By law any Ag aircraft conducting aerial application for the purpose of spraying MUST have a smoker system on board to check the current wind conditions.
This allows the pilot to see current drift condition to better aid in on target application of the chemical. Much better letting some smoke out and knowing it is safe to spray as wind is blowing away from the road.
The smoke is a lot safer to let out then the chemical for the pollution aspect of the job.
Not sure about aerobatic aircraft at air shows. They have the same system set up.