Rain does reduce air pollution, but unevenly. It clears coarse particles better than the fine PM2.5 that reaches deep into the lungs, it needs enough rain to matter, and light winter rain has been linked to higher readings. A wet ride is usually cleaner than a dry one in the same week, but not clean.
The first drops hit the visor and the smell of the road changes. Dust goes quiet, the exhaust haze lifts a little, and it feels like the city has been rinsed. Riders ask a fair question at that moment: is the air I am breathing now actually better?
Measured studies give a split answer. Rain is one of the main ways the atmosphere cleans itself, yet the size of the effect depends on how hard it rains, how long, and what was in the air before the clouds opened. This post sets wet rides against dry ones using five peer-reviewed studies and the WHO guideline values.
Key takeaways
- In Guangzhou and Xi'an, rain washed out the coarse fraction (PM2.5 to 10) better than PM2.5, and the PM2.5 drop lasted 3 and 6 days (Guo et al., 2016).
- Rainfall needed to pass a threshold before it helped: 7 mm in Guangzhou and 1 mm in Xi'an (Guo et al., 2016).
- Light winter rain in Qinhuangdao raised pollutant concentrations instead of lowering them (Liu et al., 2020).
- In Jakarta, PM2.5 moved opposite to rainfall in every season measured (Istiana et al., 2023).
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.
Rainfall washout in Guangzhou and Xi'an
- What they measured
- Daily PM2.5 and coarse particle (2.5 to 10 micrometre) readings with weather data in two Chinese cities, 2013 to 2014, using a distributed lag model.
- What they found
- Rain washed out the coarse fraction better than PM2.5. Thresholds were 7 mm in Guangzhou and 1 mm in Xi'an, and the PM2.5 drop lasted 3 and 6 days.
- What it can't tell you
- Daily data from two Chinese cities, so it cannot show what happens within a single shower or on a motorcycle.
Precipitation and wind in Qinhuangdao
- What they measured
- Hourly rain, wind, PM2.5 and PM10 in one Chinese city, 2016 to 2018.
- What they found
- Rain scavenged PM10 more than PM2.5, the effect rose with intensity and duration, and light winter rain increased concentrations.
- What it can't tell you
- One coastal city; the abstract does not give a full mechanism for the winter increase.
Monsoon and winter particles in Delhi
- What they measured
- PM10, PM2.5 and PM1 mass in Delhi from August to December 2007, with washout modelled by size.
- What they found
- Readings ranged from 20 to 180 µg/m³ in the monsoon and 100 to 500 µg/m³ in winter, and PM10 washed out differently from PM2.5 and PM1.
- What it can't tell you
- One city and one year, with the three size fractions reported together.
PM2.5 and weather in Jakarta
- What they measured
- PM2.5, other pollutants and weather at two Jakarta sites, analysed by season with correlation and convergent cross mapping.
- What they found
- PM2.5 was negatively correlated with precipitation, relative humidity and wind speed in all seasons.
- What it can't tell you
- Correlation and causality modelling, not a direct measurement of rain washing particles out.
What the studies actually measured
None of these studies put a monitor on a motorcycle in the rain. They are all fixed-station or fixed-site records of ambient air, taken a few metres above the ground. That is the honest limit of this post, and it is worth stating before any number is quoted.
Guo and colleagues used two years of daily readings (2013 to 2014) from Guangzhou and Xi'an. They fitted a model that links each day's rainfall to PM2.5 and to the coarse fraction, PM2.5 to 10, over the following days (Guo et al., 2016).
Liu and colleagues worked at hourly resolution. Their record covers 2016 to 2018 in Qinhuangdao, so they could compare air before and after individual showers (Liu et al., 2020).
Tiwari and colleagues measured PM10, PM2.5 and PM1 in Delhi from August to December 2007, across the monsoon and into winter (Tiwari et al., 2012). Istiana and colleagues analysed two sites in Jakarta, matching PM2.5 against rainfall, humidity and wind in each season (Istiana et al., 2023).
Kusumaningtyas and colleagues looked at Jakarta over 2000 to 2016 and tied the long-run improvement partly to a rising trend in rainfall (Kusumaningtyas et al., 2018).
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Rain clears coarse dust faster than fine smoke
Two of the studies agree on the main result. Rain removes the larger particles more efficiently than the small ones.
Guo and colleagues found a better washout for the 2.5 to 10 micrometre fraction than for PM2.5 (Guo et al., 2016). Liu and colleagues reported the same pattern: the scavenging effect on PM10 was greater than on PM2.5 (Liu et al., 2020).
Tiwari and colleagues went a step further and modelled how each size fraction decays during rain. They described a clear difference in washout between PM10 and the smaller PM2.5 and PM1 particles over different rainfall durations (Tiwari et al., 2012).
Why a raindrop catches big particles and misses small ones
Do not read a clean-looking sky after rain as clean air for fine smoke. The big dust is gone first. The PM2.5 you cannot see is what stays.
You need enough rain before it helps
Rain is not a switch. Guo and colleagues estimated that rainfall had to pass a threshold before PM2.5 and the coarse fraction fell: 7 mm in Guangzhou and 1 mm in Xi'an (Guo et al., 2016). Rain below those amounts was not enough for their model to register a washout effect.
Liu and colleagues found the same logic in hourly data. The change in PM2.5 across a shower tracked three things: the PM2.5 level before the rain, how hard it fell, and how long it lasted (Liu et al., 2020).
The starting level matters. For PM10, Liu and colleagues found that the higher the concentration before the rain, the more the rain removed (Liu et al., 2020).
Light winter rain can make readings worse
This is the result that surprises people. In Qinhuangdao, light rain in winter increased pollutant concentrations (Liu et al., 2020). The paper reports the finding without a full mechanism in its abstract, so treat the cause as open.
The practical point is simple. A drizzle on a cold, stagnant morning is not evidence of cleaner air. It shows why one rule of thumb should not be trusted.
Wet season versus dry season
The clearest rider-sized comparison is the season, not the shower.
In Delhi in 2007, concentrations of PM10, PM2.5 and PM1 ranged from 20 to 180 µg/m³ during the monsoon and from 100 to 500 µg/m³ in winter, with a spike to 1,200 µg/m³ in November during Deepavali fireworks (Tiwari et al., 2012). Those ranges pool three particle sizes, so read them as a seasonal contrast, not a PM2.5 forecast.
Delhi particle ranges, monsoon versus winter (µg/m³)
Range of PM10, PM2.5 and PM1 readings, August to December 2007.
| Season | Low (µg/m³) | High (µg/m³) |
|---|---|---|
| Monsoon | 20 | 180 |
| Winter | 100 | 500 |
Jakarta points the same way. Istiana and colleagues found PM2.5 negatively correlated with precipitation, relative humidity and wind speed in every season they tested, at two sites (Istiana et al., 2023). That is a correlation, not a measurement of washout, and the same paper found that rising humidity in September to November coincided with higher PM2.5.
Over the long run, Kusumaningtyas and colleagues linked Jakarta's improving air between 2000 and 2016 partly to an increasing trend in rainfall, alongside policy changes (Kusumaningtyas et al., 2018).
Wet ride versus dry ride, side by side
| Question | What the studies found | What to assume on the bike |
|---|---|---|
| Coarse dust or fine smoke? | Coarse fraction washed out better than PM2.5 (Guo et al., 2016; Liu et al., 2020) | Dust drops first. Fine particles linger. |
| A short shower? | Thresholds of 1 mm and 7 mm in two cities (Guo et al., 2016) | A few drops may change little. |
| Heavy, long rain? | Removal rose with intensity and duration (Liu et al., 2020) | Cleaner air, but only while and shortly after it falls. |
| Light winter rain? | Pollutant levels rose in Qinhuangdao (Liu et al., 2020) | Do not assume relief. |
| How long does it last? | PM2.5 stayed lower for 3 and 6 days (Guo et al., 2016) | The benefit fades within a week. |
| Wet season versus dry? | Monsoon ranges well below winter ranges in Delhi (Tiwari et al., 2012) | Wet-season rides are usually the cleaner ones. |
What rain does not fix
Rain changes the air in the sky. It does not change the air in your lane. Exhaust from the truck ahead is released close to your face, and a shower does not stop engines from running.
No study here measured riders in the rain, so there is no number for how much a wet ride cuts what you breathe in traffic. Fixed stations also sit away from the kerb. The seasonal gap in Delhi and the Jakarta correlations describe the background. Your exposure is background plus whatever is directly ahead.
Haze events work differently again. Smoke arrives with the wind from burning land, and monsoon winds shape where it comes from. Rain on the day helps, but the source decides how long the smoke keeps coming. The live haze map for riders shows what the air is doing right now.
What to do on a wet ride
- Check the reading, not the sky. Open a live air quality map. If PM2.5 is still above 15 µg/m³, the WHO 24-hour guideline, the rain has not cleared the air (WHO, 2021).
- Wait out the first drizzle. Short, light rain may be under the washout threshold. Heavier, longer rain does more.
- Time the dry spell after a storm. The PM2.5 drop in two cities lasted 3 and 6 days (Guo et al., 2016), so the best window is the day or two after heavy rain.
- Keep your own filtration on. The lane around you stays the same wet or dry, so a filter on your helmet still earns its place.
Rain makes the background better for a few days. It does not take the traffic out of your lane.
Frequently asked questions
Does rain reduce air pollution?
Yes, on average, but unevenly. Studies in China found rain lowered PM2.5 and cleared coarse particles even better, with effects lasting several days (Guo et al., 2016; Liu et al., 2020).
Is it safe to ride a motorcycle in rain when the air is polluted?
Rain lowers some particles, but it does not make polluted air safe. Check a live PM2.5 reading and compare it with the WHO guideline of 15 µg/m³ for 24 hours (WHO, 2021).
Does light rain clean the air?
Not reliably. The Guangzhou and Xi'an model needed rain above a threshold (Guo et al., 2016), and in Qinhuangdao light winter rain was followed by higher pollutant levels (Liu et al., 2020).
How long does air stay cleaner after heavy rain?
In Guangzhou and Xi'an the PM2.5 reduction lasted 3 and 6 days (Guo et al., 2016). Expect the benefit to fade within a week.
Is the rainy season better for riders than the dry season?
In Delhi and Jakarta the data point that way. Monsoon-season readings ran well below winter readings in Delhi (Tiwari et al., 2012), and Jakarta PM2.5 moved opposite to rainfall (Istiana et al., 2023).
Sources
- Guo L, Zhang Y, Lin H, Zeng W, Liu T, Xiao J, Rutherford S, You J, Ma W. (2016). The washout effects of rainfall on atmospheric particulate pollution in two Chinese cities. Environmental Pollution, 215. Open study
- Liu ZQ, Shen L, Yan C, Du J, Yang L, Zhao H. (2020). Analysis of the influence of precipitation and wind on PM2.5 and PM10 in the atmosphere. Advances in Meteorology. Open study
- Tiwari S, Chate DM, Pragya P, Ali K, Bisht DS. (2012). Variations in mass of the PM10, PM2.5 and PM1 during the monsoon and the winter at New Delhi. Aerosol and Air Quality Research, 12. Open study
- Istiana T, Kurniawan B, Soekirno S, Nahas AC, Wihono A, Nuryanto DE. (2023). Causality analysis of air quality and meteorological parameters for PM2.5 characteristics determination: evidence from Jakarta. Aerosol and Air Quality Research. Open study
- Kusumaningtyas SDA, Aldrian E, Wati T, Atmoko D, Sunaryo S. (2018). The recent state of ambient air quality in Jakarta. Aerosol and Air Quality Research. Open study
- World Health Organization (2021). What are the WHO air quality guidelines? Open page
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.