I am considering a holidy in Aruba in November; however, I am concerned about the fact that there is an oil refinery on the island. Any comments about the air quality? Has it been a problem for asthmatics?

You won't notice it at all.
The refinery is many miles east from the Eagle and Palm Beach areas, and the breezes are constant. They typically blow the opposite direction from the beach, toward the refinery. There is no stagnant air over Aruba, that's for sure.

It's my understanding that the refineries are no longer operating. But in any case, we went to Aruba last week and the air was heavenly. Go, and don't give it another thought.

As far as I know the refinery is still operating, albeit at a reduced level.

We only noticed the odor in San Nicolas (e.g. Charlie's Bar). Baby Beach, while near the refinery, is upwind of it. I believe the prevailing winds are from the east, which would be TOWARD the resort area, but it's a long way from San Nicolas to Palm Beach.

You might want to avoid any daysails that visit the south side of the island (most don't) because you may encounter smoke there from the burning landfill.