Above an AQI of 100, the air itself hasn’t necessarily changed much since the day before, but what your body does with it during a hard set of hill repeats changes dramatically. Sprint up an incline eight or ten times and your breathing rate can leap from a resting 12 to 15 breaths a minute to something closer to 40 or 60, with air volume moving through your system rising from roughly 12 litres a minute to around 100 litres. When you exercise, your body uses more oxygen and produces more carbon dioxide, and to meet this demand, your breathing rate increases from a resting average of about 15 breaths per minute to somewhere closer to 40–60 breaths per minute during exercise. That single physiological shift is the reason a “bit of haze” on a moderate pollution day becomes something far more consequential once you start working hard.
Key takeaways
- Your breathing rate multiplies during hill repeats, but your nose’s natural filter gets bypassed
- Fine particles during intense exercise deposit in your lungs at 4.5 times the resting rate—and it gets worse the harder you push
- An AQI of 100 isn’t just “a bit of haze”—it’s the threshold where hard training becomes genuinely risky
Your nose stops doing its job
Quiet, resting breathing runs almost entirely through the nose, and the nose is genuinely good at its job: narrow passages, sticky mucus, tiny hairs, all designed to trap the coarser stuff before it gets anywhere near your airways. Push the pace, though, and most of us switch to mouth breathing without even noticing. Many of us transition to mouth breathing during exercise, bypassing the fine-tuned filtration mechanisms of the nasal passageway, further increasing the amount of particulate matter inhaled with each breath. The particles that matter most here are the fine ones. Larger PM10 particles are easily inhaled and tend to get embedded on the surface of the lungs or along the major airways of the upper lungs, but PM2.5, being smaller in size, is light enough to penetrate further into the lungs, where it can get deposited in the alveoli, the tiny sacs where oxygen exchange takes place. That’s the entire problem in one sentence: the particles small enough to dodge your nose’s defences are precisely the ones capable of reaching the gas-exchange surfaces of your lungs.
Depth of breath matters too, not just speed. Deeper breathing promotes peripheral particle pollution deposition in small airways and alveoli, while fast shallow breathing favours more central deposition in the trachea and large bronchi. Hill repeats produce exactly the deep, forceful breathing pattern that drives particles into the periphery rather than letting them settle harmlessly in the upper airway where a cough or a tissue can deal with them.
The numbers behind “just a bit of haze”
Researchers have modelled this rather precisely. Studies have shown that around 9% of inhaled fine particulate matter deposits in the lung, with roughly 6% reaching the alveolar region. That’s the baseline. Now factor in effort. One report found that moderate-intensity exercise, at a minute ventilation of 38 litres per minute, can produce a 4.5-fold increase in the fractional deposition of particulate matter compared with rest. Hill repeats push well past “moderate.” Modelling work using Monte Carlo simulations across exercise intensities found that compared with moderate and severe intensities, heavy exercise at 75 to 115 litres per minute produces greater lung deposition in the bronchiolar region, while at the severe intensity domain of 115 to 145 litres per minute there is greater deposition per minute of exercise. In plain terms: the harder and more explosively you’re working, the more efficiently your lungs are hoovering up whatever is floating in the air, and the deeper it settles.
There’s also a detail worth knowing if you train alongside a partner of the opposite sex. Modelling of urban cycle commuters found that females have higher bronchiolar and alveolar deposition than males regardless of ventilation rate, a difference linked to sex-related structural and functional differences in airway anatomy and breathing pattern during exercise. It’s not a huge margin, but it does mean two runners doing the identical session in the identical smog aren’t necessarily absorbing an identical dose.
Making sense of the number on your app
British runners checking an app before a session will often see two different scales, which causes genuine confusion. The 0-500 AQI familiar from American apps and many international ones classifies 0-50 as good, with it being safe to exercise outdoors, while 51-100, the moderate band, is acceptable but raised pollutant levels may affect breathing, especially for people with asthma or allergies. The American Lung Association is blunt about where the line sits for training: any day with an AQI of 100 or above should see exercise and other activities shifted indoors.
The UK’s own Defra Daily Air Quality Index works differently, running 1 to 10 rather than 0 to 500. On this scale, Low (1-3) means you can enjoy your usual outdoor activities, Moderate (4-6) suggests at-risk adults and children with lung problems should consider reducing strenuous physical activity, and High (7-9) calls for reducing strenuous exertion outdoors, particularly if symptoms appear. A DAQI reading of around 7 tends to correspond roughly to the AQI-100 territory referenced in the title, so if your local council’s index nudges into the High band before a planned session of repeats, treat it the same way you’d treat an AQI reading north of 100.
What to actually do about it
None of this means abandoning hard training whenever a city has a slightly grubby day. It means being selective about when you do the hardest sessions. Checking the forecast the evening before, rather than the pollution reading at the exact moment you lace up, gives you the option to swap a hill-repeat day for an easy indoor spin or a gentle jog through a park, since intensity is precisely the variable that determines how deep particles travel. Morning training isn’t automatically the safer choice either, since traffic-related pollutants can be surprisingly persistent depending on local conditions and time of day. If a hard session has to happen outdoors on a hazy day, choosing a route away from busy roads, keeping the effort just a notch below all-out, and accepting a slightly slower recovery run instead of your planned repeats is a reasonable trade-off, not a compromise on fitness. Your GP is the right person to speak to if you have asthma, COPD, or heart disease and want tailored advice on exercising during poor air quality days, rather than adjusting your training purely by feel.
Sources : sciencedirect.com | lung.org