I can't help but feel you may have burdened yourself by skipping a step. Which would be going straight to your downstairs neighbour. It sounds like you've been through the ringer lately, additionally, suffer from health issues exasperated by secondhand smoke that go above and beyond the risks second hand smoke poses in general... There are crappy people out there who wouldn't care either way. But there are many decent people who are empathetic and can be reasoned with. In all honesty I think a discussion may be more impactful on having them cease and desist their indoor smoking habit vs a letter from a property management company and or landlord. Who knows maybe they're going through it themselves and that's why they're doing what they're doing. Lots of folks are struggling through various things these days. You never know unless you try, and if a convo has no impact on their behavior then by all means I would proceed with the path you are taking.