I live in a low rental, had complained since I have moved in on, June,26/2010... have called city of winnipeg was told private property. Called the water waste line.. they did nothing and the smell is still in the unit. The development, was build in 2007.. the air vents where the heat comes from, that is where the gas and sour smell is coming from also smelling like a big toilet.. can winnipeg housing rehabiltation corporation, still rent out these units.. have called property manager .N/A RESPONDE.. wHEN TURNING ON HEAT IT ONLY GETS WORSE... can't breath plus having windows and window a/c on .the wheather is only 8degrees outside can't inmagine when it gets colder.. property manager names is Sharon the owner of the building is Peters.. Am really hoping that you can HELP ME! Sooo that I can bring it to there attention of the owner.. can't get anywhere with manager.. PLEASE HELP ME..

Private should have nothing to do with the Health Dept. PUT IT IN WRITING. That way you have a trail if you need to withhold rent.
Call the Gas Company to see if natural gas is supplied to the building. They will treat that as an emergency if it is.
Natural gas has a smell added to the odorless gas that is distinctively sort of sweet and bitter at the same time.
There's also sewer gas.
And a dead animal, possibly down at the bottom of the heating duct, possibly rotting away in water too. They smell so awful that this is what it might be, especially of no one else smells it in their apt.
Do some sleuthing yourself! Outside, in the basement if there is one, ask people below you and around you, put up signs in the lobby.
See what pipes come in and where and try to find help identifying them.
Cover heating vents one at a time with cardboard and tape to see if you can make it go away to pinpoint the source. That's what I would do 3rd, after the Health Dept letter and the call to the gas co.
Worse comes to worse, go to the ER. I'm not a big fan of ER abuse but if you are breathing toxic fumes you need to find out. THEY will have to contact the Health Dept.

Call your local building department and ask if an inspector can come out. My guess is that the plumbing in your building is not properly vented. There has to be an escape from the roof of the building for sewer gases or they come back into the building. Sounds like someone cut some corners building the place and it's probably a violation of building code. As another option, hire a plumber and get documentation of what's wrong and then pursue the building code violation once you know exactly what is wrong and have expert opinion in writing.