Should you exercise outdoors during wildfire smoke?

Wildfire season raises an obvious question for anyone who trains outdoors: at what point does bad air make exercise do more harm than good? Two physicians who host the Barbell Medicine podcast worked through the actual research, and the short answer is that the tipping point is much higher than most people assume, though the picture changes depending on whether the exercise is lifting or cardio.

What the air quality numbers actually mean

PM2.5 refers to particles smaller than 2.5 microns, small enough to bypass the nose and throat and reach deep into the lungs and bloodstream. On a clean day, that number sits around 5 micrograms per cubic meter; wildfire smoke can push it past 100, sometimes above 150. The Air Quality Index (AQI) translates that concentration into a 0 to 500 scale, where anything under 50 is considered good and above 100 starts to raise concern. A frequently cited 2016 modeling paper estimated that roughly 90 minutes of cycling at 100 micrograms per cubic meter is the point where pollution exposure could outweigh the benefit of the exercise, but that paper modeled ongoing city commuting pollution, not a short, heavy wildfire smoke event, and its own authors gave three different numbers for the same tipping point across different sections of the paper.

The evidence leans toward still exercising

A more direct dataset comes from a study in BMC Medicine that combined seven cohorts, about 1.5 million adults followed for a median of 12 years. Below an annual average of 25 micrograms per cubic meter, roughly an AQI of 80, meeting exercise guidelines cut all-cause mortality by about 30%. Above that threshold, in cities with worse air than Los Angeles on average, the same exercise habits still helped, but the benefit dropped to roughly 12 to 15%. Data extended up to a dose equivalent to an AQI of about 137 and exercise remained protective throughout. Separately, a modeling study across six cities, including Beijing and Delhi, tested what happens when active commuters switch to transit or working from home on high pollution days: restricting activity did not lower mortality in any of the six cities, and in Beijing and Delhi the model actually showed mortality rising with reduced activity.

Lifting is different from cardio here

Minute ventilation, the total volume of air moved per minute, is the key variable connecting exercise intensity to pollutant exposure. At rest it's roughly 10 liters per minute, walking pushes it to about 23, and cycling to around 43. Research on resistance training, including in people with COPD, found that breathing rate barely changed between sets done at 40% and 80% of a max effort, even though perceived effort rose substantially. That means indoor lifting carries meaningfully lower particulate exposure risk than outdoor cardio during a smoke event, with one caveat: high-rep circuit-style training that pushes ventilation up starts to look more like conditioning work from an exposure standpoint.

Practical guidance for a smoky week

For a healthy person, the recommendation is to favor exercise most of the time, shift outdoor cardio indoors when possible, and reserve real caution for days bad enough that leaving the area becomes a consideration. Indoor air isn't a perfect shield either; during past California wildfires, indoor particulate matter in monitored buildings nearly tripled despite filtration, simply because outdoor air quality got so much worse. For people with pre-existing lung or cardiovascular disease, including COPD, asthma, or a history of hospitalization during past smoke events, the calculus shifts toward strongly favoring indoor exercise and closer symptom monitoring rather than skipping activity altogether.

Effective reps: how much do fractional sets actually count?

The second half of the episode tackles a training theory question: if a set only "counts" toward muscle growth when taken within five reps of failure, does a triceps' secondary contribution during a heavy bench press actually count, and should a warm-up set count toward the primary muscle? A 2024 meta-regression pulling 140 results from 26 studies on muscle growth found the relationship between proximity to failure and growth is closer to a straight line than a hard cutoff, running continuously from failure out past 20 reps in reserve. The difference between stopping at 5 reps in reserve versus going to failure was small, and the confidence range included the possibility of no difference at all. A separate analysis of 67 studies and roughly 2,000 people modeled indirect (fractional) sets as counting zero, half, or one full set toward growth, and found that counting them as half a set fit the data best, meaning secondary muscle involvement provides real but reduced stimulus.

What this means for programming

For hypertrophy, working somewhere around 7 to 9 reps in reserve is a reasonable general target for most sets, particularly on machines and isolation work, since the growth curve near failure is a gradient rather than a switch. For strength, staying further from failure, roughly 4 to 6 reps in reserve, tends to preserve bar speed and force quality better, provided the load is heavy enough, generally above 60 to 70% of a one-rep max, to drive the adaptation in the first place. Both hosts agree that individual variation in training response outweighs fine-tuning reps in reserve by one or two, and that missing workouts or sets consistently over a year matters far more than optimizing proximity to failure on any single set.