It’s not picture perfect all over the island, all the time so it’s best to consult Purple Air that provides real time air measurements around the entire island.
https://www.purpleair.com/map?&zoom=9&lat=19.815527505170323&lng=154.94598183598066&clustersize=33&orderby=L&latr=1.2185727284505852&lngr=2.4785156920552254

Hawaii is huge so it’s easy to do many things and avoid Pele’s petulant behavior. Most often, the air is worst around Kailua on the Kona side of the island. Hilo, where it has over 300 inches of rain annually is lush and fresh and wet.

Highlights of our trip July 2018:

Rent a kayak from Bay Side Adventures https://www.baysideadventures.com
Skip a tour, get a $50 Groupon and kayak yourself to Captain Cook Monument for the best corals we saw on the island. We snorkeled/drifted down twice, towing the kayak and saw 4 turtles. Look in the bay for a cluster if kayaks because they are probably with the resident pod of Spinner Dolphins. It’s a large pod and even the very young porpoise and spin for your delight. They were circling when we visited so we didn’t have to paddle much.
Be sure to arrive early, before 8am, for easy parking, quieter waves, clearer air.

Sunset cruise with manta rays Kamanu Charters https://www.kamanu.com
We were on their large catamaran with an excellent crew. My sister was one of the first overboard and saw two manta rays immediately. I saw one before dark. Then we all grabbed the modified paddle-board outfitted with lights. There was steady number of mantas that somersaulted next to us, scooping up the plankton attracted to the lights. Bring a jacket for your return.

Budget tips: Costco has the cheapest gas. Also great sashimi and the usual items.
Excellent and reasonable sushi at Hayashi’s in Kailua; California roll $6.50. Visit Sam’s across the walkway for well drinks for $3.50 if you have a 45 min wait for your order like we did.