Thus van is a business which runs their engine all day every day

This van parks up every day and runs a business from within. A hair cutting business. They run the engine non stop all day long and remove the front number plate. The rear one can't be seen due to being so close to the bike storage. I have no issue with the parking or business but the fumes are polluting and they run it for 8 hours every day. They could use a storage battery instead which would solve the problem. When I tried to speak to them about it they were fairly threatening as they just stared me out till I left. I wasn't afraid at all but they aren't up for a talking cure. Why do the parking and environmental officers ignore them? Surely they should be either ticketed or fined for the engine running constantly?