After decades of gardening, I have never thought about the gender of trees and shrubs sold by nurseries. Turns out, they're almost all-male (customers don't like messy, stinky fruits on their shade trees and evergreen shrubs).
So our landscapes are packed with pollen makers. Knowing this doesn't help your allergies, nor is there a quick solution. But the nursery industry can slowly wean itself from selling trees that are notorious for provoking allergic reactions--junipers, oaks, birches.
There's a fellow in Canada who just drove across his nation inventorying trees and shrubs in public landscapes. The result: get out your handkerchiefs. He also points out that growing these plants on rooftops guarantees a wide-spread pollen shower every Spring. Yikes!
For more, including a scary video of a juniper wafting clouds of pollen into the air, see his website.
Hmm I think I've seen photomicrographs of juniper pollen. Each grain is covered with tiny sharp spikes. Wait, nope, that's ragweed pollen.