Carbon Dioxide is heavier than air so it is not absolutely necessary to have the lid on but i am sure that it is more effective it the lid is closed.
Carbon Dioxide is also the natural gas in the atmosphere that every green plant on this planet must have in order to live. CO2 is as important to the lives of you and your chickens as Oxygen is. Without CO2 all forms of food on this planet will cease to grow.
The smoke and fog that you see on stage during plays and concerts is CO2 in the form of hot or dry ice that has been dropped into a vat of water so that it will flash off faster generating a white cloud of CO2 gas. I have yet to hear of any audience member, one of the production crew, or an actor dying from CO2 poisoning. Besides keeping Eskimo Pies frozen at ball game, hot ice (Carbon Dioxide) is also used to put down the excess cockerels that hatcheries are unable to sell..
You normally breath in about 400 molecules of CO2 out of a total of 1,000,000 molecules of air with every breath of fresh mountain air that you take. When you go to the movies you are exposed to around 8,000 parts CO2 out of every 1,00,000 air molecules and despite machines to scrub the air, a sub-mariner is exposed to 12,000 parts of CO2 per million million parts of air. Carbon Dioxide is not the big danger some in this country make it out to be.