Head for the hills

Clean-Air Credentials: According to Russell Schnell, Hawaii can receive significant pollution from China but still it manages to record the cleanest air on earth. How? By rising above it, literally. Pollution particles stop climbing when they meet the inversion layer, an atmospheric boundary of warmer air. At 11,145 feet, the Mauna Loa observatory is above the clouds and, therefore, virtually free of pollution.

Take a Breather: To sample the subtle difference, start at sea level, paddling a traditional outrigger canoe. Next, hop in a car (a hybrid, of course) and go where the air is truly clear: the Mauna Loa scenic trail—one of the few locations on earth where it’s possible to drive above the inversion layer—and hike the six miles to the summit.

(http://www.travelandleisure.com/slideshows/
the-worlds-cleanest-air/2)

Meanwhile, far below the inversion layer and near the coast where most of the island's population lives, most of the western side of the island is regularly smothered in a thick grey haze of airborne volcanic particulates. But even during regular tradewind weather, the air blowing in from sea on the eastern cape is also usually very clean and refreshing. It's when the persistent winds stall that islands as far away as Oahu begin feeling the effects of the volcanic pollution and Honolulu begins to resemble a typical North American city.