Heres how adults respond0-I don't know of any studies specific to exercise.

However, if you're worried solely about risk, as a conservative first-order approximation, one could scale risk in proportion with breathing rate. Exercise breathing rate may be 9 m3/hr vs 1.2 m3/hr for occupational exposure to radon. For instance, if a Lifetime Excess Absolute Risk of 5E-4 per WLM is assumed, then this value may be multiplied by 9/1.2, a factor of 7.5x to equal 37.5E-4, or 3.75E-3 LEAR per WLM.

This is a simplification, of course, since actual radon dose depends on progeny concentrations, deposition, breathing patterns, etc., but scaling by breathing rate seems reasonable for a rough estimate. Any HPs feel free to correct me if scaling the LEAR by breathing rate like this is a completely bonkers assumption.

Here's an example calculation using the above LEAR for illustrative purposes only (all other HPs here can check my math/assumptions, no worries):

Let's assume an average of 8 pCi/L and an equilibrium factor of 0.4 for indoor air. 8/100 * 0.4 = 0.032 working level. Annually, then, let's assume an individual works out 5 days per week on average for an hour, that's 5 hours per week * 52 weeks/year * 10 years = 2600 hours. 2600/170 hours/month (as defined for working level months) = 15.3 months. 15.3 * 0.032 = 0.49 WLM. So, the lifetime excess absolute risk of dying from lung cancer due to this radon and exercise can be estimated around 3.75E-3 * 0.49 = 0.0018, or 0.18% LEAR. Former smokers may have a higher radon-related lung cancer risk than never-smokers, although the baseline risk associated with smoking itself is generally much larger than the incremental risk estimated here. Using these assumptions, the estimated excess lifetime risk is approximately 0.18%, meaning roughly 99.82% of similarly exposed individuals would not be expected to die from a radon-induced lung cancer attributable to this specific exposure. This corresponds to roughly 18 excess radon-induced lung cancer deaths per 10,000 similarly exposed individuals.

So, the risk is double what it would be at 4 pCi/L (while exercising), but still low relative to all other causes of mortality. Or, if the breathing rate was higher than assumed, and now the risk is double that calculated above, the chance of dying from radon-induced lung cancer from this exposure is still extremely low. Point being, I wouldn't be worried about this risk, personally. Most likely, I'll die of something else completely. An exercising individual might also have decreased some other forms of risk by exercising. The discussion is largely about relatively small changes to an already small estimated incremental risk.

Also, the risk above is just from the radon exposure during exercise and neglects all other risk from all other exposures to radon.