Back in the day the only weed available to me was crappy brown brick weed likely from Mexico.
I can remember getting headaches very often from that stuff.
Does anyone else have recollections of that? I wonder
what it was about it that gave the headaches. Pesticides?
Discuss. 
Sitting in a car, “smoking out” with the windows rolled up for a couple hours will do it. Drive in theater days.
Most likely, pesticides. The govt’s of US and Mexico used used Paraquat in eradication efforts in the 70’s, and the farmers used all sorts of bad stuff on their crops.
There is also a terp or cannabinols or something that can give me headaches. Cuz it happens still and pesticide free homegrown is all I smoke nowadays. It seems to frequently associate with whatever causes the burnt out, blah feeling that some strains give me…
My son calls that “hot boxing”
Or in the extreme, especially in hot climates (AZ) we called it a victory hotbox… Smoke several bowls, joints, blunt with windows up and AC off till everyone is hot and sweaty, eyes burning, can’t see out through the smoke. Then everyone simultaneously rolls all 4 windows down right as we pull out, with smoke billowing out the windows, and everyone hanging out saying “Victory” as fresh air rushes in…
Fond foggy memory’s of HS at the start of this millennium.
Sounds like Moldanene
We were doing Hawaiian hotboxes box in hs. Blast a shower on full hot, steam the room out. Then proceed to get ripped. The goofy shit we did back when 
I have fond memories of using a frisbee or double album cover to de-seed my brown Mexican brick weed. I never had sensimilia until college and even then it was a tiny bit of Northern Lights a buddy brought back from Amsterdam after his semester abroad in Europe. I can remember some bad headaches and the reddest eyes you’ve ever seen. I didn’t know any better and we were just happy to have anything during the late summer drought when weed dried up in our area. It’s crazy to see how far it’s come in just 40 years. Now I hope to find seeds and would kill for some of those old Mexican seeds.
Search for a strain high in tylenolilene 
i always thought headaches from brick weed came from not cleaning completely from seeds and you ended up smoking a grinded seed or a whole seed (i remember they would pop inside the joint lol - and i would have a headache).
there are a lot of urban legends regarding brick weed, a common one would be that dealers added pesticide when dried to give a sense of more potent weed, but would give you headaches or chills. dunno if thats tru.
I got headaches back in the day a couple of times but only from black market weed. I rarely got it from organic grown weed. @Floyd We call it hotbox as well done it sometimes a couple of years ago, I personally don’t see any use in it other than making your throat hurt and being unhealthy af.
Paraquat, back in the day.
Oh yeah, I remember the weed from the 70’s. One particular green Schwag we actually called ‘Headaches’. That being said commercial weed actually improved in the late 70’s onward as relatively decent Columbian weed totally displaced the commercial Mexican
I do not think any one single reason is the culprit.
From the things I saw roll out of kilo’s when cut down, would have you eye rolling into a coma!
In my first couple buys in 1970, just a copping a couple of fingers, and I would whine about the seed.
When I started getting ounces, I whined more about the seeds.
My first kilo,(blue cello) had a bird body, encased in SEEDS, in the center of it!
There was no separating the males and females, or ridding the plant of all the starchy fan leaves!
I only ever got to buy a bale, on 2 occasions, and I was not there for the hack n sake.