What we plant often has a direct effect on our own health and the health of those near us. A pollen-producing male tree in our own yard will easily expose us to ten times more pollen than would a similar tree growing just down the block. This can be compared to second-hand smoke. It is possible to inhale some smoke from a person smoking a block or two away from you, but it is hardly the same as someone smoking right next to you. It is the same with plants. If your own yard is full of allergenic plants, then you will be exposed most.