Poison Ivy Itchier

I've been wondering what it is that has been making my skin itchy recently. Since I haven't been to the woods i can cross out poison ivy as a culprit. This article although speculative upon higher co2 levels makes me wonder what is happening in normal non-poisonous plants or plants which cause mild reactions in allergic people? Like say a hive reaction to ragweed, or dandelion spores? Maybe increased co2 levels boost it's allergic producing properties... it gets the whole city itching, sneezing and whatever!