This June had record rainfall where I live in western Montana, which means record weed growth.
As a result I have spent many hours pulling Canadian Thistle and other noxious weeds and have decided to burn them in my fire pit rather than haul them to the dump like I usually do.
Is there any danger breathing the thick smoke from a fire of burning weeds?
FWIW, I don’t think there there is any poison oak on my property to worry about. 
For the most part, there wouldn’t be danger. But if there was any chance of even a little poison ivy being in there (or some other toxin-containing weed), breathing or getting smoke/residue on your skin could cause a lot of trouble. There is a chilling 19th century account in one of Bernard Roueche’s stories, about a traveler whose train stopped in an area where they were burning brush, and the smoke came in through the open windows. It turned out there was poison ivy being burned, and the resulting extensive reaction suffered by the victim was severe, including temporary blindness.
Burning also degrades air quality and risks fires (it can’t be all that wet in your part of the world now).
Much better to compost the weeds rather than burn them - as a bonus, you get a useful soil additive.
if you compost then you want to do hot compost to destroy the viability of all the plant material and seeds.
Canada thistle I wouldn’t worry about, but if you have any wild parsnip in your burn pile you might want to reconsider. I’m not sure how burning this nasty invasive would work out, but I’m well aware of its effects on skin contact with sun exposure. While mowing the lawn early this year I apparently got some plant matter on the skin of my forearms, and I’ve been enjoying the resultant severe rash and sun sensitivity ever since—and I had a mild case of it. Just do an image search on “wild parsnip rash” to see the more severe cases. (Warning: NSFTS.)
Oh totally poison oak and ivy make the best fires. Be sure to stand right next to the conflagration so you get the most of that great smoky smell.
Thanks Jaledin. Snarky comments aren’t really needed here.
I am well aware of not burning poison oak and ivy and I only pick specific types of weeds, like thistles, that I believe are safe to burn… but I wanted to make sure.
In the future I will consider composting instead of burning.