Im trying to find some info on inverted poses and smoking but i cant find any. Are you in danger of getting toxins to your brains if you practice for instance shoulder stand or the plow pose when youre a smoker?
Toxins from smoking are transferred from your lungs to your blood stream. Your brain needs a rich supply of blood, so whether you go upside down or not, the toxins go to your brains.
Uh best not to smoke during inverted poses. 
Generally speaking the risk factors in asana relate more specifically to alignment and action rather than what’s in your bod. The rest is common sense - like don’t do strenuous exercise in high heat for long periods of time unless you’ve checked with your cardiologist:
You already are at risk for many diseases from smoking so do not do any physical activity unless you have a doctors permission. Namaste.