This could also be because people don't go out as much - perhaps for fear of pollution.

Generally, it takes a lot of pollution to counter the positive health effects of active commuting. It was a while since I did the research, but someone estimated that you need to cycle for more than sixteen hours per day for the pollution to outweigh the cardio benefits in the 90th percentile of cities ordered by pollution. There was no city for which any mount of cycling would have made you worse off.

Granted, walking is probably slightly lower in intensity, but still.

Sure, that might be a factor but I honestly have no clue where to get out. Me and my wife just always land in the local restaurant which is 10-15 minutes away because there's literally nothing walkable in 30 mins foot radius.

So yeah we gave up and just go eat out. :|

So not only is the air bad, the walkable places are few and far between, strongly discouraging people to even go where the better air is or, as you said, just have the cardio from walking/cycling to improve your health.

Any chance you remember the source of this estimation ?

It’s been 15 years I commute daily by bike and worry about the side effect on my lungs… sometimes the road stinks so much, especially when too much trafic jam. Trying to slow my pace to breathe « less deep » but not a fun things to do.

Perhaps chronic illness is more prevalent now because in previous times you would have just died from those conditions. Anyway, if you're living 20 years longer on average than your grandparents, it stands to reason that your health is better for at least some of those years, since you're not dead.