We do. In specific circumstances. The conspiracy of chemtrails is that all airplanes are releasing chemical agents other than combustion products for nefarious purposes. They are getting downvoted for suggesting the existence of a technology proves its constant use.

All Airplanes release chemicals - True (exhaust)

Airplanes can be used to release other chemical agents - True (crop dusting, air show smoke, cloud seeding, firefighting slurry)

United Flight 237 is used to spread chemical agents - False