Rucking in the Haze: Why PM2.5 Matters
Updated: Sep 30
For ruckers (and exercisers) in Singapore, haze presents a particular problem: the harder and longer you exercise outdoors, the more polluted air you breathe in.
PM2.5 refers to fine particulate matter measuring 2.5 micrometres or less in diameter. These particles are small enough to travel deep into the lungs, reaching the alveoli where oxygen passes into the bloodstream. They are not simply "dust" - PM2.5 can contain combustion-derived chemicals, metals and other substances capable of triggering biological reactions.
As ever, please talk to your doctor or medical practitioner most familiar with your medical history before implementing any changes in diet, exercise or lifestyle, especially if you are under treatment. Links to all studies at bottom of page.
What does PM2.5 do to the body?
Once deposited in the lungs, PM2.5 can stimulate oxidative stress and inflammation. Immune cells respond to the particles and release inflammatory signalling molecules. Reactive oxygen species can also damage cellular structures and activate further inflammatory pathways.
Importantly, the effects don't necessarily stop in the lungs. Inflammatory signals can enter the circulation, while exposure can affect the cardiovascular and autonomic nervous systems. Research has identified pathways involving vascular inflammation, impaired endothelial function, changes in blood pressure and increased blood-clotting tendency. These mechanisms help explain the associations between particulate pollution and cardiovascular disease.
Why are smaller particles more concerning?
Size matters because larger particles are more effectively trapped in the nose and upper airways. PM2.5 can penetrate much further into the respiratory system.
There is also an even smaller category, ultrafine particles (PM0.1), which are less than 0.1 micrometres. Their very small size and large surface area relative to their mass mean they are receiving increasing scientific attention.
But particle size isn't the whole story, composition matters too. PM2.5 from traffic, biomass burning, industrial combustion and other sources can contain different mixtures of chemicals, so particles with the same mass concentration aren't necessarily biologically identical.
Why rucking changes the equation
The important concept for anyone exercising during haze is inhaled dose. Your exposure isn't determined solely by the concentration of PM2.5 in the air. It also depends on:
PM2.5 concentration × Breathing rate × Duration = Inhaled dose
At rest, you breathe relatively little air. During a brisk ruck, your ventilation increases considerably. You may also shift towards mouth breathing, reducing the filtering provided by the nose. A prolonged ruck therefore combines two factors that increase exposure: you are breathing more air and doing so for longer.
A systematic review of exercise and air pollution found evidence that exercising in polluted air can influence respiratory and cardiovascular responses, while research examining inhaled pollution dose highlights the importance of increased ventilation during physical activity.
This doesn't mean that exercise suddenly becomes harmful whenever PM2.5 is detectable. Exercise has substantial health benefits. The issue is that a hard 90-minute ruck is a very different pollution exposure from sitting outside for 90 minutes.
Built to Ruck
If you are training for a loaded march, across 20 units our online course "Built to Ruck The Science and Practice of Rucking" covers everything the research tells us about load carriage training, pacing, heat management, hydration, fueling, foot care and more.
It is built for ruckers and tabbers at every level of experience, and draws directly on the peer-reviewed research.
What should a rucker do?
When PM2.5 rises, think about managing your pollution dose rather than simply asking whether you can exercise. If conditions deteriorate, reducing the intensity or duration of a session reduces the amount of polluted air you inhale. Moving a hard training session indoors or postponing it can reduce exposure further. This is particularly relevant to Singapore because NEA guidance takes account of both air quality and the length and intensity of outdoor activity. During elevated pollution, healthy people are advised to reduce prolonged or strenuous outdoor physical exertion at specified air-quality levels.
For ruckers that provides a useful principle:
When the air gets worse, don't just look at the PM2.5 number. Look at the dose you're creating by exercising in it, and don't be afraid to back-off.
A short easy walk and a hard, prolonged ruck may occur in exactly the same air, but they don't produce the same inhaled dose. For a training group, the practical response to worsening haze may therefore be to reduce pace, shorten the route, reduce training intensity, move indoors or postpone the session.
The objective isn't to fear every particle of pollution; it is to avoid unnecessarily large doses when air quality is poor. And that's an important distinction: exercise is good for health, but exercising hard in polluted air isn't necessarily the best time to chase a training target.
Stay Healthy,
Alastair
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Related Studies and Resources
Josa-Culleré A, Cakmak-Onal A, Gimeno-Santos E, Alcaraz-Serrano V, Buekers J, Delgado-Ortiz L, Marin A, Rodríguez-Chiaradia DA, Garcia-Aymerich J, Rivas I, Koch S. Effects of the inhaled dose of air pollution on health: a systematic review. Eur Respir Rev. 2025 Sep 26;34(177):250042. doi: 10.1183/16000617.0042-2025. PMID: 41005809; PMCID: PMC12464717.
Feng S, Huang F, Zhang Y, Feng Y, Zhang Y, Cao Y, Wang X. The pathophysiological and molecular mechanisms of atmospheric PM2.5 affecting cardiovascular health: A review. Ecotoxicol Environ Saf. 2023 Jan 1;249:114444. doi: 10.1016/j.ecoenv.2022.114444. Epub 2022 Dec 21. PMID: 38321663.
Qin F, Yang Y, Wang ST, Dong YN, Xu MX, Wang ZW, Zhao JX. Exercise and air pollutants exposure: A systematic review and meta-analysis. Life Sci. 2019 Feb 1;218:153-164. doi: 10.1016/j.lfs.2018.12.036. Epub 2018 Dec 21. PMID: 30582950.
Other
Giles LV, Thomson CJ, Lesser I, Brandenburg JP. Running Through the Haze: How Wildfire Smoke Affects Physical Activity and Mental Well-Being. J Phys Act Health. 2024 Nov 6;21(12):1435-1445. doi: 10.1123/jpah.2024-0305. PMID: 39504954.
Ho AFW, Hu Z, Woo TZC, Tan KBK, Lim JH, Woo M, Liu N, Morgan GG, Ong MEH, Aik J. Ambient Air Quality and Emergency Hospital Admissions in Singapore: A Time-Series Analysis. Int J Environ Res Public Health. 2022 Oct 16;19(20):13336. doi: 10.3390/ijerph192013336. PMID: 36293917; PMCID: PMC9603816.
Hahad O, Kuntic M, Frenis K, Chowdhury S, Lelieveld J, Lieb K, Daiber A, Münzel T. Physical Activity in Polluted Air-Net Benefit or Harm to Cardiovascular Health? A Comprehensive Review. Antioxidants (Basel). 2021 Nov 8;10(11):1787. doi: 10.3390/antiox10111787. PMID: 34829658; PMCID: PMC8614825.
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