DENVER — Oh say, can you see across the Grand Canyon? Not as well as you used to on some days.

The question of how clean the air is in the American West has never been an easy one to answer, strange to say. And now scientists say it is getting harder, with implications that ripple out in surprising ways, from the kitchen faucets of Los Angeles to public health clinics in canyon-land Utah to the economics of tourism.

It is at least partly about dust, something that has been entwined with Western life for a long time, and now appears to be getting worse.

In the 1800s, the high deserts stretching west and south of the Rockies became a famed destination for respiratory sufferers like “Doc” Holliday, the gunfighter-dentist (and tuberculosis patient), who came to take what was called the desert cure.

But cattle and sheep by the tens of thousands were at the same time trampling across those fragile landscapes, loosening once stable soils to the four winds and creating a kind of parallel — but equally true — Western mythology around the tumbleweed and the dusty trail.