Interesting question! It seems that the benefits of cycling for cardiovascular health are well-established, but the potential respiratory risks from air pollution are a valid concern. One perspective is that the benefits of exercise may outweigh the risks, but it's important to note that this might not be the case for everyone, especially those who are already vulnerable to respiratory issues.
As for the minimum intensity and duration thresholds, research on this topic is still evolving. Some studies suggest that cycling at a moderate intensity for at least 30 minutes a day could have a positive impact on lung health, while others argue that even low-intensity exercise can offer benefits.
In the end, it might be a matter of balancing the benefits of cycling with the risks of air pollution exposure. Cyclists could consider riding in less-polluted areas, using air quality apps, or wearing masks to minimize their exposure.