These are just examples of the sorts of answers you might think of

X → Y : Ozone damages the lungs and predisposes people to asthma.

Y → X : Asthmatics are more likely to move to built up, and therefore polluted areas, where doctors are readily available. (This is a bit more like Z → X & Y, with Z as the availability of doctors)

Z → X & Y : Asthma is caused by the immune system of children in industrialised (and therefore polluted) countries going haywire because of a lack of healthy exposure to dirt during childhood. (Z is the polluted but over-sanitised lifestyle of the developed world).