- A lab test in dry air. Tropical humidity is higher. It tested clothing, not helmets alone.
What the studies actually measured
Five pieces of research carry this post. None of them measured a rider inside a helmet at a red light. Together they cover the parts: the heat on the body, the smoke in the air, and how often the two now arrive together.
The heat load comes from a lab. de Rome et al. (2016) put 12 men in motorcycle protective clothing on exercise bikes for 90 minutes. At 35 °C their core temperature climbed 0.02 °C every minute.
The heat and smoke combination comes from death records. Rahman et al. (2022) studied California from 2014 to 2019. Stafoggia et al. (2023) pooled 22.6 million deaths from 620 cities in 36 countries.
The closest match to our riders is Uttajug et al. (2024). They studied fire smoke and extreme heat in Upper Northern Thailand, where the smoke is from burning vegetation, as it is in Sumatra and Kalimantan.
The forecast comes from Cai et al. (2014). They ran 20 climate models and counted extreme El Niño events: 101 per 2,000 model years in the old climate, 212 in the warmer one.
The three death studies do not fully agree, and that disagreement is part of the story below. They do agree that heat and smoke each raise risk on their own.
Ride with filtered air
Filters haze PM2.5 on every ride
Easi Breezi mounts on your scooter or motorbike and feeds H11 HEPA-filtered air into your helmet. Bike-powered, no batteries, no charging.
Your helmet is hot because its vents run on road speed
Helmet vents have no fan. They rely on ram air: the wind of your own speed pressing into the openings. The push is called dynamic pressure, and it rises with the square of speed (NASA Glenn).
In warm air at 32 °C, that works out to about 160 pascals at 60 km/h and about 4.5 pascals at 10 km/h. That is 1/36 of the push. At a red light it is zero (NASA Glenn). This is our arithmetic from the textbook formula, not a helmet test.
Air speed through an opening rises with the square root of the push (NASA Glenn, Bernoulli). So the air moving through a vent at 10 km/h is about one sixth of what moves at 60 km/h. Either way, the vents quit when you slow down.
That is the worst possible timing. Crawling traffic is where you sit longest, next to hot engines and exhaust, with the sun on your shell. We covered the body-heat side of this in do motorcycle helmets get hot.
Why the vents quit when you stop
Side view. Moving, ram air feeds the vents. Stopped, the only airflow left is the gap you open yourself.
So riders open the visor or the chin vent. It works for heat. In haze season it opens a path for smoke that skips any filter or mask you are wearing. A mask only cleans the air that goes through it, which we showed in our best face mask for motorcycle riding comparison.
At a long red light in haze, close the visor for the wait and open it once you are moving again. Moving air at speed cools you better than a stalled gap does.
Heat in riding gear builds up faster than it feels
A hot helmet is easy to write off as discomfort. The lab data says the load on the body is bigger than that.
Predicted time to moderate hyperthermia (core above 38.5 °C) in motorcycle protective clothing at 35 °C.
Source: de Rome et al., Ergonomics, 2016In that study, profound hyperthermia (above 40 °C) was predicted within 180 minutes at 35 °C. At 25 °C, the same gear caused no predicted hyperthermia at all (de Rome et al., 2016).
The test air was dry, at 40% humidity (de Rome et al., 2016). Sweat evaporates well in dry air. In a humid tropical city it evaporates less, so the same temperature is harder on the body.
The heat is also rising. Copernicus (2026) reports that August 2026 was the joint hottest month ever recorded, level with July 2023, and 1.65 °C above the pre-industrial average.
The haze years were also the hot years
Haze in Southeast Asia follows drought, and drought follows El Niño. In 2015, a strong El Niño gave the most severe fire season in the NASA satellite record. Fires grew sharply once dry-season rain stayed below 4 mm a day (Field et al., 2016).
That smoke was deadly. A modelling study estimated 100,300 excess deaths across Indonesia, Malaysia and Singapore from the 2015 haze, more than double the 2006 event (Koplitz et al., 2016).
Worldwide, smoke exposure has been creeping up. In 2010 to 2019, about 2.18 billion people had at least one day of heavy fire smoke a year, averaging 9.9 days each. The number of people exposed was up 6.8% on the decade before. Southeast Asia was among the most exposed regions (Xu et al., 2023).
Now 2026. NOAA gives a 75% chance that October to December will be stronger than any El Niño since 1950 (NOAA CPC, 2026). The WMO expects it to be very strong, with near 100% likelihood of lasting through February 2027 (WMO, 2026).
The fires followed. Indonesia logged more than 9,000 hotspots in August, the highest August since records began in 2015. Singapore had unhealthy air in 5 regions on 15 September, the first time since 2019 (Kompas, 2026).
The clinics are filling too. Acute respiratory infection cases in Indonesia's seven haze provinces rose fast through September (Indonesian Ministry of Health, via The Jakarta Post, 2026):
For a rider these are not two news stories. The hottest weeks and the smokiest weeks are the same weeks, set by the same ocean. How often heat and haze hit together is what we tracked in is haze getting worse every year.
Heat and smoke on the same day raise risk, but the studies split on how much
The question is whether a hot, smoky day is just the two risks added up, or something worse. Three large studies give two different answers.
California says worse. Rahman et al. (2022) found the combined day raised deaths 21.0%, more than the sum of the separate effects. Heart deaths rose 29.9% on those days.
The 620-city study leans the same way. The death rise per 10 µg/m³ of PM10 went from 0.54% on the coolest days to 1.21% on the hottest, about 2.2 times as large (Stafoggia et al., 2023).
Thailand says no. In fire-smoke country, Uttajug et al. (2024) found fire PM2.5 raised deaths 0.9% per 10 µg/m³ and extreme heat raised them 12.8%. They found no sign that the two multiplied.
The fair reading: nobody has settled whether heat makes smoke more dangerous. Every study agrees each one is harmful on its own. A rider in haze season now gets both on the same afternoon.
By 2031, a hot and hazy season is about a 2 in 5 bet
Cai et al. (2014) found that in a warming climate, extreme El Niño events go from about one every 20 years to one every 10. Those are the years that bring the deep droughts and the worst fires.
We turned that into a rider's five-year view. At 1 in 20 a year, the odds of at least one extreme season in 2027 to 2031 are 1 minus 0.95 to the fifth power: about 22.6%, using the rates from Cai et al., 2014.
At the modelled 1 in 10 (Cai et al., 2014), it is 1 minus 0.9 to the fifth: about 41.0%.
Chance of at least one extreme El Niño season, 2027 to 2031
Cumulative odds by year. Blue: the old rate of 1 in 20 years. Orange dashed: the modelled warmer-climate rate of 1 in 10. Tap or focus a dot for its value.
| By year | Old rate, 1 in 20 | Modelled rate, 1 in 10 |
|---|---|---|
| 2027 | 5.0% | 10.0% |
| 2028 | 9.8% | 19.0% |
| 2029 | 14.3% | 27.1% |
| 2030 | 18.5% | 34.4% |
| 2031 | 22.6% | 41.0% |
Put plainly: a rider who starts commuting in Southeast Asia today has roughly a 2 in 5 chance of riding through another extreme El Niño season before 2031. That is close to double the 20th-century odds, on the rates from Cai et al., 2014.
Three caveats. The sum treats each year as independent, and El Niño years are not, since a big event is often followed by a La Niña. The rate comes from high-emission model runs. And it is a frequency, not a forecast for any named year.
It also leaves out the heat trend. Even an ordinary year now starts from a hotter baseline, with August 2026 at 1.65 °C above pre-industrial (Copernicus, 2026).
Airflow that does not need speed or an open visor
The trap exists because the helmet's only airflow depends on speed and comes through an unfiltered gap. Easi Breezi separates the two. The unit runs off the bike's own power and feeds air into the helmet under the chin bar through an H11 HEPA filter that stops 95% of PM2.5.
It keeps moving that air at a red light, so the visor can stay shut. It is a filter and a fan, not an air conditioner: it will not lower the temperature of the air, but the air it moves is filtered.
Frequently asked questions
Why is my motorcycle helmet so hot?
Because the vents depend on road speed. The push into a vent drops to about 1/36 at 10 km/h compared with 60 km/h, and to zero at a stop (NASA Glenn). Slow traffic is when you get the least air and the most engine heat.
Are motorcycle helmets hot in tropical weather?
Yes, and riding gear adds to it. In a lab test at 35 °C, riders in protective clothing were predicted to reach moderate hyperthermia within 105 minutes, in drier air than a tropical city (de Rome et al., 2016).
What is the best motorcycle helmet for hot weather?
One with large, adjustable vents and a removable, washable liner, because vents work best when moving. In haze season, add a way to get filtered air in when stopped. Our guide to a motorcycle helmet for hot weather walks through the options.
Is it safe to ride with the visor open in haze?
It lets smoky air reach your face with no filter in the way. Deaths rose on days with fire smoke in Northern Thailand, and more on days with both heat and smoke in California (Uttajug et al., 2024; Rahman et al., 2022). Close it when readings are high.
What to do this week
- Close the visor at long stops in haze. Open it once you are moving and the vents work again.
- Check PM2.5 and the temperature together. Look at a live reading before the ride, since this season the hottest days can also be the smokiest.
- Break up long rides in the heat. In the lab, moderate hyperthermia was predicted within 105 minutes at 35 °C (de Rome et al., 2016). Stop, drink and cool down before that.
- Filter the air inside your helmet. The Easi Breezi unit feeds air through an H11 HEPA filter at any speed, so you are not choosing between heat and smoke at every light.
The models say this squeeze gets more common, not less. The habits you build this season are the ones you will need in 2031. Our helmet temperature protocol for haze season ranks the rest.
Sources
- NASA Glenn Research Center. Dynamic pressure. grc.nasa.gov
- NASA Glenn Research Center. Bernoulli's equation. grc.nasa.gov
- Cai W, Borlace S, Lengaigne M, et al. (2014). Increasing frequency of extreme El Niño events due to greenhouse warming. Nature Climate Change 4:111. doi:10.1038/nclimate2100
- Rahman MM, McConnell R, Schlaerth H, et al. (2022). The effects of coexposure to extremes of heat and particulate air pollution on mortality in California: implications for climate change. American Journal of Respiratory and Critical Care Medicine 206(9):1117. doi:10.1164/rccm.202204-0657OC
- Stafoggia M, Michelozzi P, Schneider A, et al. (2023). Joint effect of heat and air pollution on mortality in 620 cities of 36 countries. Environment International 181:108258. doi:10.1016/j.envint.2023.108258
- Uttajug A, Seposo X, Phosri A, et al. (2024). Effects of coexposure to air pollution from vegetation fires and extreme heat on mortality in Upper Northern Thailand. Environmental Science & Technology. doi:10.1021/acs.est.3c08074
- de Rome L, Taylor EA, Croft RJ, et al. (2016). Thermal and cardiovascular strain imposed by motorcycle protective clothing under Australian summer conditions. Ergonomics 59(4):504. doi:10.1080/00140139.2015.1082632
- Copernicus Climate Change Service (2026). August was world's joint-hottest month on record. 10 September 2026. climate.copernicus.eu
- Field RD, van der Werf GR, Fanin T, et al. (2016). Indonesian fire activity and smoke pollution in 2015 show persistent nonlinear sensitivity to El Niño-induced drought. PNAS 113(33):9204. doi:10.1073/pnas.1524888113
- Koplitz SN, Mickley LJ, Marlier ME, et al. (2016). Public health impacts of the severe haze in Equatorial Asia in September-October 2015. Environmental Research Letters 11:094023. doi:10.1088/1748-9326/11/9/094023
- Xu R, Ye T, Yue X, et al. (2023). Global population exposure to landscape fire air pollution from 2000 to 2019. Nature 621:521. doi:10.1038/s41586-023-06398-6
- NOAA Climate Prediction Center (2026). ENSO diagnostic discussion, 10 September 2026. cpc.ncep.noaa.gov
- World Meteorological Organization (2026). El Niño set to become very strong, raising risks of extreme weather into 2027. 3 September 2026. wmo.int
- Kompas (2026). Indonesian fire haze worsens air quality in Singapore. 22 September 2026, reporting SiPongi and NEA Singapore data. kompas.com
- Indonesian Ministry of Health, via The Jakarta Post (2026). Respiratory problems climb past 170,000 cases in Indonesia. 24 September 2026. thejakartapost.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.