This happens almost every day in my neighborhood, usually in the late afternoon or early evening. It appears to be an attempt to kill mosquitoes. It might kill some of them, but not all. Not by a long shot.
A guy runs around with a fogging machine. He shoots the stuff into the drainage sewers, where I guess the mosquitoes hide. It leaks out from drains further down the street.
The smell is pretty noxious. Every time we hear the lawnmower-like sound, we run around and turn off all the air conditioners. We don’t want the fumes to get sucked into the house. If it kills mosquitoes, it can’t be good for other living things. Like us, for example.