Shortwave, a short daily science podcast looked at how eczema and other allergies may be aggravated or caused by air pollution in some people. Here is a link to the show with its transcript.
My wife’s sister lives here in Indiana, the state with the 5th worst Air Quality Index in the nation(1), and travels quite a bit. She has a number of allergies and when she is traveling they rarely bother her, but back home again in Indiana they flare up.
Suggest that your sister spend her summer vacation at Olympic National Park, on the pristine peninsula where I live. The air is crystalline pure and any temperature over 80 degrees is considered a heat wave.
Yes, my asthma is much better here. 
Wendy
I have a similar problem in reverse. When I leave the cozy confines of coastal NH - southern Maine I have all sorts of sinus and throat issues.
JimA
There are few regions of the country that offer the majestic beauty of the Pacific Northwest. With mountains, forests, rivers, bays, and access to the ocean, this is a paradise for anyone who loves the outdoors.
But it can also be a difficult place for those with seasonal allergies. If you suffer from allergic rhinitis, you understand just how difficult the pollen season can be. With knowledge and a plan, however, you can control and reduce your allergic reactions.
When you’re used to almost constant humidity, spending some time in a dry place causes some suffering. And when you’re used to almost constant dryness, spending some time in a humid place causes some suffering. Probably because humans are somewhat adaptable, but that adaptation takes time.